Relationship of chlorophyll, seed moisture and ABA levels in the maturing Brassica napus seed and effect of a mild freezing stress

Chlorophyll (Chl) retention by mature seed of canola as the result of an early frost or other environmental factors (the "green seed problem") causes serious economic losses. The relationship of seed degreening to rate of moisture loss by seed and silique and the role of ABA in this process were investigated as a function of developmental age. During the normal predesiccation stage (28-45 days after pollination), seed of Brassica napus (cv. Westar) loses Chi rapidly but seed moisture slowly. After a mild freezing stress, there is a rapid loss of moisture from silique walls, followed by accelerated loss of seed moisture. Chi degradation ceases at 35-45% seed moisture. ABA levels in silique walls of frozen plants (determined by enzyme-linked immunosorbant assay) increased after freezing, apparently in response to moisture loss. In contrast, ABA levels in the seed increased dramatically 1 day after freezing, then decreased to control levels. The influence of the rate of seed moisture loss on Chi degradation was investigated by fast and slow drying of isolated seed under controlled humidity conditions. Seed dried rapidly at 22% RH retained most of its Chi, whereas seed dried slowly at 86% RH lost Chi as fast or faster than seed on control (unfrozen) plants. In all treatments, Chi loss stopped at about 40% seed moisture.