Learning the basic morphology and mechanics of a plant’s reproductive system 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—that is, when the quality and quantity of seed are below expectations. Seed growers who view learning the ins and outs of reproductive biology as mysterious or “too complicated” are selling themselves short. The biology behind these processes is elegant, yet very utilitarian to the job at hand, and is easily learned by anyone interested in the wonders of nature and in understanding how a seed is formed.
Flower Morphology
The flower is a modified shoot that contains the reproductive parts of the angiosperms. Most flowers have four distinct floral parts—sepals, petals, stamens, and carpels—which form four whorls that make up the individual flower. There is a common pattern in the organization of these floral appendages, although there may be variations in the details of their arrangement based on the method of pollination and seed dispersal for specific families, genera, or individual species.
The sepals and petals are leaf-like and usually form the two outer whorls of the flower. The sepals are often green and thicker than the petals. Collectively the sepals are known as the calyx, and they constitute the outermost structure of most flowers. They generally serve as protection to the developing flower bud before it opens. The petals form the next whorl, which is known as the corolla. Petals are often brightly colored, are scented, and have nectaries at their base, all of which lure insects and other animals to the task of efficiently pollinating flowers. In some wind-pollinated species like beets and spinach, which don’t require insects for pollination, the petals can be small, dull, and inconspicuous.
The two inner whorls of most flowers comprise the stamens and carpels. These are the fertile parts of the flower, with the stamens being the male, pollen-bearing appendages, collectively known as the androecium. Each stamen consists of a stalk or stem, called a filament, upon which the anther is borne. The anther contains two pairs of pollen sacs in which pollen is formed and develops. The female corollary, the carpels, are the ovule-bearing appendages that are collectively known as the gynoecium. Carpels can occur individually, but are often several are fused together to form the gynoecium. The pistil has three parts: the ovary, the style, and the stigma. The ovary contains the ovules, which become seed after fertilization; the style, a tube connecting the ovary to the stigma; and the stigma, a nutrient-rich surface where the pollen grains germinate on their way to the ovary.
The Journey of
the Male Gametes
The male gametes or sex cells are produced in large numbers in the pollen sacs of the anthers. Meiotic divisions result in single-celled microspores or pollen grains so small, individual grains are only discernible by the human eye through the use of magnification. At maturity the pollen is released, either through pores in the anther walls or by a splitting or tearing of the anther (referred to as dehiscence of the anther). In species that are insect-pollinated, the pollen often has barbs or is sticky. Wind-pollinated plants have light, smooth pollen that is designed to be airborne. When pollen is released, a specific set of circumstances must exist for the pollen to successfully complete its journey, ultimately fertilizing the ovule in an ovary of the same species.
Pollination: The first step in what can be considered a journey frequently fraught with obstacles is pollination, the movement of pollen from the anthers to the stigma. Upon release the environmental conditions must be such that the pollen grain remains viable and reaches the stigma of a flower of the same species. If ambient conditions are too hot, the pollen may be denatured; too dry, and the pollen may desiccate and lose viability before reaching a receptive stigma. If it is too cool or rainy the activity of pollinating insects can be greatly impeded: Honeybees are especially sensitive to these conditions and will not fly when it is too cool or wet. Everyone who knows someone with an appreciable number of fruit trees in cooler climates has heard the hard-luck stories of poor “fruit set” in a year when there was a prolonged cool, wet period during the relatively short period that fruit trees flower. Rainy conditions can also impede the movement of pollen in wind-pollinated species by soaking pollen as anthers open and washing much of it to the ground.
Another condition that can impede pollination for most self-pollinated and all wind-pollinated species is to have little or no airflow at the time of pollen maturation and release. In selfers the anthers are always borne in close proximity to the stigma within the cleistogamous or closed flowers common to all selfers. In some cases the anthers are so close to the stigma that just the act of dehiscence (splitting open) will cause the pollen to cascade onto the stigmatic surface with little or no prompting. However, in many cases this short journey requires some type of external movement to literally shake the pollen from the anthers onto the stigma. In many selfing species (tomatoes, peppers, common beans, peas) this is easily accomplished by wind moving and jostling the plant. In some selfers (runner beans and fava beans are good examples) insect visits actually increase seed set by the movement they cause when landing on the flower in an attempt to get pollen, even when they don’t get pollen! Any grower who has ever grown tomatoes through to fruiting in a greenhouse knows that you have to shake them daily during flowering to ensure good fruit set in a greenhouse when they are shielded from the wind. While wind-pollinated crossers like corn, spinach, or beets are grown in sheltered locations, away from the prevailing wind and airflow during flowering, there are times when several days of unusually still air can hinder full seed set (or random mating across the plants in a population that increases the genetic mixing that is essential for the health of crossers) due to low pollen flow in the air.
