The Critical Period for the Effect of Intercepted Solar Radiation On Sunflower Oil Does Not Depend On Grain Filling Duration.

The intercepted solar radiation per plant (ISR) affects weight per seed and oil concentration in sunflower grains. A reduction of ISR during the 250-450ºC day after flowering period (the critical period) accounted for most of this effect in the Dekalb G-100 traditional hybrid. However, it is unknown whether this critical period is similar in hybrids with different duration of their grain filling period. To unravel this issue, two experiments were conducted at Balcarce, Argentina, during 2007-2009 using two hybrids with different grain filling period duration. ISR was modified by both, shading (80%) and thinning plants (50%), either throughout grain filling or by applying short period treatments. The response of oil concentration to accumulated ISR from R6 to physiological maturity (R6-PM) and for 100ºC day segments after flowering was analyzed by fitting an exponential rise to maximum equation to experimental data. The critical period for the effect was defined by those 100ºC day segments where r² for the fitting was higher than the one obtained with ISR accumulated during R6-PM. The critical periods for both hybrids were located between 400 and 650ºC day after flowering, i.e. in the second half of the grain filling period. No correlation between the critical period and R6-PM stage duration was found. This result could be used to improve the performance of sunflower oil yield models as well as guide management practices intended to increase resources use efficiency during the critical period.