Studies On The Mechanism Of The Effects Of Nitrogen On Plant Growth And Photosynthesis In Wheat Plants

The increment in wheat yield is of great importance for food production,especially at the context of limited arable land.Photosynthesis is the basis for the formation of dry matter,which is essential to plant growth and yield formation.As an important nutrient element,nitrogen is the main component of several important compounds in plants.Nitrogen application can increase the content of Rubisco enzyme（Ribulose-1,5-bisphosphate carboxylase/oxidase）,carboxylation efficiency,the maximum electron transfer rate,and photosynthesis.Our previous studies showed that with the increase of leaf nitrogen content,photosynthesis increased significantly,while photosynthetic nitrogen use efficiency rate（PNUE）decreased dramatically.The decreased PNUE under high N supply is related to the change in chloroplast development,which can affect the CO₂ diffusion through mesophyll cells.Under the same nitrogen supply,there was a large intraspecific variation in PNUE,and there is still some knowledge gap on the influence of nitrogen on crop growth and photosynthesis.Therefore,a hydroponic experiment with different nitrogen levels was conducted in wheat plants,the objectives were:（1）to study the influence of nitrogen on photosynthesis with the aspects of the maximum carboxylation efficiency,the maximum electron transfer rate,hydraulic conductivity,and stomata structure;（2）to investigate the threshold of leaf nitrogen content in leaf growth and photosynthesis;（3）to analyze whether the decrease in PNUE under high N supply is related to stomatal conductance in 15 wheat genotypes.The results are as follows:1.In this study,the wheat height,stem and leaf dry weight,and leaf area of XM7,XM823,JM22,and NM13 were not significantly different between high（40 mg/L）and medium（10 mg/L）N supply.But they were significantly decreased under low N supply（2.5 mg/L）.There was no significant difference in root weight among the three nitrogen treatments,but the root/shoot ratio increased significantly with the decrease in nitrogen supply.Leaf photosynthesis was more sensitive to N deficient than leaf area.With the decrease in leaf nitrogen content,leaf photosynthesis showed a linear decrease while leaf area showed an exponential decrease.And the threshold leaf nitrogen showing the restricted leaf growth was 3.99%(1.62g m^-2).2.The decrease of leaf nitrogen content significantly reduced leaf photosynthesis in all genotypes,however,stomatal conductance showed no significant decrease in nitrogen deficit treatments.The non-significant decrease in stomatal conductance may be related to the non-significant influence of N on stomata size and stomata density.g_m、V_cmax、J_maxwere drastically decreased with the decrease in nitrogen concentration.The positive correlations between leaf photosynthesis and g_m,V_cmax and J_max indicated that the decreases in CO₂ transport capacity and biochemical function under nitrogen-deficient conditions are the major reasons for the decrease in leaf photosynthesis.The influence of nitrogen supply on K_leaf was genotypic dependent,but there was no significant correlation between K_leaf and g_s,which indicated that the correlation between K_leaf and g_s is genotypic dependent.3.There were significant differences in A,g_s,g_m,LMA,and leaf nitrogen content among 15 wheat varieties.The largest leaf photosynthesis A of 30.7μmol m^-2 s^-1was found in SL375,and the lowest photosynthesis of 20.5μmol m^-2 s^-1 was found in XM943.There was a significant positive correlation between A and g_s and g_m,but the correlation between leaf photosynthesis and leaf nitrogen content was not significant.The results showed that the genotypic variation in leaf photosynthesis was related to the differences in CO₂diffusion capacity,but not to the difference in leaf nitrogen content.The negative correlation between A_mass and LMA indicated that the changes in the A_mass among wheat varieties was related to the differences in LMA among varieties.4.There were significant differences in PNUE among different varieties,and the variation in PNUE in 15 wheat varieties was 12.4-21.9μmol g^-1 s^-1.PNUE was negatively correlated with LMA and was positively correlated with g_s,which indicated that the differences in PNUE among different varieties may be related to CO₂ diffusion capacity and leaf structures.Nitrogen deficiency significantly reduced A and g_m,but significantly increased PNUE.The response ranges of A,g_s,g_m,PNUE to nitrogen changes differed among varieties.The response of net photosynthetic rate to nitrogen is significantly positively correlated with the changes in stomatal conductance and mesophyll conductance.The response degree of PNUE to nitrogen changes was positively correlated with the variation of g_s,which indicated that the difference in the change of PNUE to nitrogen is related to the difference in CO₂ supply among varieties.