BASIC BOTANY, PHYSIOLOGY, AND ENVIRONMENTAL EFFECTS ON PLANT GROWTH In order to gain a working knowledge of horticulture, it is necessary to understand the structure and function of plants and the environmental factors that affect plant growth. In the greatly diversified kingdom of plants, all flowering plants have certain structures and functions in common. These similar ities are the basis for this chapter. Higher flowering plants are divided into two groups, monocotyledons (monocots) and dicotyledons (dicots). Although monocots and dicots are similar in many ways, differences with respect to number of seed leaves, number of flower parts, leaf vein pattern, and root structure exist. In addition, physiological dissimilarities exist which, for example, result in different responses to herbicides. BOTANY: PLANT PARTS AND FUNCTIONS The parts of a plant can be divided into two groups, sexual reproductive parts and vegetative parts. Sexual reproductive parts are those involved in the production of seed. They include flower buds, flowers, fruit, and seeds. The vegetative parts include leaves, roots, leaf buds, and stem. Although the vegetative parts are not directly involved in sexual reproduction, they are often used in asexual or vegetative forms of reproduction, such as cuttings. PRINCIPAL PARTS OF A VASCULAR PLANT STEMS Stems are structures which support buds and leaves and serve as conduits for carrying water, minerals, and sugars. The three major internal parts of a stem are the xylem, phloem, and cambium. The xylem and phloem are the major components of a plant's vascular system. The vascular system transports food, water, and minerals and offers support for the plant. Xylem tubes conduct water and minerals, while phloem tub es conduct food. The vascular systems of monocots and dicots differ. While both contain xylem and phloem, they are arranged differently. In the stem of a monocot, the xylem and phloem are paired into bundles; these bundles are dispersed throughout the stem. But in the stem of a dicot, the vascular system forms ring s inside the stem. The ring of phloem is near the bark or external cover of the stem and is a component of the bark in mature stems. The xylem forms the inner ring; it is the sapwood and heartwood in woody plants. The difference in the vascular system of the two groups is of practical interest to the horticulturist because certain herbicides are specific to either monocots or dicots. The cambium is a meristem, which is a site of cell division and active growth. It is located between the xylem and phloem inside the bark of a stem and is the tissue responsible for a stem's increase in girth, as it produces both the xylem and phloem tissues. Stems may be long, with great distances between leaves and buds (branches of trees, runners on strawberries), or compressed, with short distances between buds or leaves (fruit spurs, crowns of strawberry plants, dandelions). Stems can be above the ground like most stems with which we are familiar, or below the ground (potatoes, tulip bulbs). All stems must have buds or leaves present to be classified as stem tis sue. DIVERSIFIED STEM DEVELOPMENT Above-ground modifications A crown is a region of compressed stem tissue from which new shoots are produced, generally found near the surface of the soil. A spur is a compressed fruiting branch. A runner is a type of stolon. It is a specialized stem that grows on the soil surface and forms a new plant at one or more of its nodes. A branch is a stem which is more than one year old. Below-ground modifications A rhizome is a specialized stem which grows horizontally at or just below the soil surface and acts as a storage organ and means of propagation in some plants. A tuber is an enlarged portion of an underground stem. A corm is a compressed stem with reduced scaly leaves. A bulb is composed of a short stem plate, closely spaced buds, and fleshy leaves. An area of the stem where leaves are located is called a node. Nodes are areas of great cellular activity and growth, where auxiliary buds develop into leaves or flowers. The area between nodes is called the internode. The length of an internode may depend on many factors. Decreasing fertility will decrease internode length. Internode length varies with the season. Too little light will result in a long internode, causing a spindly stem. This situation is known as stretch or etiolation. Growth produced early in the season has the greatest internode length. Internode length decreases as the growing season nears its end. Vigorously growing plants tend to have greater internode lengths than less vigorous plants. Internode length will vary with competition from surrounding stems or developing fruit. If the energy for a stem has to be divided between three or four stems, or if the energy is diverted into fruit growth, internode length will be shortened. Modified Stems: Although typical stems are above-ground trunks and branches, there are modified stems which can be found above ground and below ground. The above-ground modified stems are crowns, stolons, and spurs, and the below-ground stems are bulbs, corms, rhizomes, and tubers. Above-ground stems: Crowns (strawberries, dandelions, African violets) are compressed stems having leaves and flowers on short internodes. Spurs are short, stubby, side stems that arise from the main stem and are common on such fruit trees as pears, apples, and cherries, where they may bear fruit. If severe pruning is done close to fruit-bearing spurs, the spurs can revert to a long, nonfruiting stem. A stolon is a horizontal stem that is fleshy or semi-woody and lies along the top of the ground. Strawberry runners are examples of stolons. Remember, all stems have nodes and buds or leaves. The leaves on strawberry runners are small, but are located at the nodes, which are easy to see. The nodes on the runner are the points where roots begin to form. The spider plant has stolons. Below-ground stems such as the potato tuber, the tulip bulb, and the iris rhizome are underground stems that store food for the plant. The tuber, like any other stem, has nodes that produce buds. The eyes of a potato are actually the nodes on the stem. Each eye contains a cluster of buds. Rhizomes are similar to stolons, but grow underground. Some rhizomes are compressed and fleshy such as those of iris; they can also be slender with elongated internodes such as bentgrass. Johnsongrass is a hated weed principally because of the spreading capability of its rhizomes. Tulips, lilies, daffodils, and onions are plants that produce bulbs - shortened, compressed, underground stems surrounded by fleshy scales (leaves) that envelop a central bud located at the tip of the stem. If you cut through the center of a tulip or daffodil bulb in November, you can see all the flower parts in miniature within the bulb. Many bulbs require a period of low-temperature exposure before they begin to send up the new plant. Both the temperature and length of this treatment are of critical importance to commercial growers who force bulbs for holidays. Corms are not the same as bulbs. They have shapes similar to bulbs, but do not contain fleshy scales. A corm is a solid, swollen stem whose scales have been reduced to a dry, leaf-like covering. Some plants produce a modified stem that is referred to as a tuberous stem. Examples are tuberous begonia and cyclamen. The stem is shortened, flattened, enlarged, and underground. Buds and shoots arise from the crown and fibrous roots are found on the bottom of the tuberous stem. In addition, some plants such as the dahlia and the sweet potato produce an underground storage organ called a tuberous root, which is often confused with bulbs and tubers. However, these are roots, not stems, and have neither nodes nor internodes. Stems are commonly used for plant propagation. Above-ground stems can be divide d into sections that contain internodes and nodes. They are referred to as cuttings, and will produce roots to form a new plant. Below-ground stems are also good propagative tissues: rhizomes can be divided into pieces; b ulbs form small bulblets at the base of the parent bulb; cormels are miniature corms that form under the parent corm ; and tubers can be cut into pieces containing eyes and nodes. All of these will produce new plants. It may sometimes be difficult to distinguish between roots and stems, but one sure way is to look for the presence of nodes. Stems have nodes; roots do not. Types of Stems. A shoot is a young stem with leaves present. A twig is a stem which is less than one year old and has no leaves, since it is still in the winter-dormant stage. A branch is a