Why Understanding the Biology of Flowering is Important
Leaning the basic mechanics of plant reproductive biology is very important in understanding how to maximize the yield and quality of the seed you produce. The inherent biological processes of the flower and its sexual parts are often the first place a seed grower looks to when a particular seed crop is not performing adequately, i.e. the quality and quantity of seed is below expectations. Many seed growers who view the ins and out of the biological process as something "too complicated" or even mysterious are selling themselves short as these concepts are really full of common sense and are actually fun to think about when you try to optimize your seed harvest.

Learning the basic flower parts and the various types of flowers is quite easy. Once you become accustomed to the kind of variation that flowers from different plant families can have, you'll be able to differentiate a stigma from an anther in almost any plant family you encounter. This will allow you to monitor crucial steps in the reproductive process like pollen formation, pollen shed, embryo and seed formation, seed development and maturity. This will enable you to troubleshoot any potential problems that may be occur in these crops, helping you to determine whether the poor performance of any particular seed crop is due to environmental, cultural, or varietal factors that can potentially avoided in subsequent growing seasons.

Flower Parts
The basic flower parts are broken down into male and female structures that hold the sex cells or gametes. The male flower parts, known collectively as the stamen, consists of the 1) filament (a type of stem), 2) anthers (which hold the pollen), and 3) pollen grains (which deliver the sperm to the female flowers). The female flower parts, known collectively as the pistil consists of the; 1) ovary (which contains the ovules which become seeds), 2) style (a tube connecting the ovary to the stigma), and 3) stigma (the nutrient rich surface where pollen germinates on its way to the ovary).

The Male Journey in Plants
The function of the stamens is to produce healthy pollen with fully formed sperm nuclei. The anthers must be able to open when the pollen is mature, allowing the pollen to be delivered via insect, wind, or simple physical motion of the plant (this is all that is necessary in many "selfers") to the surface of the stigma so that it can enter the pistil. If the pollen is viable and the environmental conditions such as temperature are "right" then the pollen can germinate and form a pollen tube that will grow down the style and deliver a sperm nucleus to an egg cell in one of the ovules in the ovary. The joining of a sperm nucleus with the egg nucleus is the fertilization event that will produce a zygote that becomes the embryo of the developing seed.

The Female Journey in Plants
The function of the pistil is to form an ovary that can have anywhere from one ovule (examples are peaches and avocados) to hundreds of ovules (as in tomatoes or watermelons) in a single ovary. The stigmatic surface of the pistil is where the pollen germinates if the surface is biochemically compatible (i.e. the same or closely related species). The growth of the pollen tube that grows down through the style can be a long and arduous journey, with the example of a corn plant's style, its silk, being a particularly striking one. The pollen tube then delivers a pair of sperm nuclei to a single embryo, one to fertilize the egg and the other to fertilize the endosperm. This double fertilization event is very important for all of the true flowering plants (Angiosperms) as the development of the endosperm of the seed is as important to the development of the embryo. All of these events must happen with minimal environmental stress in order for a healthy, vigorous seed to be formed.

Floral Biology and Things That Can Go Wrong
A number of very routine environmental conditions can occur that will interrupt the seed forming and growing process. Certainly insuring the optimum growing conditions to minimize the environmental stresses is very important during seed development.

Pollen development and production: Seed producing plants must have ample pollen to insure a good seed set. The amount of pollen produced by a particular plant is influenced by two major factors.

1) Environment: If the plant is stressed due to an inadequate supply of nutrients, water, sunlight, etc., it will at some point depress the development and production of pollen. Also, if the temperatures are too hot or too cold for a particular crop, it can definitely lower or even stop the amount of pollen that is produced.

2) Genetic: There are definite differences in the amount of pollen produced by different varieties of the same crop. As an example the table beet variety 'Ruby Queen' produces copious quantities of pollen that on a clear day can actually be seen as faint yellow clouds of dust rolling out of a large field of 'Ruby Queen' that's flowering. On the other hand the popular gold colored beet 'Burpee's Golden Beet' has such poor pollen production that it often sets much less seed than many other table beet varieties.

Pollen movement: Pollen moves in three ways. In selfers with perfect flowers that remain closed until pollination the plant must be shook for the pollen to release from the anthers and land on the stigma. Usually a little bit of wind in the field will accommodate this, but in the greenhouse it is important to shake the plants each day to release the pollen. In crossers with perfect flowers or monoecious flowers (separate male and female flowers on the same plant) insects are almost always crucial for getting pollen from anthers to stigmas. A shortage of insect pollinizers may then be a serious setback for getting the optimum yields in insect pollinated crossers. Lastly are the crossers that are wind pollinated like beets (with perfect flowers), corn (with monoecious flowers), or spinach (with dioecious flowers, i.e. males and female flowers are on separate plants. The wind does all the work blowing the pollen from plant to plant and it is therefore important to have both a good size population of plants so that a lot of potential pollen is present and to have a variety that is a good pollen producer to insure a good seed set.

Pollen growth: Once the pollen lands on the stigma it must grow down through the style to the ovary. However, if the temperature is either too hot or too cold it may stop the pollen from either germinating on the style or more commonly stop its growth as it goes down the style. Once the pollen tube stops growing it usually doesn't start again and dies before fertilizing an ovule in the ovary. In tomatoes there are some varieties that will set fruit under colder temperatures than others, i.e. the Sub-Arctic types, where the pollen is able to grow through cold night temperatures without stopping. Conversely, there are some southern tomato types sold by southern seed companies with names like "Hot Set" where the plants can set fruit even once night time temperatures hit the 90s (F). This phenomenon happens in all species of flowering plants to some degree and can explain why extremes in temperature can often ruin the seed yield of a crop.

Embryo/endosperm development: Any part of the female flower can suffer under stress. While ovules can be malformed under extremes in environmental stress at any time it is usually during the development of the embryo after fertilization that stress can cause problems that result in poor seed or seed that aborts. Heat, cold, drought or fertility can play a part in this. Heat can be especially detrimental during the rapid growth of the endosperm right after fertilization. Spinach seed is notoriously sensitive to hot weather conditions right as the seed is forming and spinach grown with temperatures much over 80 F usually produce smaller seed and seed that has a lower than usual germination percentage. Many speculate that this is probably due to poor endosperm development under heat stress.

Maturation of seed: As the seed matures it can be damaged by environmental stresses. Certainly heat and sunscald can damage the germination rate and shelf life of seed. Far more common is the effects of excessive moisture on any of the dry seeded crops (most seeds not borne in fruits) as it can cause bacterial and fungal diseases to spread across the seed bearing portions of the crop that will either affect the quality of the seed outright or will infect the seed with inoculum of the pathogen that can cause disease next year. These seed borne diseases are especially insidious as the grower and seed company may not even know it is there, but when the seed is planted next year the disease may spread through the crop quickly with devastating consequences.

Copyright 2003 by John P. Navazio