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Sunflower (Helianthus annuus L.) yields are strongly reduced by late sowing dates. The objectives of this study were to investigate the physiological bases of the sowing date (S), genotype (G), and genotype × sowing date (G × S) interaction effects on sunflower yield and contribute to development of ideotype‐based selection strategies for improving yield at late plantings. Nine differentially adapted hybrids were evaluated during two seasons at normal and late planting dates at Venado Tuerto, Argentina, by means of the framework that defines oil yield as the product of grain number and weight and oil concentration. Sowing date was the major source of variation for oil yield. The G × S interaction, in turn, accounted for a portion of the total variability four times higher than the contribution of G. The S and G × S interaction effects on grain number were mostly determined by grain set in the central portion of the capitulum. Duration of grain filling was the main determinant of the S and G × S interaction effects on grain weight and grain oil concentration. Variation in grain oil concentration between sowing dates was largely due to changes in kernel oil proportion, rather than changes in kernel percentage. Relative changes in rate of grain filling and kernel percentage also contributed to the observed G × S interactions for grain weight and oil concentration, respectively. Although some degree of compensation among oil yield components underlie the observed G × S interactions, it was found that seed set in the central portion of the head can be used as an indicator of adaptation to late planting dates.

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 × 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 × S interaction, in turn, accounted for a portion of the total variability three times higher than the contribution of G. Both S and G × 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 × 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 be 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.

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