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David W. Still

Brassica crops have indeterminate growth and flower over an extended period of time. Harvested seed is therefore comprised of seed of varying degrees of physiological maturity and quality. Using population-based threshold models, broccoli (Brassica oleracea L. Group Italica), brussels sprouts (B. oleracea L. Group Gemmifera), red cabbage (B. oleracea L. Group Capitata), and rapeseed (B. napus L.) were characterized during seed development with respect to sensitivity to abiotic stress (reduced water potential) and shelf life. Using these models our data suggests that the physiological patterns of seed development are the same in all brassica crops we have tested to date. These population-based models can be used to provide a biological basis in which to evaluate cultural, postharvest and storage practices to ensure the production and maintenance of seed vigor.

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David W. Still and Kent J. Bradford

Brassica crops have an extended flowering period due to both progressive development within a given raceme and variability among multiple racemes. Early harvest can result in poor seed quality due to immaturity, while delayed harvest may sacrifice yield due to shattering. To characterize the development of seed quality, we measured maturity indices and conducted vigor tests on hybrid red cabbage (Brassica oleracea var capitata) seed harvested weekly starting 33 days after full bloom (DAF). Viability and germination rate increased from the top to the bottom of the raceme, and were maximal by 40 and 48 DAF, respectively. After 48 DAF, there was little difference in seed quality due to position on the raceme. Seed dry weight also reached a plateau by 48 DAF, when rapid dehydration began. Sensitivity of germination to inhibition by reduced water potential or abscisic acid (ABA) was assessed using a threshold model based upon germination rates. Germination became less sensitive to both factors and more uniform during maturation, with -1.0 MPa or 50 μM ABA being required to inhibit germination by 50% after about 48 DAF Seed ABA content reached a peak of 10 μg/g dry wt. by 40 DAF, then declined linearly to 1.5 μg/g dry wt. by 68 DAF Overall, optimal seed quality was attained at 54 DAF

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David W. Still and Kent J. Bradford

Lettuce seeds were germinated at 20 C in the dark in water and sampled at various intervals during the first 18h of germination to determine quantitative and qualitative differences in proteins. The soluble protein fraction was partitioned into albumins and globulins by dialysis and the proteins of the globulin fraction were visualized by SDS-PAGE. Heat stable proteins were obtained by boiling the proteins, cooling on ice, and resuspending in buffer.

The soluble protein content remained constant during the first 8h of germination. Thereafter protein content decreased and was 6% of the amount present in unimbibed seed in 21 day old seedlings. The ratio of heat stable to heat unstable proteins decreased during the germination process. No differences in banding patterns were observed when the soluble protein fraction were run on SDS-PAGE. However, on gels run with the globulin fraction a 57 kD protein appeared 4 and 8 h after imbibition and had disappeared by 12 h after imbibition. The role of proteins and heat-stable proteins during germination and prevention of dessication during early seedling growth is discussed.

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David W. Still and Kent J. Bradford

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.