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The effects of a non-lethal freezing stress on chlorophyll content, moisture level and distribution, and abscisic acid (ABA) levels were examined in siliques and seeds of Brassica napus (canola). A non-lethal freezing stress resulted in the retention of chlorophyll in seed at harvest that was most pronounced for seeds 28, 32 and 36 days after flowering (DAF). This increase was primarily due to an increased retention of chlorophyll a relative to chlorophyll b. Chlorophyll retention in seeds exposed to a non-lethal freezing stress correlated with an increased ABA catabolism, as measured 1, 3 or 7 days after the stress treatment. Although the non-lethal freezing stress had no significant effect on moisture content in seeds of siliques stressed at 28–44 DAF, moisture distribution, as viewed by magnetic resonance imaging, showed an uneven drying of 32 and 40 DAF siliques after exposure to the non-lethal freezing stress. Moisture was initially lost more rapidly from the silique wall between seeds, than in control non-stressed siliques. Increased moisture loss was not due to structural changes in the vasculature of the silique/seed of stressed tissues. These results are consistent with the hypothesis that a non-lethal freezing stress-induced decrease in ABA level, during seed maturation, effects an inhibition of normal chlorophyll a catabolism resulting in mature but green B. napus seed. [ABSTRACT FROM AUTHOR]

Modification of the structure of abscisic acid (ABA) has been reported to result in modification of its physiologic activity. In this study we tested the effect of removing methyl groups from the ring and of chirality of ABA on activity in microspore-derived embryos of oilseed rape (Brassica napus L.). The natural (+)-ABA molecule induced growth inhibition and an increase in the amount of erucic acid accumulated in the oil at medium concentrations less than 1 μM. (-)-ABA showed similar effects. Removing the T-methyl group resulted in a dramatic decrease in activity: (+)-7'-demethyl-ABA retained some activity as a growth inhibitor; a 10-100 μM concentration of this compound was needed for a response, and (-)-7'-demethyl-ABA was almost completely inactive. Similar effects were observed with regard to elongase activity, which catalyzes erucic acid biosynthesis from oleic acid. Removal of the 8'- and 9'-methyl groups resulted in a more complex response. These compounds all showed intermediate activity; for growth inhibition, the presence of the 9'-methyl was the more important determinant, whereas chirality dominated the response on erucic acid accumulation, with the (+)-enantiomers being more active. [ABSTRACT FROM AUTHOR]