High temperatures during the grain-filling period do not reduce the potential grain dry matter increase of rice

High temperatures during the grain-filling period (GFP) of rice (Oryza saliva L.) increase the grain dry matter increase rate (GIR), but this increase in GIR is insufficient to completely compensate for the concomitant reduced GFP, and as a result, grain yield decreases. The shortfall in GIR as temperatures increase has been believed to signify a reduction of the potential GIR as a sink capacity. However, we suspect that lack of assimilate supply to the grain, rather than the decreased potential GIR, lowers the GIR and causes reduced grain weight. Our objective was to determine if the grain weight could reach full potential under higher temperature conditions if assimilate supply during the GFP was sufficient to sustain the increased GIR. Rice was grown at three locations in western Japan over 3 yr. At one location, plots were covered with plastic film during the GFP to increase temperature. Spikelet filling percentages (F%) at maturity varied between 70 and 90% when mean temperatures ranged between 23 and 29°C during the GFP. When plots were thinned to half density during GFP, all F% were approximated by a single logistic equation based on accumulated temperature, with a ceiling of 90%. Hence, thinning can overcome the lower F%. These results suggest that potential GIR in rice is not reduced by high temperatures during the GFP. Yield reductions commonly associated with such conditions are likely due to the failure of assimilate supply to the grain to meet the requirements of the accelerated GIR.