ste m which is more than one year old, and typically has lateral stems. A trunk is a main stem of a woody plant. Most tree s have a single trunk. Trees are perennial woody plants, usually have one mail trunk, and are usually more than 12 feet tall at maturity. Shrubs are perennial woody plants, may have one or several main stems, and are usually less than 12 feet tall at maturity. A vine is a plant which develops long, trailing stems that grow along the ground unless they are supported by another plant or structure. Some twining vines circle their support clockwise ( hops or honeysuckle) while others circle counter-clockwise (pole beans or Dutchman's pipe vine). Climbing vines a re supported by aerial roots (English ivy or poison ivy), slender tendrils which encircle the supporting obj ect (cucumber, gourds, grapes, and passion-flowers), or tendrils with adhesive tips (Virginia creeper and Japanese creeper). Texture and Growth of Stems. Woody stems contain relatively large amounts of hardened xylem tissue in the central core, and are typical of most tree fruits and ornamental trees and shrubs. A cane is a stem which has a relatively large pith (the central strength-giving tissue of stem) and usually lives only one or two years. Examples of plants with canes include rose, grape, blackberry , and raspberry. Herbaceous or succulent stems contain only small amounts of xylem tissue and us ually live for only one growing season. If the plant is perennial, it will develop new shoots from the root. Plants are classified by the number of growing seasons required to complete a life cycle. Annuals pass through their entire life cycle from seed germination to seed production in one growing season, and then die. Biennials are plants which start from seeds and produce vegetative structures a nd food storage organs the first season. During the first winter, a hardy evergreen rosette of basal leaves pers ists. During the second season, flowers, fruit, and seeds develop to complete the life cycle. The plant then dies. Carro ts, beets, cabbage, celery, and onions are biennial plants. Hollyhock, Canterbury Bells, and Sweet William are bienni als which are commonly grown for their attractive flowers. Plants which typically develop as biennials may, in some cases, complete the cycle of growth from seed germination to seed production in only one growing season. This situation occurs when droug ht, variations in temperature, or other climatic conditions cause the plant to physiologically pass through the e quivalent of two growing season, in a single growing season. This phenomenon is referred to as bolting. Perennial plants live for many years, and after reaching maturity, typically produce flowers and seeds each year. Perennials are classified as herbaceous if the top dies back to the ground each winter and new stems grow from the roots each spring. They are classified as woody if the top persists, as in shrubs or trees. Stems as Food. The edible portion of cultivated plants such as asparagus and k ohlrabi is an enlarged succulent stem. The edible parts of broccoli are composed of stem tissue, flower buds, and a fe w small leaves. The edible part of potato is a fleshy underground stem called a tuber. Although the name suggests otherwise, the edible part of the cauliflower is proliferated stem tissue. LEAVES Parts of a Leaf. The blade of a leaf is the expanded, thin structure on either side of the midrib. The blade is usually the largest and most conspicuous part of a leaf. The petiole is the stalk which supports the leaf blade; it varies in length, and may be lacking entirely in some cases where the leaf blade is described as sessile or stalkless. The principal function of leaves is to absorb sunlight for the manufacturing of plant sugars in a process called photosynthesis. Leaves develop as a flattened surface in order t o present a large area for efficient absorption of light energy. The leaf is supported away from the stem by a stem- like appendage called a petiole. The base of the petiole is attached to the stem at the node. The small angle formed between the petiole and the stem is called the leaf axil. An active or dormant bud or cluster of buds is usually lo cated in the axil. The leaf blade is composed of several layers. On the top and bottom is a layer of thickened, tough cells called the epidermis. The primary function of the epidermis is protection of leaf tissue. The way in which the cells in the epidermis are arranged determines the texture of the leaf surface. Some leaves have hairs that are an extension of certain cells of the epidermis. The African violet has so many hairs that the l eaf feels like velvet. Part of the epidermis is the cuticle, which is composed of a waxy substance called cutin that protects the leaf from dehydration and prevents penetration of some diseases. The amount of cutin is a direct response to sunlight, increasing with increasing light intensity. For this reason, plants grown in th e shade should be moved into full sunlight gradually, over a period of a few weeks, to allow the cutin layer to build and to protect the leaves from the shock of rapid water loss or sunscald. The waxy cutin also repels water and can shed pesticides if spreader-sticker agents or soaps are not used. This is the reason many pesticide manufacturers i nclude some sort of spray additive to adhere to or penetrate the cutin layer. On the underside of leaves, some epidermal cells are capable of opening and clo sing. These cells guard the interior of the leaf and regulate the passage of water, oxygen, and carbon dioxide throu gh the leaf. These regulatory cells are called guard cells. They protect openings in the leaf surface called stoma. The opening and closing of the cells is determined by the weather. Conditions that would cause large water losses fr om plants (high temperature, low humidity) stimulate guard cells to close. Mild weather conditions leave guard c ells in an open condition. Guard cells will close in the absence of light. The middle layer of the leaf is the, mesophyll and is located between the upper and lower epidermis. This is the layer in which photosynthesis occurs. The mesophyll is divided into a dense upp er layer, called the palisade, and a spongy lower layer that contains a great deal of air space, called the parenc hyma layer. The cells in these two layers contain chloroplasts which are the actual site of the photosynthetic process. LEAF PARTS Types of Leaves. A number of rather distinct types of leaves occur on plants. Leaves commonly referred to as foliage are the most common and conspicuous, and, as previously stated, serve a s the manufacturing centers where the photosynthetic activity of the plant occurs. Scale leaves or cataphylls are found on rhizomes and are also the small, leathery, protective leaves which enclose and protect buds. Seed leaves, or cotyledons, are modified leaves which are found on the embryonic plant and commonly serve as storage organs. Spines and tendrils, as found on barberry and pea, are specialized modified leaves which protect the plant or assist in supporting the stems. Storage leaves, as are found in bulbous plants and succulents, serve as food storage or gans. Other specialized leaves include bracts, which are often brightly colored. The showy structures on dogwoods and poinsettias are bracts, not petals. Venation of Leaves. The vascular bundles from the stem extend through the peti ole and spread out into the blade. The term venation refers to the patterns in which the veins are distributed in the blade. Two principal types of venation are parallel-veined and net-veined. Parallel-veined leaves are those in which there are numerous veins which run essentially parallel to each other and are connected laterally by minute, straight veinlets. Possibly the most common type of parallel-veining is that found in plants of the grass family where the veins run from the base to the apex of the leaf. Another type of parallel-venation is found in plants such as banana, calla, and pickerel-weed, where the parallel veins run laterally from the midrib. Parallel-veined leaves occur on plants which are part of the monocotyledon group. Net-veined leaves, also called reticulate-veined, have veins which branch from the main rib(s) and then subdivide into finer veinlets which then unite in a complicated network. This system of e nmeshed veins gives the leaf more resistance to tearing than most parallel-veined leaves. Net-venation may be eit her pinnate or palmate. In pinnate venation, the veins extend laterally from the midrib to the edge, as in apple, cherry and peach. Palmate venation occurs in grape and maple