Effects of planting date, genotype, and their interactions on sunflower yield: I. Determinants of oil-corrected grain yield

Sunflower (Helianthus annuus L.) yields are strongly reduced when normal sowing dates are delayed. The objectives of this study were to investigate the physiological bases of the sowing date (S), genotype (G), and G X S interaction effects on sunflower yield, and to contribute to the formulation of ideotype-based selection strategies for improving yield at late plantings. Nine hybrids differentially adapted to northern and central Argentina were evaluated during two seasons in October (normal) and December (late) planting dates at Venado Tuerto, Argentina. Yield was defined as the product of total biomass and harvest index. Sowing date accounted for most of the yield variation. The G X S interaction, in turn, accounted for a portion of the total variability three times higher than the contribution of G. Both S and G X S interaction effects on yield mostly involved the variation of attributes and processes expressed postanthesis. Biomass differences between planting dates were the dominant determinant of the S effect on yield. The genotype-specific responses for harvest index were the dominant determinant of the G X S interaction, and were mostly associated with changes in the rate of harvest index increase. Variations in biomass and harvest index were strongly associated with the amount of intercepted radiation during grain filling which, in turn, was associated to duration of grain filling and green leaf area. Canopy stay green proved to he associated with adaptation to late planting dates. This indirect selection criterion appears to be a more reliable attribute for use in breeding for adaptation to late plantings than some other genotype characteristics linked to yield.