With many seed crops, the most difficult production decision is when to harvest. In indeterminate crops such as Brassica species, early harvests result in immature seed of low vigor while late harvests risk seed deterioration and seed loss due to shattering. To provide a biological basis on which to determine harvest timing, we have characterized seed development in rape seed (Brassica napus L. `Weststar') and red cabbage (Brassica oleracea L. Group Capitata) using population-based hydrotime 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 uniformity of germination, viability, and the sensitivity of germination to water potential and ABA. Indices derived from these models, along with maximum germination and t50 values, were used to determine physiological maturity (maximum seed quality) of the seeds during development. The overall trends in seed development were similar in both species: as seeds matured, germination became more uniform and less sensitive to low Ψ and externally applied ABA. The models accurately described germination time courses and final germination percentages except for seeds imbibed at very high concentrations of ABA. In rape seed, physiological maturity was attained several days after maximum seed dry mass, while in red cabbage physiological maturity occurred at or after maximum seed dry mass. Vigor indices were correlated with easily discerned traits such as moisture content and silique phenotypic characteristics. The results of these experiments suggest that hydrotime and ABA-time models can be successfully used to provide a biological basis on which to determine harvest in brassicas.
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