leaves, where the principal veins extend outward, lik e the ribs of a fan, from the petiole near the base of the leaf blade. Net-veined leaves occur on plants which are pa rt of the dicotyledon group. TYPES OF VENATION Leaves as a Means of Identifying Plants. Leaves are useful in identifying spec ies and varieties of horticultural plants. The shape of the leaf blade and the type of margin are of major importa nce as identifying characteristics. Simple leaves are those in which the leaf blade is a single continuous unit. A compound leaf is composed of several separate leaflets arising from the same petiole. A deeply lobed leaf may appear similar to a compound leaf, but if the leaflets are connected by narrow bands of blade tissue, it may be classified as a simple leaf. If the leaflets have separ ate stalks and, particularly, if these stalks are jointed at the point of union with the main leaf-stalk, the leaf is conside red to be compound. Some leaves may be doubly compound, having divisions of the leaflets. Shape of the leaf blade: The following are some common shapes which are found in leaves and leaflets. Linear: Narrow, several times longer than wide; approximately the same wid th throughout. Lanceolate: Longer than wide; tapering toward the apex and base. Elliptical: 2 or 3 times longer than wide; tapering to an acute or rounded ape x and base. Ovate: Egg-shaped, basal portion wide; tapering toward the apex. Cordate: Heart-shaped, broadly ovate; tapering to an acute apex, with the base turning in and forming a notch where the petiole is attached. Shape of the leaf apex and base: The following are common shapes found in leaves. Acuminate: Tapering to a long, narrow point. Acute: Ending in an acute angle, with a sharp, but not acuminate, point. Obtuse: Tapering to a rounded edge. Sagittate: Arrowhead-shaped, with two pointed lower lobes. Truncate: Having a relatively square end. Leaf margins: Studying leaf margins is especially useful in the identification of certain varieties of fruit plants. Entire: A smooth edge with no teeth or notches. Serrate: Having small, sharp teeth pointing toward the apex. Dentate: Having teeth ending in an acute angle, pointing outward. Crenate: Having rounded teeth. Sinuate: Having a pronounced sinuous or wavy margin. Incised: Margin cut into sharp, deep, irregular teeth or incisions. Lobed: Incisions extend less than halfway to the midrib. Cleft: Incisions extend more than halfway to the midrib. Leaf arrangement along a stem: The various ways leaves are arranged along a stem are also used to help identify plants. Rosulate arrangement is one in which the basal leaves form a rosette ar ound the stem with extremely short nodes. Opposite leaves are positioned across the stem from each other, two leaves at each node. Alternate or spiral leaves are arranged in alternate steps along the stem with only one leaf at eac h node. Whorled leaves are arranged in circles along the stem. Leaves as Food. The leaf blade is the principal edible part of several horticultural crops including chive, collard, dandelion, endive, kale, leaf lettuce, mustard, parsley, spinach, and Swiss chard. The edible part of leek, onion, and Florence fennel is a cluster of fleshy leaf bases. The petiole of the leaf is the edible product in celery and rhubarb. In plants like Brussels sprout, cabbage, and head lettuce, the leaves - in the form of a large, naked bud - are the edible product. BUDS A bud is an undeveloped shoot from which embryonic leaves or flower parts arise . The buds of trees and shrubs of the temperate zone typically develop a protective outer layer of small, leat hery, bud scales. Annual plants and herbaceous perennials have naked buds in which the outer leaves are green and s omewhat succulent. Buds of many plants require exposure to a certain number of days below a critic al temperature (rest) before they will resume growth in the spring. This time period varies for different plants. The flower buds of forsythia require a relatively short rest period and will grow at the first sign of warm weather. Many peach varieties require from 700 to 1,000 hours of temperatures below 45F (7C) before they will resume growth. D uring rest, dormant buds can withstand very low temperatures, but after the rest period is satisfied, buds b ecome more susceptible to weather conditions, and can be damaged easily by cold temperatures or frost. A leaf bud is composed of a short stem with embryonic leaves, with bud primordia in the axils and at the apex. Such buds develop into leafy shoots. Leaf buds are often less plump and more po inted than flower buds. A flower bud is composed of a short stem with embryonic flower parts. In some c ases, the flower buds of plants which produce fruit crops of economic importance are called fruit buds. This terminology is objectionable because, although flowers have the potential for developing into fruit, this development may never occur because of adverse weather conditions, lack of pollination, or other unfavorable circumstances. The structure is a flower bud, and should be so designated since it may never set fruit. Types of Buds. Buds are named for the location which they inhabit on the stem surface. Terminal buds are those which are located at the apex of a stem. Lateral buds are borne on the sides of the stem. Most lateral buds arise in the axis of a leaf and are called axillary buds. In some instances, more tha n one bud is formed. Adventitious buds are those which arise at sites other than in the terminal or axillary posi tion. Adventitious buds may develop from the internode of the stem; at the edge of a leaf blade; from callus tissue at the cut end of a stem or root; or laterally, from the roots of a plant. Buds as Food. Enlarged buds or parts of buds form the edible portion of some h orticultural crops. Cabbage and head lettuce are examples of unusually large terminal buds. Succulent axillary buds of Brussels sprouts become the edible part of this plant. In the case of globe artichoke, the fleshy basal por tion of the bracts of the flower bud are eaten along with the solid stem portion of the bud. Broccoli is the most import ant horticultural plant in which edible flower buds are consumed. In this case, portions of the stem as well as small l eaves associated with the flower buds are eaten. ROOTS A thorough knowledge of the root system of plants is essential if their growth, flowering, and fruiting responses are to be understood. The structure and growth habits of roots have a pronounce d effect on the size and vigor of the plant, method of propagation, adaptation to certain soil types, and response to cultural practices and irrigation. The roots of certain vegetable crops are important as food. Roots typically originate from the lower portion of a plant or cutting. They possess a root cap, have no nodes, and never bear leaves or flowers directly. The principal functions of roots are to absorb nutrients and moisture, to anchor the plant in the soil, to furnish physical support for the stem, and to serve a s food storage organs. In some plants, they may be used as a means of propagation. Types of Roots. A primary (radicle) root originates at the lower end of the embryo of a seedling plant. A taproot is formed when the primary root continues to elongate downward into the soil an d becomes the central and most important feature of the root system, with a somewhat limited amount of secondary branching. Some trees, especially nut trees like pecan, have a long taproot with very few lateral or f ibrous roots. This makes them difficult to transplant and necessitates planting only in deep, well-drained soil. The ta proot of carrot, parsnip, and salsify is the principal edible part of these crops. A lateral, or secondary, root is a side or branch root which arises from another root. A fibrous root system is one in which the primary root ceases to elongate, lead ing to the development of numerous lateral roots, which branch repeatedly and form the feeding root system of the plant. A fibrous root is one which remains small in diameter because of a lack of significant cambial activity. On e factor which causes shrubs and dwarf trees to remain smaller than standard trees is the inactivity of the camb ium tissue in the roots. If plants that normally develop a taproot are undercut so that the taproot is s evered early in the plant's life, the root will lose its taproot characteristic and develop a fibrous root system. This is done commercially in nurseries so that trees, which naturally have tap roots, will develop a compact, fibrous root sys tem. This allows a higher rate of transplanting success. The quantity and