Pollen Germination: The next step in the journey of the male gametes is the germination of the pollen on the stigma. When the pollen comes in contact with the stigma, a biochemical signal from the moist, rich nutrient medium of the stigma surface promotes hydration and subsequent germination of the pollen grain. Problems can arise that hinder pollen germination if the ambient temperatures are too high or if the stigmatic surface is too dry from extremely low relative humidity. Low relative humidity has been found to be the culprit in cases of poor seed set in several instances of vegetable seed production in the western United States, where either the pollen or the stigma was desiccating before the pollen could properly germinate. Cold temperatures can also hinder pollen germination at different thresholds for different crops.
Pollen Tube Growth: As each pollen grain germinates, it forms a pollen tube that begins to grow down into the porous stigma. A pollen grain contains a vegetative nucleus and a generative nucleus. The vegetative nucleus migrates to the tip of the pollen tube, where it controls the growth and development of the tube, which is rapidly growing through the stigma and into the style. Meanwhile the generative nucleus divides, producing two sperm within the pollen tube. Each growing pollen tube eventually grows through the style with the purpose of delivering the two sperm cells to an individual ovule in the ovary.
This process of the pollen tube germinating and growing through the stigma and style is actually an entire alternate generation of the plant (the gametophyte generation) from the generation of the plant that we are most familiar with (the sporophyte generation). The pollen tube is essentially a free-living plant! For most of the crop plants we grow, the pollen tube life cycle lasts for only 18 to 36 hours, depending on the species. It must complete its journey, delivering the sperm cells to an ovule, in that period of time or it will run out of stored energy and die. In the time that it is alive the pollen tube also has environmental parameters in which it can grow much like a plant does. If the ambient temperature becomes too hot or too cold while the pollen tube is growing, then the tube can stop growing; in most species it will not start to grow again. The pollen tube’s growth parameters are much akin to the plant in which it’s found. In a heat-loving crop like watermelon, which grows prolifically at temperatures above 80°F (27°C), watermelon pollen tubes grow prolifically as well, and both are able to maintain good, steady growth with temperatures upward of 95°F (35°C). By contrast, in a cool-weather crop such as spinach, which produces its most luxuriant growth at temperatures between 60 and 65°F (16 to 18°C), the pollen tube also expresses optimal growth. But when temperatures rise above 75°F (24°C), pollen tube growth will stop and spinach seed crops can suffer severe drops in yield.
The Development of the Embryo and Endosperm
The embryo is formed when the pollen tube finally delivers the two sperm nuclei to the mature embryo sac of an ovule. This last critical step in the forming of the seed is known as fertilization. The fertilized ovule will become the seed. In the angiosperms this step is a double fertilization, with one of the sperm entering into the egg cell in the embryo sac and fusing with the egg nucleus to form a zygote and the other sperm traveling to the center of the embryo sac, where it fuses with two polar nuclei to form the primary endosperm nucleus, which has three sets of chromosomes. These two fertilization events happen concurrently and must occur for the seed development to progress normally. The endosperm starts to divide before the first mitotic division of the zygote; it divides and grows at a more rapid pace than the embryo to develop the storehouse of food for the seed in the form of endosperm starches, lipids, fats, and proteins.
This steady, rapid growth of the endosperm is important for two reasons:
1. The normal, healthy growth of this tissue is crucial to the development of the embryo, which can abort if anything interferes with the development of the endosperm.
2. There is only a short developmental window for the endosperm to be formed, and the more endosperm nutritive tissue is formed during this period, the better equipped the seed will be, with ample food reserves if its initial growth occurs under adverse conditions.
As with pollen tube growth, this endosperm growth is most readily done when the climatic conditions are most favorable for the particular plant’s growth and development. Heat or cold during this period can arrest growth or slow it to the point where the resultant seed is much smaller due to the lower amount of endosperm that was formed. This can lead to lower germination rates in the seed crop and less vigor in the seedlings even if it does germinate. This is why it is so important to grow seed crops in suitable climates!