Thomas G. Chastain
Grass seed crops are biologically inefficient in the production of seed. Many flowers are produced by these grasses yet relatively few of the flowers become seed, thus the potential seed yield may be many times greater than the actual seed yield harvested. Losses due to inadequate pollination and fertilization, abortion during seed development, and seed shattering all contribute to the relative low numbers of seed that are harvested compared to the crop’s yield potential.
Pollen is the delivery system for transfer of genes from one plant to another. Pollination is the process of pollen transfer and is essential to seed production in most grass seed crops. Grass seed crops are wind pollinated but they are also cross-pollinated because most have high levels of self-incompatibility.
The time course of pollination varies among grass seed crops, but there are some general patterns. Typically, spikelets release pollen first in the apical part of the inflorescence and the release of pollen proceeds downward from there to the basal spikelets. Pollination on an individual inflorescence can be spread over a 4-10 day period. Florets open and release pollen beginning with the proximal florets of the spikelet and ending with the distal florets.
The stamen is the male part of the grass floret. The upper portion of the stamen is known as the anther and is the site of pollen production. Anthers start releasing pollen within about 10 minutes after the stamen emerges from the floret. The pistil is the female part of the grass floret. The upper portion of the pistil is known as the stigma. The stigma is feathery and sticky in grasses and catches the airborne pollen. The stigma is the only part of the pistil visible outside the floret.
After pollen land on the stigma, they germinate to form pollen tubes. These tubes grow down through the style to the ovary. Pollen tube enters the ovule through the micropyle. Sperm cells emerge from the pollen tube and fuse with the egg cell and the two polar nuclei. The fusion of one sperm cell and polar nuclei forms the endosperm nucleus. The fusion of the 2nd sperm cell with the egg cell gives rise to the zygote. This is known as double fertilization and marks the start of seed development.
From beginning to end, pollination in a grass seed field can be spread out over a 21-day period or longer reaching a maximum known as peak anthesis. Pollination does not take place at the same time each year. In a cool spring, pollination is shifted later in the season while in a warm spring it is shifted earlier. The magnitude of the shift depends on the warmth or coolness of the season but typically the shift is only a few days to a week. Cloudy cool weather can extend the length of pollination while hot, dry weather will shorten the pollination period.
Pollen viability is the ability of pollen to germinate and produce a pollen tube. Tall fescue pollen can remain viable for up to 48 hours while most grass pollen is viable for only a few hours. Grass pollen quickly loses viability in high temperatures. Cold or heat during pollen development can reduce pollen viability. Low minimum temperatures during pollination have been shown to reduce seed yield in perennial ryegrass and possibly tall fescue.
Pollen is generally released from mid-morning until afternoon. Pollen viability is highest in the morning and is low later in the afternoon. A second, lower peak in viability in the afternoon has been observed in creeping bentgrass. Factors such as drought stress and dehydration (low humidity), N supply, and others can adversely affect pollen viability. The stigma can dry out due to drought or excessively high temperatures during flowering making pollination more problematic.
Too much rainfall in spring increases lodging and reduces pollination, causing reduced seed yield. High rainfall reduces pollen in the air as evident in pollen counts. When growers encounter high rainfall in mid to late May and the first two weeks in June, especially if there are few or no dry periods, there is a possibility that pollination is reduced. Timing is also important as late-maturing crops may avoid rain problems unlike early-maturing crops if the rain comes early.
Lodging reduces pollination but this effect can be offset by plant growth regulator applications.