distribution of plant roots is very important because these two factors have a major influence on the absorption of moisture and nutrients. The depth and spread of the roots is dependent on the inherent growth characteristics of the plant and the texture and structure of the soil. Roots w ill penetrate much deeper in a loose, well-drained soil than in a heavy, poorly-drained soil. A dense, compacted laye r in the soil will restrict or stop root growth. During early development, a seedling plant absorbs nutrients and moisture from the few inches of soil surrounding it. Therefore, the early growth of most horticultural crops which are seeded i n rows benefits from band applications of fertilizer, placed several inches to each side and slightly below the seeds. As plants become well-established, the root system develops laterally and usual ly extends far beyond the spread of the branches. For most cultivated crops, roots meet and overlap between the rows. The greatest concentration of fibrous roots occurs in the top foot of soil, but significant numbers of lat erals may grow downward from these roots to provide an effective absorption system several feet deep. Parts of a Root. Internally, there are three major parts of a root. The meristem is at the tip and manufactures new cells; it is an area of cell division and growth. Behind it is the zone of elon gation, in which cells increase in size through food and water absorption. These cells, by increasing in size, push the root through the soil. The third major root part is the maturation zone, in which cells undergo changes in order to be come specific tissues such as epidermis, cortex, or vascular tissue. The epidermis is the outermost layer of cells surrounding the root. These cells are responsible for the absorption of water and minerals dissolved in water. Cortex cells are involved in the movement of water from the epidermis and in food storage. Vascular tissue is lo cated in the center of the root, and conducts food and water. Externally, there are two areas of importance; root hairs are found along the m ain root and perform much of the actual work of water/nutrient absorption. The root cap is the outermost tip of the root, and consists of cells that are sloughed off as the root grows through the soil. The root cap covers and protec ts the meristem. Roots as Food. The enlarged root is the edible portion of several vegetable cr ops. The sweet potato is a swollen root, called a tuberous root, which serves as a food storage area for the plant . Carrot, parsnip, salsify, and radish are elongated taproots. FLOWERS The sole function of the flower, which is generally the showiest part of the pl ant, is sexual reproduction. Its attractiveness and fragrance have not evolved to please man, but to ensure the continuance of the plant species. Fragrance and color are devices to attract pollinators - insects that play an i mportant role in the reproductive process. Binomial nomenclature is the scientific system of giving a double name to plant s and animals. The first, or genus name, is followed by a descriptive, or species, name. Modern plant classificati on, or taxonomy, is based on a system of binomial nomenclature developed by the Swedish physician, Carl von Linne (Li nnaeus). Prior to Linnaeus, people had tried to base classification on leaf shape, plant size, flower color , etc. None of these systems proved workable. Linnaeus's revolutionary approach was to base classification on the f lowers and/or reproductive parts of a plant, and to give plants a genus and species name. This has proven to be the best system, since flowers are the plant part least influenced by environmental changes. For this reason, a knowle dge of the flower and its parts is essential to plant identification. Parts of the Flower. As the reproductive part of the plant, the flower contain s the male pollen and/or the female ovule plus accessory parts such as petals, sepals, and nectar glands. Sepals are small, green, leaf-like structures on the base of the flower which p rotect the flower bud. The sepals collectively are called the calyx. Petals are highly colored portions of the flower. They may contain perfume as w ell as nectar glands. The number of petals on a flower is often used in the identification of plant families and genera. The petals collectively are called the corolla. Flowers of dicots typically have sepals and/or petals in multiples of four or five. Monocots typically have these floral parts in multiples of three. The pistil is the female part of the plant. It is generally shaped like a bowli ng pin and located in the center of the flower. It consists of the stigma, style, and ovary. The stigma is located at the top, and is connected to the ovary by the style. The ovary contains the eggs, which reside in the ovules. After th e egg is fertilized, the ovule develops into a seed. The stamen is the male reproductive organ. It consists of a pollen sac (anther ) and a long, supporting filament. This filament holds the anther in position so the pollen it contains may be dis bursed by wind or carried to the stigma by insects or birds. Types of Flowers. If a flower has a stamen, pistils, petals, and sepals, it is called a complete flower. If one of these parts is missing, the flower is designated incomplete. If a flower contains functional stamens and pistils, it is called a perfect flo wer. (Stamen and pistils are considered the essential parts of a flower.) If either of the essential parts is lacking, the flower is imperfect. Pistillate (female) flowers are those which possess a functional pistil(s) but lack stamens. Staminate (male) flowers contain stamens but no pistils. Because cross-fertilization combines different genetic material and produces st ronger seed, cross-pollinated plants are usually more successful than self-pollinated plants. Consequently, more plants reproduce by cross-pollination than self-pollination. As previously mentioned, there are plants which bear only male flowers (stamina te plants) or bear only female flowers (pistillate plants). Species in which the sexes are separated into stam inate and pistillate plants are called dioecious. Most holly trees are dioecious; therefore, to obtain berries, it is necessary to have a female tree. Monecious plants are those which have separate male and female flowers on the s ame plant. Corn plants and pecan trees are examples. Some plants bear only male flowers at the beginning of the growing season, but later develop flowers of both sexes; examples are cucumbers and squash. How Seeds Form. Pollination is the transfer of pollen from an anther to a stig ma. This may occur by wind or by pollinators. Wind-pollinated flowers lack showy floral parts and nectar since t hey don't need to attract a pollinator. Flowers are brightly colored or patterned and contain a fragrance or nectar whe n they must attract insects, animals, or birds. In the process of searching for nectar, these pollinators will transf er pollen from flower to flower. The stigma contains a chemical which excites the pollen, causing it to grow a l ong tube, down the inside of the style, to the ovules inside the ovary. The sperm is released by the pollen grain and f ertilization typically occurs. Fertilization is the union of the male sperm nucleus (from the pollen grain) an d the female egg (in the ovule). If fertilization is successful, the ovule will develop into a seed. Types of Inflorescences. Some plants bear only one flower per stem and are cal led solitary flowers. Other plants produce an inflorescence, a term which refers to a cluster of flowers and how t hey are arranged on a floral stem. Most inflorescences may be classified into two groups, racemes and cymes. In the racemose group, the florets, which are individual flowers in an inflores cence, bloom from the bottom of the stem and progress toward the top. Some examples of racemose inflorescence inclu de spike, raceme, corymb, umbel, and head. A spike is an inflorescence in which many stemless florets are attach ed to an elongated flower stem, or peduncle, an example being gladiolus. A raceme is similar to a spike except the florets are borne on small stems attached to the peduncle. An example of a raceme inflorescence is the snapdrago n. A corymb is made up of florets whose stalks, pedicels, are arranged at random along the peduncle in such a way that the florets create a flat, round top. Yarrow has a corymb inflorescence. An umbel is similar except that the ped icels all arise from one point on the peduncle. Dill has an umbel inflorescence. A head, or composite, infloresce nce is made up of numerous stemless florets which is characteristic