Soybean Flower Production as Related to Plant Growth and Seed Yield

Flowers are directly related to seed yield through the fruits that form from flowers, but the physiological factors that regulate flower production after induction of the flowering state are not well understood for soybean [Glycine max (L.) Merr.], where the connection of flowering to yield is complicated by abortion of potential fruits. The objectives of this research were to (i) evaluate methods of obtaining flower count data and characterizing flowering in terms of rate of flower production (FRATE), duration of flowering (FPERIOD), and total flowers (FTOTAL) in soybean genotypes differing in numbers of flowers and levels of reproductive abortion; (ii) determine relationships between these characteristics and seed yield; and (iii) measure effects of plant factors on the flowering characteristics when both plant growth and flowering are altered by variation in planting date. Growth and flowering were measured in five plantings during 2 yr at Brookings, SD in Vienna (fine-loamy, mixed Udic Haploboroll) soil, using cultivars Dassel and Corsoy 79, which have few flowers per raceme, and the many-flowered lines SD87001 and IX93-100. Flowering was measured by counting blossoms three times weekly throughout the bloom period (Method I) and by counting fruits and abscission scars for aborted flowers 2 wk after final blossoming (Method U). Cumulative flowering curves determined by Method I were closely fit by the logistic function. Total flowers by Method I was lower than by Method II, but only minor differences were observed between methods for detecting flowering differences caused by planting date or genotype. Flowering traits measured by Method I accounted for 60 to 77% of the variation in seed yield. Genotypes differed markedly in the relationship of plant characteristics to flowering traits, but rate of increase in height in vegetative stage generally had significant negative correlation to FPERIOD, had positive correlation to FTOTAL and FRATE, and was identified as one of several factors that had major influence on flowering traits. It is concluded that large and rapidly growing soybean plants tend to have short FPERIOD with high FRATE and FTOTAL, but genotypes also may have additional traits that determine flower production characteristics. Contribution from USDA-ARS and the South Dakota Agric. Exp. Stn. Journal Series no. 2671.