Effects of light intensity and nitrogen fertilizer interaction on carbon and nitrogen metabolism at grain-filling stage and its relationship with yield and quality of southern soft japonica rice

Abstract

Southern soft japonica rice Nanjing 9108 and Yangnongxiang 28 were selected as the experimental materials, and two light intensity treatments, and four nitrogen treatments were set. Light intensity treatments [100% natural light intensity (L1) and 50% natural light intensity (L2)] and four nitrogen treatments [no nitrogen fertilizer (N1) in the middle and late growth stages, one-time nitrogen fertilizer applied at the top sixth leaf stage (N2), one-time nitrogen fertilizer applied at the top fourth leaf stage (N3), and one-time nitrogen fertilizer applied at the top second leaf stage (N4)] were conducted at grain-filling stage. The difference of carbon and nitrogen metabolism at grain-filling stage and its effects on rice yield and quality under the conditions of different light intensity and nitrogen application period as well as light-nitrogen interaction conditions were investigated. The results showed that with the decrease of light intensity at grain-filling stage the net photosynthetic rate of flag leaf decreased by 7.35%-42.36% on average, sucrose phosphate synthase (SPS), and sucrose synthase (SS) had low activity, the C/N ratio of leaves decreased by 3.98-6.49, the transportation of photosynthetic products to grains decreased, and the content of grain starch (including amylose) decreased. Meanwhile, the activities of nitrate reductase (NR), glutamine synthetase (GS), and glutamate synthetase (GOGAT) increased, plant nitrogen concentration increased, and the accumulation of protein increased relatively, which were not conducive to the formation of yield and good quality. After the application of nitrogen fertilizer at the middle and late growth stages, the activities of key enzyme in carbon and nitrogen metabolism in leaves were significantly increased, the aging of leaves was slowed down, and the grain-filling period of rice was prolonged, which were conducive to the increase of yield. With the delay of nitrogen fertilizer application period, nitrogen metabolism became more vigorous, and the protein content in grain had a relative significant increase, resulting in the decrease of the ratio of starch to protein and the ratio of amylose to protein, and the decrease of taste value. Under the experimental condition, normal light intensity combined with nitrogen fertilizer treatment (L1-N3) at the top fourth leaf stage synergistically improved the activities of key enzymes of carbon and nitrogen metabolism in leaves, thus the photosynthetic products and nitrogen-containing compounds were transported to grains in the appropriate proportions. Ultimately, the ratio of starch to protein in grain ranged from 11.43 to 12.03, and the ratio of amylose to protein ranged from 1.34 to 1.50, the rice had low hardness, high viscosity, and balance as well as good taste, high yield, and excellent quality could be obtained simultaneously.

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