of daisy inflorescence. In the cyme group, the top floret opens first and blooms downward along the ped uncle. A dischasium cyme has florets opposite each other along the peduncle. Baby's breath inflorescence is an example. A helicoid cyme is one in which the lower florets are all on the same side of the peduncle, examples being freesia and statice inflorescences. A scorpioid cyme is one in which the florets are alternate to each other along the peduncle. Examples are tomato and potato inflorescences. Parts of fruit: Fruit consists of the fertilized and mature ovules called seed s and the ovary wall, which may be fleshy, as in the apple; or dry and hard, as in a maple fruit. The only parts o f the fruit which are genetically representative of both the male and female flowers are the seeds (mature ovules ). The rest of the fruit arises from the maternal plant, and is therefore genetically identical to that parent. Some fruits have seeds enclosed within the ovary (apples, peaches, oranges, squash, cucumbers). Others have seeds that are situated on the periphery of fruit tissue (corn, strawberry). Types of fruit: Fruits can be classified as simple fruits, aggregate fruits, o r multiple fruits. Simple fruits are those which develop from a single ovary. These include cherries and peaches (drupe), pears and apples (pome), and tomatoes (berries). Tomatoes are a botanical fruit since they develop from the flower, as do squash, cucumbers, and eggplant. All of these fruits develop from a single ovary. Other types of simpl e fruit are dry. The fruit wall becomes papery or leathery and hard. Examples are peanut (legumes), poppy (capsule), ma ple (samara), and walnut (nut). Aggregate fruits, such as raspberries, come from a single flower which has many ovaries. The flower appears as a simple flower, with one corolla, one calyx, and one stem, but with many pistils or ovaries. The ovaries are fertilized separately and independently. If ovules are not pollinated successfu lly, the fruit will be misshapen and imperfect. Strawberry and blackberry are also aggregate fruits with the addition of an edible, enlarged receptacle. For this reason, they are sometimes termed aggregate-accessory fruits. Multiple fruits are derived from a tight cluster of separate, independent flowe rs borne on a single structure. Each flower will have its own calyx and corolla. Examples of multiple fruits are pin eapple, fig, and the beet seed. Multiple fruits are not common in Virginia. Kinds of Fruit The seed, or matured ovule, is made up of three parts. The embryo is a miniatur e plant in an arrested state of development. Most seeds contain a built-in food supply called the endosperm (or chid is an exception). The endosperm can be made up of proteins, carbohydrates, or fats. The third part is the hard outer covering, called a seed coat, which protects the seed from disease and insects and prevents water from entering the seed (this would initiate the germination process before the proper time). Parts of a Seed Seedlings. Germination is the resumption of active embryo growth. Prior to any visual signs of growth, the seed must absorb water through the seed coat. In addition, the seed must be in the p roper environmental conditions; that is, exposed to oxygen, favorable temperatures, and for some, correct light. The radicle is the first part of the seedling to emerge from the seed. It will develop into the primary root from which root hairs and lateral roots will develop. The portion of the seedling between the radicle and the first leaf-like structure is called the hypocotyl. The seed leaves, cotyledons, encase the embryo and are usually different in shape from t he leaves that the mature plant will produce. Plants producing one cotyledon fall into the group of monocotyledons or monocots. Plants producing two seed leaves are called dicotyledons or dicots. PHYSIOLOGY: PLANT GROWTH AND DEVELOPMENT The three major plant functions that are the basics for plant growth and develo pment are photosynthesis, respiration, and transpiration. HOW A PLANT GROWS PHOTOSYNTHESIS One of the major differences between plants and animals is the ability of plant s to internally manufacture their own food. To produce food for itself, a plant requires energy from sunlight, carbon dioxide from the air, and water from the soil. If any of these ingredients is lacking, photosynthesis, or food produ ction, will stop. If any factor is removed for a long period of time, the plant will die. Photosynthesis literally means " to put together with light." 673 kcal of radiant energy Chlorophyllous cells CARBON DIOXIDE + WATER --------------------------> SUGAR + OXYGEN 6CO2 + 6H2O ----------------------------------------------->C6H12O6 + 6O2 Plants store the energy from light first in simple sugars, such as glucose. This food may be converted back to water and carbon dioxide, releasing the stored energy through the process called resp iration. This energy is required for all living processes and growth. Simple sugars are also converted to other sug ars and starches (carbohydrates) which may be transported to the stems and roots for use or storage, or they may be used as building blocks for more complex structures, e.g. oils, pigments, proteins, cell walls, etc. Any green plant tissue is capable of photosynthesis. Chloroplasts in these cel ls contain the green pigment, which traps the light energy. However, leaves are generally the site of most food pr oduction due to their special structure. The internal tissue (mesophyll) contains cells with abundant chloroplasts in an arrangement that allows easy movement of water and air. The protective upper and lower epidermis (skin) lay ers of the leaf include many stomata that regulate movement of the gases involved in photosynthesis into and out of the leaf. Photosynthesis is dependent on the availability of light. Generally speaking, a s sunlight increases in intensity, photosynthesis increases. This results in greater food production. Many garden crops, such as tomatoes, respond best to maximum sunlight. Tomato production is cut drastically as light intensities dr op. Only two or three varieties of "greenhouse" tomatoes will produce any fruit when sunlight is minimal in late f all and early spring. Water plays an important role in photosynthesis in several ways. First, it main tains a plant's turgor, or the firmness or fullness of plant tissue. Turgor pressure in a cell can be compared to air i n an inflated balloon. Water pressure or turgor is needed in plant cells to maintain shape and ensure cell growth. Se cond, water is split into hydrogen and oxygen by the energy of the sun that has been absorbed by the chlorophyll in th e plant leaves. The oxygen is released into the atmosphere and the hydrogen is used in manufacturing carbohyd rates. Third, water dissolves minerals from the soil and transports them up from the roots and throughout the plant, where they serve as raw materials in the growth of new plant tissues. The soil surrounding a plant shou ld be moist, not too wet or too dry. Water is pulled through the plant by evaporation of water through the leaves (t ranspiration). Photosynthesis also requires carbon dioxide (CO2) which enters the plant throug h the stomata. Carbon and oxygen are used in the manufacture of carbohydrates. Carbon dioxide in the air is plen tiful enough so that it is not a limiting factor in plant growth. However, since carbon dioxide is consumed in making sug ars and is not replenished by plants at a rapid rate, a tightly closed greenhouse in midwinter may not let in enough outside air to maintain an adequate carbon dioxide level. Under these conditions, improved crops of roses, carnations, tomatoes, and certain other crops can be produced if the carbon dioxide level is raised with CO2 generators or, in small greenhouses, with dry ice. Although not a direct component in photosynthesis, temperature is an important factor. Photosynthesis occurs at its highest rate in the temperature range 65 to 85F (18 to 27C) and decreases w hen temperatures are above or below this range. RESPIRATION Carbohydrates made during photosynthesis are of value to the plant when they ar e converted into energy. This energy is used in the process of building new tissues (plant growth). The chemi cal process by which sugars and starches produced by photosynthesis are converted into energy is called respira tion. It is similar to the burning of wood or coal to produce heat (energy). This process in cells is shown most simp ly as: C6H12O2 + 6O2 --------------- 6CO2 + 6H2O + Energy This equation is precisely the opposite of that used to illustrate photosynthes is, although more is involved than just reversing the reaction. However, it is appropriate to relate photosynthesis to a building process, while respiration is a breaking-down process. Photosynthesis 1. Produces food. 2. Stores energy. 