Agriculture Reference
In-Depth Information
temperatures, and male-sterile lines should be selected under high temperatures.
When producing hybrid seed, male-sterile lines must be rogued for male-fertiles
during blooming, and these must be destroyed to prevent pollen contamination.
Pollen tube growth and seed set are adversely affected by temperatures above
40°C. Temperatures 20°C above ambient air temperatures have been recorded in
densely packed onion umbels in strong insolation with a low wind speed. A
circular area of aborted flowers on the insolated side of the umbel, which can
occur in these conditions, is termed 'dollar spot' in the USA. Ovary wall
temperatures of 52°C give near 100% seed abortion, and such excessive
temperatures are sometimes responsible for seed crop failures in the USA.
Seed ripening, harvesting and storage
The rate of progress towards seed ripening increases with temperature, and
about 600 degree-days above 10°C are needed to progress from anthesis to the
stage when ripe, black seeds start to be shed from the capsules. At seed
ripening, seeds have attained their full dry weight and seed and umbel
moisture content decline quite rapidly. Harvest must be timed just before the
capsules start to shatter and lose seeds (see Fig. 6.17).
Seeds are hand-harvested when 25-30% of the umbels show ripe seed due
to dehiscence of the capsules. Umbels are cut with about 15 cm of scape
attached. To avoid seed losses, mechanized harvesting is best about 10 days
earlier, when ripe seed can be seen on just 2% of the umbels and when the seed
moisture content is 30-40%. After harvest the umbels may be dried in a variety
of ways. In sunny climates they may be dried outdoors on plastic sheeting, in
layers about 20 cm thick. The umbels should be turned regularly to avoid
Fig. 6.17.
The growth, drying and development of germinability of onion seeds on
ripening umbels in relation to umbel shattering, harvesting time and seed quality
(from Steiner and Akintobi, 1986. Courtesy of
HortScience
).
