USING HYDROTIME AND ABA-TIME MODELS TO QUANTIFY SEED QUALITY OF BRASSICAS DURING DEVELOPMENT

With many seed crops, the most difficult production decision is when t o harvest. In indeterminate crops such as Brassica species, early harv ests result in immature seed of low vigor while late harvests risk see d deterioration and seed loss due to shattering. To provide a biologic al basis on which to determine harvest timing, we have characterized s eed development in rape seed (Brassica napus L. 'Weststar') and red ca bbage (Brassica oleracea L. Group Capitata) using population-based hyd rotime and ABA-time models. These models provide information relevant to assessing physiological maturity, and therefore, seed quality. The hydrotime and ABA-time models quantify germination rate, the uniformit y of germination, viability, and the sensitivity of germination to wat er potential and ABA. Indices derived from these models, along with ma ximum germination and t(50) values, were used to determine physiologic al maturity (maximum seed quality) of the seeds during development. Th e overall trends in seed development were similar in both species: as seeds matured, germination became more uniform and less sensitive to l ow W and externally applied ABA. The models accurately described germi nation time courses and final germination percentages except for seeds imbibed at very high concentrations of ABA. In rape seed, physiologic al maturity was attained several days after maximum seed dry mass, whi le in red cabbage physiological maturity occurred at or after maximum seed dry mass. Vigor indices were correlated with easily discerned tra its such as moisture content and silique phenotypic characteristics. T he results of these experiments suggest that hydrotime and ABA-time mo dels can be successfully used to provide a biological basis on which t o determine harvest in brassicas.