3. Occurs in cells containing chloroplasts. 4. Releases oxygen. 5. Uses water. 6. Uses carbon dioxide. 7. Occurs in sunlight. 1. Uses food for plant energy. 2. Releases energy. 3. Occurs in all cells. 4. Uses oxygen. 5. Produces water. 6. Produces carbon dioxide. 7. Occurs in darkness as well as light. By now, it should be clear that respiration is the reverse of photosynthesis. U nlike photosynthesis, respiration occurs at night as well as during the day. Respiration occurs in all life forms and in all cells. The release of accumulated carbon dioxide and the uptake of oxygen occurs at the cell level. In animals, b lood carries both carbon dioxide and oxygen to and from the atmosphere by means of the lungs or gills. In plants, there is simple diffusion into the open spaces within the leaf, and exchange occurs through the stomata. TRANSPIRATION Transpiration is the process by which a plant loses water, primarily from leaf stomata. Transpiration is a necessary process that involves the use of about 90% of the water that enters the plant t hrough the roots. The other 10% of the water is used in chemical reactions and in plant tissues. Transpiration is necessary for mineral transport from the soil to the plant parts, for the cooling of plant parts through evaporation , to move sugars and plant chemicals, and for the maintenance of turgor pressure. The amount of water lost from the p lant depends on several environmental factors such as temperature, humidity, and wind or air movement. An increase in temperature or air movement decreases humidity and causes the guard cells in the leaf to shrink, o pening the stomata and increasing the rate of transpiration. Plant growth and distribution are limited by the environment. If any one enviro nmental factor is less than ideal, it will become a limiting factor in plant growth. Limiting factors are also respon sible for the geography of plant distribution. For example, only plants adapted to limited amounts of water can live in deserts. Most plant problems are caused by environmental stress, either directly or indirectly. Therefore, i t is important to understand the environmental aspects that affect plant growth. These factors are light, temper ature, water, humidity, and nutrition. LIGHT Light has three principal characteristics that affect plant growth: quantity, quality, and duration. Light quantity refers to the intensity or concentration of sunlight and varies with the season of the year. The maximum is present in the summer and the minimum in winter. The more sunlight a plant receives (up to a point), the better capacity it has to produce plant food through photosynthesis. As the sunlight quantity decreases, the photosynthetic process decreases. Light quantity can be decreased in a garden or greenhouse by using cheesecloth shading above the plants. It can be increased by surrounding plants with white or reflective material, or supplemental lights. Light quality refers to the color or wavelength reaching the plant surface. Sun light can be broken up by a prism into respective colors of red, orange, yellow, green, blue, indigo, and violet. On a rainy day, raindrops act as tiny prisms and break the sunlight into these colors, producing a rainbow. Red and blue lig ht have the greatest effect on plant growth. Green light is least effective to plants as they reflect green light an d absorb none. It is this reflected light that makes them appear green to us. Blue light is primarily responsible for veg etative growth or leaf growth. Red light, when combined with blue light, encourages flowering in plants. Fluoresce nt, or cool-white, light is high in the blue range of light quality and is used to encourage leafy growth. Such light would be excellent for starting seedlings. Incandescent light is high in the red or orange range, but generally produces too much heat to be a valuable light source. Fluorescent "grow" lights have a mixture of red and blue colors that attempts to imitate sunlight as closely as possible, but they are costly and generally not of any g reater value than regular fluorescent lights. Light duration, or photoperiod, refers to the amount of time that a plant is ex posed to sunlight. When the concept of photoperiod was first recognized, it was thought that the length of periods of light triggered flowering. The various categories of response were named according to the light length (i.e., short-day and long-day). It was then discovered that it is not the length of the light period but the length of unin terrupted dark periods that is critical to floral development. The ability of many plants to flower is controlled by photo period. Plants can be classified into three categories, depending upon their flowering response to the duration of da rkness. These are short-day, long-day, or day-neutral plants. Short-day plants form their flowers only when the day length is less than about 12 hours in duration. Short-day plants include many spring- and fall-flowering plants such as chrysanthemum and poinsettia. Long-day plants form flowers only when day lengths exceed 12 hours (short nights). They include alm ost all of the summer-flowering plants, such as rudbeckia and California poppy, as well as many vegetables incl uding beet, radish, lettuce, spinach, and potato. Day-neutral plants form flowers regardless of day length. Some pla nts do not really fit into any category but may be responsive to combinations of day lengths. The petunia will flower r egardless of day length, but flowers earlier and more profusely under long daylight. Since chrysanthemums flower und er the short-day conditions of spring, or fall, the method for manipulating the plant into experiencing short days is very simple. If long days are predominant, a shade cloth is drawn over the chrysanthemum for 12 hours daily t o block out light until flower buds are initiated. To bring a long-day plant into flower when sunlight is not prese nt longer than 12 hours, artificial light is added until flower buds are initiated. TEMPERATURE Temperature affects the productivity and growth of a plant, depending upon whet her the plant variety is a warm- or cool-season crop. If temperatures are high and day length is long, cool-sea son crops such as spinach will bolt rather than produce the desired flower. Temperatures that are too low for a wa rm-season crop such as tomato will prevent fruit set. Adverse temperatures also cause stunted growth and poor quality; for example, the bitterness in lettuce is caused by high temperatures. Sometimes temperatures are used in connection with day length to manipulate the flowering of plants. Chrysanthemums will flower for a longer period of time if daylight temperatures are 59F (15C). The Christmas cactus forms flowers as a result of short days and low temperatures. Temperatur es alone also influence flowering. Daffodils are forced to flower by putting the bulbs in cold storage in October at 35 to 40F (2 to 4C). The cold temperatures allow the bulb to mature. The bulbs are transferred to the greenho use in midwinter where growth begins. The flowers are then ready for cutting in 3 to 4 weeks. Thermoperiod refers to daily temperature change. Plants and produce maximum gro wth when exposed to a day temperature that is about 10 to 15 higher than the night temperature. This allo ws the plant to photosynthesize (build up) and respire (break down) during an optimum daytime temperature, and to curt ail the rate of respiration during a cooler night. High temperatures cause increased respiration, sometimes above the rate of photosynthesis. This means that the products of photosynthesis are being used more rapidly than they are being produced. For growth to occur, photosynthesis must be greater than respiration. Low temperatures can result in poor growth. Photosynthesis is slowed down at lo w temperatures. Since photosynthesis is slowed, growth is slowed, and this results in lower yields. N ot all plants grow best in the same temperature range. For example, snapdragons grow best when nighttime temperatur es are 55F (12C); the poinsettia prefers 62F (17C). Florist cyclamen does well under very cool condit ions, while many bedding plants prefer a higher temperature. Recently it has been found that roses can tolerate much lower nighttime temperatures than was previously believed. This has meant a conservation in energy for green house growers. However, in some cases, a certain number of days of low