Effect of Light Intensity on the Growth and Photosynthesis of Rice under Different Sulfur Concentrations

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Rice (Oryza sativa L. cv IR 72) plants were grown hydroponically in 1.0 mM SO_4^2- for 1 week and transferred to 0, 0.01, 0.03, 0.1, 0.3, or 3 mM SO_4^2- under two light treatments, 1,200 (high light) and 550 (low light) μmol quanta m^-2 s^-1. When the plants were grown under S-deficient conditions, the biomass production of the shoot was more strongly suppressed in the high-light-grown plants than in the plants grown under low light conditions. The low-light-grown plants showed a lower relative growth rate (RGR), higher leaf area ratio (LAR), and a low net assimilation rate (NAR) that was not affected by S nutrition. Although total S contents of the leaf blades under both light treatments were similar, S allocation to the leaf blades under low light conditions was greater. A large decrease in the Rubisco content was observed in the high-light-grown, S-deficient plants. In contrast, the low-light-grown plants showed relatively high contents of Rubisco even under S-deficient conditions. Chlorophyll content was significantly higher in the plants grown under low light conditions. Irrespective of S and light treatments, leaf photosynthesis could be accounted for by the Rubisco content. The results indicated that S-deficient effects were relatively small in the low-light-grown plants and that high light conditions stimulated S-deficiency.
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