Physiological mechanism of the effects of two nitrogen forms on yield and photosynthetic characteristics of rice

【Objective】 The purpose of the study was to investigate the effects of two nitrogen forms on the yield and photosynthetic characteristics of rice，and to provide a reference basis for the rational application of nitrogen fertilizer and the improvement of nitrogen fertilizer utilization efficiency.【Method】 Using rice variety Zhonghua No. 11 as test material， a pot experiment was conducted with calcium nitrate （NO₃^--N）and urea（NO₄⁺-N）treatments at the same nitrogen application rate， yield per plant，tillering number per plant，effective panicle number per plant and seed setting rate per plant were investigated. Nutrient solution experiments with three treatments of the same nitrogen concentration of（NH₄）₂SO₄（NO₄⁺-N），Ca（NO₃）₂（NO₃^--N） and NH₄NO₃（NO₄⁺-N/NO₃^--N）were set up to determine the biomass of rice，total nitrogen content，chlorophyll content，photosynthetic characteristics and content of metal cations after cultivation for 21 d. The correlation between net photosynthetic rate and chlorophyll content，stomatal conductance，nitrogen content and iron ion content of leaves under different treatments was analyzed.【Result】 Compared with calcium nitrate treatment，the rice yield of urea treatment increased about 1.8 times，and tillers number per plant，effective number of tillers per plant and seed setting rate per plant increased 75.4%，97.4% and 26.1% respectively. The differences were significant（P＜0.05，the same below）or extremely significant（P＜0.01，the same below）.The leaf chlorophyll content，photosynthetic efficiency， nitrogen content and nitrogen uptake of rice cultured with Ca（NO₃）₂ were significantly lower than that of NH₄NO₃ and （NH₄）₂SO₄ treatments. However，the root to shoot ratio and nitrogen physiological utilization efficiency were significantly higher than those of NH₄NO₃ and（NH₄）₂SO₄ treatments. In addition，the ionome of rice under the 2 nitrogen nutrients changed significantly，especially the iron ion content of rice leaves under Ca（NO₃）₂ treatment decreased significantly. The result of correlation analysis showed that the net photosynthetic rate of leaves was extremely significantly correlated with chlorophyll content，stomatal conductance，nitrogen content and iron ion content under the two nitrogen forms.【Conclusion】Under the same nitrogen application rate，compared to NO₃^--N，NO₄⁺-N can significantly increase the number of tillers and seed setting rate of rice，thereby improving rice yield. NO₄⁺-N is beneficial for nitrogen and iron absorption with higher chlorophyll content，while NO₃^--N can increase the root to shoot ratio and nitrogen physiological utilization efficiency of rice. Therefore， the mixed nutrition of NO₄⁺-N and NO₃^--N is more conducive to rice growth.