Inheritance and Timing--The Physical Ability to Produce Seed
Effective pollination mechanisms are only the first step toward successful seed propagation. Many herbaceous plants are well adapted to seed production; their genetic makeup ( ploidy ) and physical characteristics allow pollination to occur and provide the opportunity for viable seeds to form. Other herbaceous plants are more selective: they may be polyploids, exhibit physical traits that prevent pollination or fertilization, or require highly specific environmental conditions under which pollination can occur. In other cases, the ability to reproduce through seed production may also be limited by either prezygotic mechanisms, which prevent pollination or fertilization, or postzygotic mechanisms, which prevent the development of the seedling.
First, consider the case of polyploidy. Polyploidy, or the presence of cell nuclei with three or more complete sets of chromosomes (triploid=3n, tetraploid=4n, etc.) occurs naturally in up to one-third of Angiosperm species. Of this percentage, approximately 40% are dicots; monocots constitute the remaining 60%. The occurrence of polyploidy is unpredictable both in nature and when an introduced substance like colchicine is used to produce it artificially. This uneven number of chromosome sets, or lack of homologous chromosome pairing, prevents meiosis during the formation of pollen and egg cells, and resulting in sterility. (See Just the Facts at Library of Crop Technology)
Polyploidy, though, can also be beneficial. Due to the extra sets of genes in their cells, polyploids may be bigger or more vigorous; have larger, thicker leaves, flowers or fruits; or display increased ability to withstand various environmental stresses—any of which may make them more desirable in the landscape. However, because the occurrence of polyploidy frequently results in sterile plants, propagation methods other than sexual reproduction must be used. Sterile plants may produce bulbs, corms, or rhizomes instead of viable seed as a means of reproduction.
An example of a postzygotic mechanism leading to decreased seed production would be when incompatibility between plant tissues occurs and causes a fertilized egg to abort. It might be that the pollinating mechanism is different or the pollen of one plant may be unable to grow through the style of another. In other cases, the parent plant may be sterile, the F1 hybrid weakened, or the offspring of succeeding generations sterile.
Barriers to successful seed propagation are not caused always by morphology or genetic makeup. For example, separation in space or time, or biological or physiological differences can result in poor seed production. Plants pollinated by either wind or insects,(Fig. 4) but located at great distances away from one another may not produce seed. Blowout penstemon, Penstemon haydenii, is an example of a penstemon species separated in space.
A different type of pollination challenge occurs when night-blooming plants are not be visited by the same insects that pollinate day-bloomers, and early-season plants are not be able to take advantage of the pollen of late-season plants. This leads us to our next topic, the effects of environment and timing on seed production.