Cultivar maturity and response of soybean to shade stress during seed filling

Soybean (Glycine max L. Merrill) cultivars with long and short growth cycles frequently exhibit similar yield potentials in nonstress environments. The objective of this experiment was to determine if cultivars with long growth cycles and large maximum vegetative masses were more resistant to stress during seed-filling. Early (Hardin and Kasota, maturity group I) and late (Essex and Hutchinson, maturity group V) cultivars were grown in the field at Lexington, Kentucky (38 degrees N), in 0.38 m rows with irrigation in 1993 to 1995. Average yield of the nonstressed late cultivars was only 6% greater than the early cultivars. Late cultivars had a longer total growth cycle (49 d longer), and their vegetative mass at the beginning of growth stage R6, early in the seed-filling period, was nearly double that of early cultivars. The larger vegetative mass contained more N and starch. Shade cloth placed over the plots at the beginning of growth stage R6 reduced yield (23-26%), seed number (8-16%) and seed size (9-21%). The yield reduction was similar for early and late cultivars. Shade stress did not accelerate leaf senescence, as seen in the decline in leaf N and chlorophyll levels, and had no effect on the timing of physiological maturity. The N and starch levels in leaves ready to abscise from the plant were not affected by cultivar maturity or shade treatment. The large quantity of potentially mobilizable N and starch in the vegetative plant of the late cultivars did not reduce the effect of stress during seed filling on yield.