Differential Responses of Soybean Genotypes Subjected to a Seasonal Soil Water Gradient1

The development of soybean [Glycine mar (L.) Merr.] cultivars specifically adapted to rainfed or irrigated production systems may require explicit selection for adaptation to drought‐prone environments or responsiveness to irrigation culture, respectively. To detect the existence of genotypic differences in soybean response to water, the agronomic performance of 16 cultivars was evaluated relative to six levels of a seasonal soil water gradient created by variable amounts of weekly irrigation. The fortuitous occurrence of minimal rainfall from early July to late August in 1983 and 1984 permitted a wide range in seed yield expression (ca. 1 to 4 Mg/ha) to be established across the gradient. As water application increased, maturity was delayed and seed quality (1983 only), plant height, lodging, seed per plant, 100‐seed weight (1983 only), seed yield, straw yield, and apparent harvest index (HI) were increased. When cultivar seed yields were regressed on June thru September water amounts, significant differences among genotypic regression coefficients were observed (range of 50 to 90 kg ha−1 cm−1 in 1983 and 27 to 82 kg ha−1 cm−1 in 1984). Consistently (across years) low coefficients for some cultivars (e.g. ‘Williams 82’, ‘Lawrence’, and ‘Amcor’), and high ones for others (e.g., ‘A3127’, ‘Century’, and ‘Fremont’), empirically characterized the former as being less sensitive to drought (and conversely, less responsive to irrigation) than the latter. Cultivar HI responses were quadratic, declining under severe water stress, but plateauing at an apparent asymptotic limit in well‐watered conditions. However, straw yield responses were relatively linear, suggesting that increases in vegetative dry matter may be the key to increasing total dry matter (and thus seed yields) in cultivars selected for use in irrigation culture.