SOYBEAN SEED QUALITY STUDIES (SCANNING ELECTRON-MICROSCOPY, PHOMOPSIS SPP., ALTERNARIA SP.)

The objectives of this thesis were to answer the following questions: (1) What is the developmental sequence of the seed coat pits and micropyle? (2) What role do natural openings on the seed surface play in field pathogen infection of the seed? (3) Why do male sterile soybeans have poor seed quality? (4) What is the relationship and genetics of reproductive growth stages and specific soybean seed quality traits?;A series of experiments was made using male sterile populations, depodding treatments and isolines to determine the effect of maturity and late reproductive growth periods on seed quality particularly infection by Phomopsis spp. The results from studies of strains from the germplasm collection and six hybrid populations suggested that the period of late seed fill, R6 to R7, was critical for seed infection by Phomopsis spp. Breeding for a short period from R6 to R7 may result in avoidance of this pathogen. Alternatively breeding for long period between R6 to R7 without seed infection may result in resistance to this pathogen. Results suggested that seeds of male sterile soybeans are of poor quality because of the long time it takes for these plants to mature. The longer the time in late seed fill, R6 to R7, the larger were the seeds. Breeding for smaller seed size may be one route to improve seed quality. Estimates of broad sense heritability for seed quality traits and late reproductive growth periods in six hybrid populations were generally below 30%. Two heritability estimates for percentage of seeds infected by Phomopsis spp. were 28.2% and 52.7%.;Observations by scanning electron microscopy of seeds, taken from the field at weekly intervals after pollination, showed that pits in the seed coat form during the dry down period. The micropyle was present after pollination as the largest natural opening and one sample was observed with the seed being invaded via the micropyle by the pathogen Alternaria sp. The structure of the micropyle, which appears to be a major route for seed invasion by pathogens, is described.