SOYBEAN SEED QUALITY RESPONSE TO DROUGHT STRESS AND POD POSITION

The effects of drought stress on soybean [Glycine max (L.) Merr.] seed quality may be influenced by morphological plant characteristics and the timing of drought stress. Determinate soybean plants were grown near Ames, IA, to ascertain the influences of the timing and frequency of drought stress and pod position on seed quality (as measured by germination, seedling dry weight, and seed leachate current). Plants were well-watered (high reproductive load) or drought-stressed at flowering (R2) to decrease potential seed number (low reproductive load). Subsequent drought stresses were implemented at the full pod (R4), seed formation (R5), and full seed (R6) stages, in addition to a nonstressed treatment. The harvested pods were grouped into the top main stem, bottom main stem, and branch positions for seed quality testing. An R2 drought-stress (low reproductive load) did not prevent reductions in seed quality from occurring when additional stress was imposed at R4, R5, or R6 plant stages. However, the R2 + R4 stress treatment increased seedling weight by 7% and weight per seed by 12% compared with the R4 drought-stress treatment. Top main stem seeds were of better quality and greater weight than seed harvested from the bottom main stem or branch positions. Branch or bottom main stem seeds were most sensitive to quality reductions when drought occurred during the R5 or R6 stages. The results indicate that variable seed quality resulting from drought stress can be attributed to the timing of stress and pod position on the plant.