Shading during the early grain filling period does not affect potential grain dry matter increase in rice

Shortage of C assimilate supply to rice grain during approximately the first 10 days after heading (DAY₁₀) has been believed to reduce potentiality of grain dry matter increase (G(P)) and decrease the final grain weight (G(F)). However, we suspect that assimilate shortage during DAY₁₀ does not determine G(F), if assimilate supply during the rest of the grain filling period (G(PERIOD)) meets subsequent requirements for realizing G(P). Our objective was to determine if G(P) is in fact affected by shortage of assimilate supply caused by shading during DAY₁₀. Plots of rice (Oryza sativa L.) grown under paddy field conditions were subjected to three strengths of shading for DAY₁₀. After the shades were removed, the plots were divided into two groups. Plants in the first group were thinned to half the plant population density, while the second group was left at normal spacing. G(F) in the unthinned group was reduced depending on the strength of the shade, but weights in the thinned group almost reached those of nonshaded control plants, except in the most heavily shaded plots, where weights were slightly reduced. In pot experiments simulating crop stand, G(F) all reached these of control plants when the pots were exposed to abundant radiation after being subjected to heavy shading during either the early or mid-G(PERIOD). These results suggest that rice plants are capable of recovering from early reduction in grain growth rate at least until the mid-G(PERIOD), and shortage of available assimilate during DAY₁₀ does not determine G(P).