temperatures are needed by plants to grow properly. This is true of crops growing in cold regions of the country. Peaches are a prime ex ample; most varieties require 700 to 1,000 hours below 45F (7 C) and above 32F (0C) before they break their rest per iod and begin growth. Lilies need 6 weeks at 33F (1C) before blooming. Plants can be classified as either hardy or nonhardy, depending upon their abil ity to withstand cold temperatures. Winter injury can occur to nonhardy plants if temperatures are too low or if un seasonably low temperatures occur early in the fall or late in the spring. Winter injury may also occur because o f desiccation (drying out) - plants need water during the winter. When the soil is frozen, the movement of water into th e plant is severely restricted. On a windy winter day, broadleaved evergreens can become water-deficient in a few mi nutes; the leaves or needles then turn brown. Wide variations in winter temperatures can cause premature bud brea k in some plants and consequent bud-freezing damage. Late spring frosts can ruin entire peach crops. If tempera tures drop too low during the winter, entire trees of some species are killed by the freezing and splitting of plant cells and tissue. Review of Temperature Effects on Plant Growth: Photosynthesis:Increases with temperature to a point. Respiration: Rapidly increases with temperature. Transpiration:Increases with temperature. Flowering: May be partially triggered by temperature. Sugar storage: Low temperatures reduce energy use and increase sugar storage. Dormancy: Warmth, after a period of low temperature, will break dormancy and the plant will resume active growth WATER As mentioned earlier, water is a primary component of photosynthesis. It mainta ins the turgor pressure or firmness of tissue and transports nutrients throughout the plant. In maintaining turgor pressure, water is the major constituent of the protoplasm of a cell. By means of turgor pressure and other changes in t he cell, water regulates the opening and closing of the stomates, thus regulating transpiration. Water also provides the pressure to move a root through the soil. Among water's most critical roles is that of the solvent for minerals moving into the plant and for carbohydrates moving to their site of use or storage. By its gradual evaporatio n from the surface of the leaf near the stomate, water helps stabilize plant temperature. Relative Humidity is the ratio of water vapor in the air to the amount of water the air could hold at a given temperature and pressure, expressed as a percent. For example, if a pound of ai r at 75F could hold 4 grams of water vapor and there are only 3 grams of water in the air, then the relative humidit y (RH) is : RH = water in the air water the air could hold (at constant temperature and pressure) so, RH = 3/4 = .75 expressed as a % = 75% Warm air can hold more water vapor than cold air; therefore, if the amount of water in the air stays the same and the temperature increases, the relative humidity decreases. Water vapor will move from an area of high relative humidity to one of low rela tive humidity. The greater the difference in humidity, the faster water will move. The relative humidity in the air space between the cells within the leaf approa ches 100%; therefore, when the stomate is open, water vapor rushes out. As the vapor moves out, a cloud of hig h humidity is formed around the stomate. This cloud of humidity helps slow down transpiration and cool the leaf . If air movement blows the humid cloud away, transpiration will increase as the stomata keep opening to balance the humidity. Cross Section of a Leaf Dots Represent Relative Humidity NUTRITION Many people confuse plant nutrition with plant fertilization. Plant nutrition r efers to the needs and uses of the basic chemical elements in the plant. Fertilization is the term used when these mater ials are supplied to the environment around the plant. A lot must happen before a chemical element supplied in a fer tilizer can be taken up and used by the plant. Plants need 17 elements for normal growth. Carbon, hydrogen, and oxygen are fou nd in air and water. Nitrogen, potassium, magnesium, calcium, phosphorous, and sulfur are found in the soil. T he latter six elements are used in relatively large amounts by the plant and are called macronutrients. There are eight other elements that are used in much smaller amounts; these are called micronutrients or trace elements. The mi cronutrients, which are found in the soil, are iron, zinc, molybdenum, manganese, boron, copper, cobalt, and chl orine. All 17 elements, both macronutrients and micronutrients, are essential for plant growth. Most of the nutrients that a plant needs are dissolved in water and then absorb ed by the roots. Ninety-eight percent of these plant nutrients are absorbed from the soil solution and only about 2% are actually extracted from the soil particles by the root. Most of the nutrient elements are absorbed as charged io ns, or pieces of molecules (which are the smallest particle of a substance that can exist and still retain the charac teristics of the substance). Ions may be positively charged cations or negatively charged anions. Positive and negative are equally paired so that there is no overall charge. For example, nitrogen may be absorbed as nitrate (NO3-) whic h is an anion with one negative charge. The potassium ion (K+) is a cation with one positive charge. Potassium nitrate (K+NO3-) would be one nitrate ion and one potassium ion. However, calcium nitrate (Ca++(NO3-)2) would have tw o nitrate ions and one calcium ion because the calcium cation has two positive charges. The balance of ions in the soil is very important. Just as ions having opposite charges attract each other, ions having similar charges compete for chemical interactions and reactions in the environm ent. Some ions are more active than others or can compete better. For example, both calcium (Ca++) and magnesium (M g++) are cations with two charges, but magnesium is more active. If both are in competition to be absorbed, the ma gnesium will be absorbed. This explains why the results of a soil test may indicate that, while there is suffi cient calcium in the soil, the plant may still exhibit a calcium deficiency because of an excess of the more active magn esium. What may be expressed as a deficiency in one micronutrient may really be caused by an excess of another. In order for the ions to be easily absorbed, they must first be dissolved in the soil solution. Some combinations of ions are easily dissolved, such as potassium nitrate. When other ions combine, they may precipitate or fall out of solution and thus become unavailable to the plant. Many of the micronutrients f orm complex combinations with phosphorous and calcium and precipitate out of the soil solution so the nutrien ts cannot be easily taken up by the plant. The pH, which is a measurement of acidity or alkalinity, greatly affects these chemical reactions. If the soil pH is extremely high (alkaline), many of the micronutrients precipitate out of the solution and are unavailable to the plant. When the soil pH is extremely low (acid), some of the micronutrients become extremely soluble and ion levels may become high enough to injure the plant. The effect of pH varies with the ion, the types of ions in the soil, and the type of soil. Therefore, not only is the amount of the nutrient importa nt, but also the soil pH. Adequate water and oxygen must be available in the soil. Water is required for nutrient movement into and throughout the roots. Oxygen is required because the mineral ions must be move d into the root cells across their membranes. This is an active absorption process, utilizing energy from respira tion. Oxygen is not transported to roots from the shoot. Without adequate oxygen from the soil, there is no energ y for nutrient absorption. This also stops active water absorption in which the water flows into the cell due to the higher concentration of nutrients that were actively absorbed. Anything that lowers or prevents the production of sugars in the leaves can low er nutrient absorption. If the plant is under stress due to low light or extremes in temperature, nutrient deficienc y problems may develop. The stage of growth or how actively the plant is growing may also affect the amount of nu trients absorbed. Many plants go into a rest period, or dormancy, during part of the year. During this dormancy, few nutrients are absorbed. Plants may also absorb different nutrients just as flower buds begin to develop. Nutrients transported from the root to the cell by the vascular system move int o