Agriculture Reference
In-Depth Information
seed set, best embryo development and most rapid pollen tube growth occurred
at 35/18 (day/night °C) (25°C mean) (Chang and Struckmeyer, 1976a, b). Air
relative humidity < 70% accelerates pollen shedding by anthers.
The ovary in each flower contains six ovules, and about half the pollen
tubes at the top of a stigma grow as far as the ovary; therefore, 12 or more
initial pollen tubes are needed to achieve maximum seed set. In fact, the
production of three to four ripe seeds per ovary is common in good seed crops.
Nectaries, easily accessible to many types of pollinating insect, are located
between the base of the ovary and the flattened, widened base of the inner
whorl of stamen filaments. The nectar is primarily a hexose sugar solution and
it fluoresces with ultra-violet light that is visible to insects (see also Chapter 6,
'Seed Production').
Fertilization and seed development
Following pollination, fertilization of ovules starts within 12 h and is complete
in 3-4 days. Pollination itself stimulates the initial development of ovules and
ovaries. The shrinkage and loss of green colour in unfertilized ovaries does not
occur until about 3 weeks after flowering, making the early assessment of seed-
set difficult.
In fertilized ovules the endosperm nuclei start to divide first, and cell
division and expansion by the embryo occurs 5-6 days later. The embryo
develops from a globular, few-celled pro-embryo, which is first visible in
microscope sections about 6 days after pollination. It then develops through an
oval to tubular, and finally to a coiled tubular structure embedded within the
endosperm. The embryo reaches its full development when the seed attains
maximum dry weight (see Fig. 2.19). Initially the endosperm is liquid, and this
is termed the 'milk stage'. At about 17 days or 330 DD (degree-days above 0°C)
after pollination, cell walls develop within the endosperm and it progresses to
the pasty 'dough stage' at about 24 days or 450 DD after flowering. At this
point the seedcoat starts to turn black. The seed attains its maximum fresh
weight at about 30 days or 570 DD after flowering.
Up to this point seed dry weight growth is near exponential and seed dry
weight is then about half its maximum. The endosperm then becomes solid and
the seed reaches its maximum dry weight, typically 3.0-3.5 mg, at 810 DD
after flowering, normally about 45 days after flowering (Gray and Ward, 1987).
At this stage capsules begin to shatter and to shed seed (see Fig. 6.17), and the
food-reserve oil globules and protein bodies can first be seen within seeds. Seed
water content declines after the attainment of maximum fresh weight.
Germinability commences just before the maximum fresh weight is attained
(570 DD after flowering) and is near maximum at seed shatter.
