| Summary: | Nitrogen is an important element that affects the growth and yield of rice obviously. To explore the effects of nitrogen (N) on the growth of double-cropping rice, four consecutive trials were conducted in South China Agricultural University. Four N forms and five N application rates were set up. Rice N uptake, leaf anatomic structure, leaf fluorescence parameters and yield were studied. The results showed that rice leaf SPAD value and N content with urea treatment were the highest among 4 N forms at the heading and flowering stage. The order of rice yield and N use efficiency treated with different N forms were urea > ammonium sulfate > ammonium nitrate > sodium nitrate. The N application rate test showed that photochemical quantum efficiency and chemical quenching coefficient of rice leaves showed an inverted-V-type changing tendency with the highest value at 180 kg/hm<sup>2</sup> N treatment. Treatment with less than 180 kg/hm<sup>2</sup> N decreased the area and perimeter of small vascular bundles of the lateral leaves. Yield of early rice and later rice with 180 kg/hm<sup>2</sup> urea treatment was 17.42 to 33.28% and 6.17 to 21.28% higher than those of other N levels, respectively. The above results suggested that 180 kg/hm<sup>2</sup> of urea N are suitable and recommended for double-cropping rice planting in South China.
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