Assessing the mechanisms underlying sunflower grain weight and oil content responses to temperature during grain filling

Temperature and solar radiation during grain filling are the main drivers of grain weight and composition in several crops. While significant progress has been done on the effect of solar radiation intercepted by the plants (ISR) on grain weight (GW, mg) and oil content (OC, mg grain⁻¹) of sunflower (Helianthus Annus L.), the effects of temperature on these traits are far from clear. As temperature affects the rate of plant development, increasing temperature would shorten grain filling duration and so, the critical period for ISR effect on GW and OC (indirect effect of temperature). However, direct effects of temperature non-mediated by ISR accumulation could also affect these traits. The objectives of this work were a) to characterize the responses of grain weight and oil content to mean temperature during grain filling in two traditional sunflower hybrids and b) to determine whether these responses can be exclusively explained by changes in ISR or any direct effect of temperature should also be considered. Two experimental approaches have been used to address these objectives: i) temperature manipulation at constant incident radiation in growth chambers; ii) modification of radiation interception in the field in different locations and years (different mean temperatures). Growth chamber experiments showed that GW and OC respond to temperature in a plateau lineal shape. At low incident radiation such as that of growth chambers, temperature effects on GW can be exclusively explained by reductions in ISR, while both direct and indirect effects of MT are evident on OC. Results obtained in field experiments confirmed this behavior at low radiation. Both GW and OC exponentially increased up to a maximum with ISR disregarding MT. Mathematical models were established to describe the responses of GW and OC to MT and ISR. These models consider MT x ISR interactive effects on GW; and MT - ISR additive effects on OC. The combination of growth chamber and field experiments observations allowed to unravel different mechanisms underlying the responses of GW and OC to temperature: at low incident radiation intensity, increasing temperature decreased GW exclusively by reducing ISR accumulation (indirect effects) while at higher radiation, temperature displays both direct (non-mediated by ISR) and indirect (ISR mediated) effects on GW. OC is affected by temperature in both direct and indirect way disregarding ISR. This work constitutes an important contribution towards understanding the effect of temperature during grain filling on sunflower yield and its relationship to solar radiation effects.