From adamf@vtaix.cc.vt.edu Fri Jul 21 07:42 EDT 1995 Return-Path: Received: from vtaix.cc.vt.edu by sunsite.oit.unc.edu (5.x/TAS/11-16-88/Jones Hack) id AA00204; Fri, 21 Jul 1995 07:42:32 -0400 Received: (from adamf@localhost) by vtaix.cc.vt.edu (8.6.12/8.6.12) id HAA54900 for adamf@sunsite.unc.edu; Fri, 21 Jul 1995 07:49:37 -0400 From: Adam Finkelstein Message-Id: <199507211149.HAA54900@vtaix.cc.vt.edu> Subject: no subject (file transmission) To: adamf@sunsite.unc.edu Date: Fri, 21 Jul 1995 07:49:37 +22300129 (EDT) X-Mailer: ELM [version 2.4 PL21] Content-Type: text Content-Length: 12109 Status: RO Path: solaris.cc.vt.edu!news.netins.net!news.eng.convex.com!news.ecn.uoknor.edu!paladin.american.edu!howland.reston.ans.net!swrinde!elroy.jpl.nasa.gov!lll-winken.llnl.gov!taco.cc.ncsu.edu!news From: Keith Mueller Newsgroups: rec.gardens Subject: FYI: Pollination Date: 21 Jul 1995 06:10:57 GMT Organization: North Carolina State University Lines: 263 Message-ID: <3ungdh$sq8@taco.cc.ncsu.edu> NNTP-Posting-Host: mario1e.ppp.ncsu.edu Xref: solaris.cc.vt.edu rec.gardens:89534 THIS IS RATHER LONG Pollination In response to many questions about pollination in rec.gardens, I have decided to post this. I've noticed people confusing pollination with fertilization (I'm just as guilty at times). So let's set it straight...pollination is the transfer of pollen from a stamen (or staminate cone) to a stigma (or ovulate cone). Fertilization is the union of an egg and a sperm cell to form a zygote. I'm sure most recognize this from a biology course. Fertilization occurs after pollination. Problems can occur at either point but I will limit myself to pollination for this discussion. Pollination in plants is of two types: self-pollination and cross pollination. Both are common. Plants that self- pollinate are not dependent on any external source for fertilizing their flowers (though insects can help as in tomato). Self-incompatible plants require pollen from another plant; in other words they are dependent on cross- pollination. The anther and stigma may be in the same flower (self-pollination), in different flowers on the same plant (self-pollination) ,in different flowers on different plants of the same cultivar (self-pollination) or in different flowers on the plants of different cultivars (cross- pollination). Floral structures often dictate which process will occur. Tomatoes and beans are self-pollinators. The structure of their flowers is such that it is far more likely that they will be pollinated by the same plant. In both cases the stigma (the receptive female part of the flower) is enclosed by the male structures. This almost eliminates the possibility of cross-pollinating. The tomato flower encloses the style and stigma (pistil) within an anther cone. The bean's pistil and stamens are enclosed within the flower itself. Furthermore, the stigma is within a sheath formed by most of the anthers. One anther is within this sheath which greatly increases the likelihood of self- pollination. Outcrossing does occur in these crops and is desirable for reasons I will explain later. Crops like hops and spinach will have only female or male flowers on one plant and are thus required to out-cross. Crops like corn have separate female and male flowers on the same plant and this may increase the likelihood of cross-pollinating as well. Some plants are self-fertile. They have the ability to produce fruit and seed without the transfer of pollen from other cultivars. Self-sterile plants require pollen from another cultivar. This is often the case in some fruit trees. This is primarily due to incompatibility, where a plant's own pollen will not grow through the style to the embryo sac and allow for fertilization to begin. Whether or not a plant is self-pollinated or cross pollinated, cross-pollination is an important process because (1) it ensures a sufficiently high genetic diversity (so that if the environment changes, at least some individuals will be successfully adapted to the new conditions and survive) and (2) it prevents inbreeding, which results in homozygosity (all the same) of the genome Inbreeding often allows for the for the expression of lethal or maladapted genetic traits (inbreeding depression). Some crops like tomatoes can withstand inbreeding. Others, like cole crops (broccoli and cauliflower), suffer from inbreeding depression and require cross-pollination to survive. Cross-pollination by insects is vitally important to the production of seeds and fruit for a very large number of horticultural plant species and cultivars. Wind also is an important mechanism of cross-pollination, but it is a random process. Therefore much pollen is wasted (think of rag weed or the loblolly pines in the South). Pollination by insects may be a much more efficient method because (1) many insects are specifically adapted for certain species of flower (e.g., fig wasps, Yucca moths, orchid pollinators); and (2) insects (especially the higher Hymenoptera [bees]) learn to use a locally abundant pollen or nectar resource. Other sources of pollen transfer can occur by water, snails, slugs, birds and bats. TYPES OF INSECT POLLINATORS* 50% Hymenoptera (e.g., social bees, solitary bees) 25% Diptera (flies [syrphid and hover flies]) 15% Beetles(solider,long-horned & metallic wood borer) 10% Lepidoptera (butterflies & moths) 5% other orders of insects *composition of species varies from crop to crop A bit on bees: Bees use color vision to locate flowers. However, receptors in compound eyes (as in bees and flies) use a different spectrum of light than in the human eye. Bees see ultraviolet light (UV), blue, yellow-green but NOT red. We see red to the violet part of the spectrum but not the short UV (< 400 nanometers). This is important to the bees and relevant to a topic on tomatoes (which I will post at a later date.) Flowers often have "hidden" (to us) UV nectar guides (or patterns) which may assist bees in finding the pollen or nectar source. There exists a good video on photography by the National Geographic Society which shows the UV guides on flowers. It is available at most video stores. Bees (particularly honeybees) are important horticulturally