Reduction of grain yield in rice shaded during the early grain filling period is not the result of inhibition of potential grain dry-matter increase

A shortage of carbon assimilate supply to rice grain during the first ten days after heading (DAY_0-10) has been considered to reduce the potential grain dry-matter increase (ΔGP). It was, however, uncertain whether assimilate supply after the shading period was sufficient to support ΔGP when the reduction of grain dry-matter increase (ΔG) was observed. Our objective was to demonstrate that ΔG after the shading treatments was reduced not through the decrease in ΔGP but through a lack of assimilate supply, based on our model analysis. In our model, ΔG is determined by assimilate supply (current dry-matter production (ΔW) plus reserved assimilate in straw (SP)) and the ΔG has an upper limitation defined by ΔGP. In this study ΔGP was estimated under a sufficient supply of assimilates which were provided by periodical thinning of the plants. By the shading during DAY_0-10, ΔG as well as ΔW were reduced even after the end of the shading treatment. Observed ΔW and SP at the heading were input to the model to calculate ΔG and straw dry-matter increase (ΔS). The calculated dry weight of the grain and straw coincided closely with the actual observed data. We herewith suggest that the grain dry-matter increase by shading during DAY_0-10 was not suppressed by inevitable reduction of ΔGP but rather by a lack of assimilate supply during the active ΔG period.