the cell through a cell membrane. There are three different ways this happens. First, an entire molecule or ion p air may move through the membrane. If the cell is using energy or active transport to absorb the ions, then only o ne of the ions in the pair is pulled into the cell. The other will follow to keep the number of positive and negative cha rges even. Most anions (negative ions) are actively absorbed. The second way of keeping the charges inside the cell balanced and absorbing a new ion is to exchange one charged ion for another ion with the same charge. A hydrogen ion (H+) is often released so that the cell can absorb another positive ion such as potassium (K+). Since this is a simple, passive exchange, absorption energy may not be required. Cations may be absorbed by this passive method. Both of the methods mentioned above may be passive or active. However, the thir d method, the carrier system, is always active absorption, requiring energy. Scientists have discovered that wit hin the cell membrane there are specialized chemicals that act as carriers. The carrier, through chemical chang es, attracts an ion from outside the cell membrane and releases it inside the cell. Once the ion is inside the cell, it is attached to other ions so that it does not move out of the cell. Complex chemical reactions are involved in the entire process. Although nutrients can be absorbed passively, research has shown that active absorption must take place i f the plant is to grow and be healthy. The factors we discussed earlier about absorption by the root are also true for absorption by the cell. A quick review of some of the factors that affect nutrient absorption: type of ion, soil pH, s olubility of ion pairs, water, soil oxygen, sugar supply, plant stress, and temperature. Foliar Absorption: A Special Case. Under normal growing conditions, plants ab sorb most nutrients, except carbon, hydrogen, and oxygen, from the soil. However, some nutrients can also be absorb ed by the leaves if they are sprayed with a dilute solution. The factors that affect absorption by the cell are stil l important because the nutrient must enter the cell to be used by the plant. Care must be taken that the concentration of the nutrient is not too high or the leaf will be injured. Also, the leaf is covered by a thin layer of wax called the cu ticle that the nutrient must get around or through before it can enter the cell. MACRONUTRIENT OUTLINE Nitrogen (N) Absorbed as NO3-, NH4+ Leaches from soil, especially NO3- Mobile in plant. Nitrogen excess: Succulent growth, dark green color, weak spindly growth, few fruits, may cause brittle growth especially under high temperatures. Nitrogen deficiency: Reduced growth, yellowing (chlorosis), reds and purples may intensify with some plants, reduced lateral breaks. Symptoms appear first on older growth. Action notes: In general, the best NH4+/NO3- ratio is 1/1. High NH4+ under low sugar conditions (low light) can cause leaf curl. Uptake inhibited by high P levels. N/K ratio extremely important. Indoors, best N/K ratio is 1/1 unless light is extremely high. In soils with high CHO/N ratio more N should be supplied. Phosphorus (P) Absorbed as H2PO4-, HPO4- Does not leach from soil readily Mobile in plant. Phosphorus excess: Shows up as micronutrient deficiency of Zn, Fe, or Co Phosphorus deficiency: Reduced growth, color may intensify, browning or purpling in foliage in some plants, thin stems, reduced lateral breaks, loss of lower leaves, reduced flowering. Action notes: Rapidly "fixed" on soil particles when applied under acid conditions fixed with Fe, Mg and Al. Under alkaline conditions fixed with Ca. Important for young plant and seedling growth. High P interferes with micronutrient absorption and N absorption. Used in relatively small amounts when compared to N and K. May leach from soil high in bark or peat. Potassium (K) Absorbed as K+ leaches from soil. Mobile in plant. Potassium excess: Causes N deficiency in plant and may affect the uptake of other positive ions. Potassium deficiency: Reduced growth, shortened internodes, marginal burn or scorch (brown leaf edges), necrotic (dead) spots in the leaf, reduction of lateral breaks and tendency to wilt readily. Action notes: N/K balance is important. High N/low K favors vegetative growth; low N/high K promotes reproductive growth (flower, fruit). Magnesium (Mg) Absorbed as Mg++. Leaches from soil. Mobile in plant. Magnesium excess: Interferes with Ca uptake. Magnesium deficiency: Reduction in growth, marginal chlorosis, interveinal chlorosis (yellow between the veins) in some species. May occur with middle or lower leaves, reduction in seed production, cupped leaves. Action notes: Mg is commonly deficient in foliage plants because it is leached and not replaced. Epsom salts at a rate of 1 teaspoon per gallon may be used 2 times a year. Mg can also be absorbed by leaves if sprayed in a weak solution. Dolomitic limestone can be applied in outdoor situations to rectify a deficiency. Calcium (Ca) Absorbed as Ca++ moderately leachable. Limited mobility in plant. Calcium excess: Interferes with Mg absorption. High Ca usually causes high pH which then pre cipitates many of the micronutrients so that they become unavailable to the plant. Calcium deficiency: Inhibition of bud growth, death of root tips, cupping of maturing leaves, we ak growth, blossom end rot of many fruits, pits on root vegetables. Action notes: Ca is important to pH control and is rarely deficient if the correct pH is maintained. Water stress, too much or too little, can affect Ca relationships within the plant causing deficiency in the location where Ca was needed at the time of stress. Sulfur (S) Absorbed as SO4-. Leachable. Not mobile. Sulfur excess: Sulfur excess is usually in the form of air pollution. Sulfur deficiency: S is often a carrier or impurity in fertilizers and rarely deficient. It may be also absorbed from the air and is a by-product of combustion. Symptoms are a general yellowing of the affected leaves or the entire plant. Action notes: Sulfur excess is difficult to control. MICRONUTRIENT OUTLINE The majority of the micronutrients are not mobile; thus, deficiency symptoms ar e usually found on new growth. Their availability in the soil is highly dependent upon the pH a nd the presence of other ions. The proper balance between the ions present is important, as man y micronutrients are antagonistic to each other. This is especially true of the heavy metals whe re an excess of one element may show up as a deficiency of another. If the pH is maintained at the proper level and a fertilizer which contains micronutrients is used once a year, deficiency symp toms (with the exception of iron deficiency symptoms) are rarely found on indoor plants. Many of the micronutrients are enzyme activators. Iron (Fe) Absorbed as Fe++, Fe+++. Iron deficiency: Interveinal chlorosis primarily on young tissue, which may become white. Fe deficiency may be found under the following conditions even if Fe is in t he soil: Soil high in Ca, poorly drained soil, soil high in Mn, high pH, high P, soil high in heavy metals (Cu,Zn), oxygen deficient soils or when nematodes attack the roots. Fe should be added in the chelate form; the type of chelate needed depends u pon the soil pH. Iron toxicity: Rare except on flooded soils. Boron (B) Absorbed as BO3- Boron excess: Blackening or death of tissue between veins. Boron deficiency: Failure to set seed, internal breakdown, death of apical buds. Zinc (Zn) Absorbed as Zn++. Zinc excess: Appears as Fe deficiency. Interferes with Mg. Zinc deficiency: "Little leaf," reduction in size of leaves, short internodes, distorted or puckered leaf margins, interveinal chlorosis. Copper (Cu) Absorbed as Cu++, Cu+. Copper excess: Can occur at low pH. Shows up as Fe deficiency. Copper deficiency: New growth small, misshapen, wilted. May be found in some peat soils. Manganese (Mn) Absorbed as Mn++. Manganese excess: Reduction in growth, brown spotting on leaves. Shows up as Fe deficiency. Found under acid conditions. Manganese deficiency: Interveinal chlorosis of leaves followed, by brown spots producing a checkered red effect. Molybdenum (Mo) Absorbed as MoO4-. Molybdenum deficiency: Interveinal chlorosis on older or midstem leaves, twisted leaves (whiptail). Chlorine (Cl) Absorbed as Cl-. Chlorine deficiency: Wilted leaves which become bronze then chlorotic then die; club roots. Chlorine toxicity: Salt injury, leaf burn, may increase succulence. Cobalt (Co) Absorbed as Co++. Needed by plants recently established. Essential for Nitrogen fixation. Little is known about its deficiency or toxicity symptoms. .