because: 1 - of there ability to learn 2 - social living (large hives) 3 - dependence on pollen 4 - they can be domesticated and manipulated easily Pesticides pose a threat to pollinators but particularly bees. Carbaryl (eg SEVIN) is extremely toxic to honeybees. Applications of pesticides should be done in the late-afternoon or evening after the flowers have closed. This is highly recommended for Cucurbits (melons, cukes, squash etc..) as they are dependent on pollinators. Pesticides like pyrtherin, sabadilla and rotenone are less toxic to bees. Naturally-occurring hover flies are very valuable, not only because of their adult pollinating ability, but because the larvae (juvenile stage) serve as important aphid predators. Thus they are doubly beneficial. In contrast, metallic wood boring beetles are not practical for use as pollinators because their immatures are pests of trees and shrubs. Nuisance flies (house and blow flies) may be good pollinators but there is a similar conflict of interest. Floral benefits received by pollinators, in return for pollination, is an example of mutualism - a form of symbiosis. Because both the plant and the insect benefit from the exchange. Selected Insect-Pollinated Hort Plants Cucurbitaceae (squash, pumpkins (Cucurbita) cucumbers and melons (Cucumis) separate male/female flowers on the same plant (for cucumbers they can be on separate plants all together). Male flowers will appear before the females (the females appear after the 6-7 leaf generally speaking). Because of separate flowers and due to the fact that pollen is too heavy for wind dispersal, insects are very important to these crops. Rosaceae (Pyrus);pears, apples (Prunus) cherries, peaches (Fragaria) strawberry (Rubus) raspberries, blackberries. Pollinators are very important for fruit set in most species and cultivars. Self-incompatibility exists in some (some cultivars of cherries for example require another cultivar to over come incompatibility.) Solanaceae (tomato, peppers, eggplant) Insects help in fruit set of tomato and can help yield but are not required (they don't necessarily cross-pollinate but vibrate the flowers which helps to shake out the pollen onto the stigma). not as important for peppers. Cruciferae (cole crops; broccoli, mustard, cabbage) Pollinators important as these crops are self-incompatible. Umbelliferae (carrots, dill and other various herbs) pollinators improve seed set. Many other families of food and ornamental crops benefit from pollinators as well. Seed Saving and Maintaining Pure Varieties Cross-pollination can be a problem where easily hybridized species and varieties (eg cucurbits) are concerned. Usually planting crops 0.5 - 2 miles apart will significantly reduce or prevent cross-pollination by flying insects. Wind can be a problem as well. With corn, a distance of at least a half a mile is needed. In major corn growing areas hand pollination and bagging may be necessary to maintain cultivar integrity. Other Aspects of Pollination If pollination - and subsequent fertilization - does not occur, fruit set and seed rarely develop. One important exception, however, is fruit that sets parthenocarpically. Parthenocarpy is the formation of fruit without the stimulation of pollination and fertilization. Without fertilization, no seeds are produced; therefore parthenocarpic fruits are seedless. There are many examples of this; "Navel" oranges, "Cavendish" banana, many figs, and Dr. Baggett's "Oregon Spring" tomato. (Not all seedless fruit are parthenocarpic, however; sometimes, as in certain seedless grapes, pollination and fertilization occur, and the fruit forms, but the embryo aborts, thus no seed is produced. Seedless watermelons are often due to a change in ploidy level (# of sets of chromosomes) which result in sterile fruit. This is not related to parthenocarpy. [True, one might find small seed but is likely not viable]). Hand Pollinating Making your own crosses can be fun. This works especially well for squashes/melons (and is relatively easy). For discussion of breeding and detailed info I'd at this point recommend the books "Breed Your Own vegetable Varieties" by Carole Deepe (Little Brown Publishers) or "Breeding Vegetable Crops" by Mark Basset (Avi) [this one is expensive so look in a good ag library first]. Both go into detail and/or provide photos illustrations as well as other pertinent information. Essentially what one needs to do is to identify the male and female flower (or parts there of). The pollen must be viable and the stigma receptive to pollen. On cucurbits (the most requested), again males will form way in advance of the females. the plant needs to get size on it before supporting fruit. Often the first fruit may abort as well. Be patient, remember that thing wants to survive and pass on its DNA by setting fruit. Problems with Pollination Sometimes high temperatures can effect pollen viability. This can occur in many crops. In tomato, another issue is that the style and stigma grow too quickly and extend out beyond the anther cone. This greatly inhibits pollination. One will often see this during hot weather. Here insects could help but, if the flowers don't drop off and the pollen isn't rendered inviable from the high temps one can collect the pollen onto their finger by gently tapping it out and then applying it gently but thoroughly to the stigma. Hey that's all there is to it. Normally wind and the action of the bees shake the pollen onto the stigma when not exerted and fruit set occurs. (Sometimes one might see a plant with no fruit and all of the flowers have styles exerting. This plant is likely male sterile, generally useless to you but favorable to breeders [ if traits are nice] to maintain exclusivity in hybrids.) I hope this helps. I'm sure I might have over-looked something. I would welcome any suggestions, additions and possible corrections (but please no spelling or grammatical errors, life's too short for that). Hopefully, one day I can get a scanner, put pics with this and make a home page. I will be posting a FYI on tomatoes and related species in the near future. And yes Randy, (my advisor) I am working on my paper as well! Keith