Physiological Mechanism Of Nitrogen Allocation Affecting Photosynthetic Capacity And Photosynthetic Nitrogen Use Efficiency In Cotton Leaves

[Objective]We aim to analyse the coupling relationship between nitrogen content and photosynthetic rate in cotton leaves,clarify the effect of leaf nitrogen allocation on photosynthetic nitrogen utilization rate,To sure the mode of nitrogen allocation in leaves and the relationship between different nitrogen components,and discuss the physiology influence mechanism of nitrogen nutrition on the photosynthetic capacity in cotton.This may provide theoretical basis and guidance for rational nitrogen application to improve utilization efficiency and promote industrial quality and efficiency.[Methods]Based on a field experiment（2021-2022）,this research studied the effects of nitrogen allocation on photosynthesis and photosynthetic nitrogen use efficiency（PNUE）by using nitrogen allocation model formula method in different cotton materials of four cotton species at flowering and boll stage.On this basis,Rubisco nitrogen content（N_r）,bioergonomic nitrogen content（N_b）,light harvesting system nitrogen content（N_L）,cell wall nitrogen content(N_cw),water-soluble nitrogen content（N_s）and water-soluble stored nitrogen content(N_os)were determined by physiological and biochemical method in leaves,the nitrogen allocation mode of cotton leaves was clarified,the main nitrogen component factors limiting the increase of photosynthetic rate were elucidated,the relationship between different nitrogen components were analyzed.The main cultivars in Xinjiang were selected to supply normal nitrogen(HN,207 kg·hm^-2nitrogen fertilizer during growth period)and free nitrogen（LN,no nitrogen fertilizer during growth period）two nitrogen supplies.The photosynthetic rate and nitrogen content of each component in different growth stages（bud,flower,boll）were studied,and the physiological mechanism of nitrogen supply affecting leaf photosynthetic capacity was discussed.[Results]（1）The effects of nitrogen allocation on photosynthetic nitrogen utilization rate in cotton leaves were studied.When leaf nitrogen content(N_area)was greater than 3.75 g·m^-2,there was no significant positive correlation between photosynthetic rate（P_n）and the nitrogen content per unit area(N_area).P_nwas positively correlated with N_rand N_bin photosynthetic apparatus（P<0.05）,a marginal positive correlation with N_L（P=0.056）.There was a significant or extremely significant positive correlation between the proportion of nitrogen components in photosynthetic apparatus and PNUE（P<0.05 or P<0.01）.The results indicated that nitrogen components of photosynthetic apparatus were more important to photosynthesis and PNUE than total nitrogen.However,the fractions nitrogen allocated to photosynthetic apparatus components were affected by N_areaand CW_area.The fraction nitrogen allocation to bioenergetics（P_b）and light harvesting system（P_L）in photosynthetic apparatus were inversely correlated with N_area（P<0.01）,The fraction nitrogen allocation to Rubisco（P_r）,P_Land P_bwere significantly,extremely significant and marginally negatively correlated with CW_area（P<0.05;P<0.01;P=0.056）.（2）The allocation mode of nitrogen in cotton leaves and the relationship between different components at boll stage were studied.And it was found that nitrogen allocation to Rubisco component,light harvesting system component,bioenergetics component,cell wall component,water-soluble storage component and other component accounted for 25%,7%,14%,14%,24%and 16%,respectively,and photosynthetic apparatus nitrogen accounted for 46%,Rubisco nitrogen accounted for 54.3%of photosynthetic apparatus nitrogen;Correlation analysis showed that P_nhad a significant positive correlation with N_rin photosynthetic apparatus（P<0.05）,a marginal positive correlation with N_b（P<0.056）,but had no significant correlation with N_L（P>0.16）,and had a significant and a marginal positive positive correlation with water-soluble nitrogen content（N_s）and water-soluble stored nitrogen content(N_os)（P<0.05;P=0.058）.The results showed that nitrogen content of Rubisco and bioenergetics in photosynthetic apparatus and water soluble stored nitrogen content in non-photosynthetic apparatus were the main factors limiting the increase of photosynthetic rate.The relationship between different nitrogen components was analyzed,and it was found that there was no correlation between P_rand the proportion of nitrogen allocation to the cell wall（P_w）（P>0.05）,but P_rhad a significant negative correlation with the fraction nitrogen allocated to non-photosynthetic apparatus(P_non)（P<0.05）.The results indicated that there was no trade-off between nitrogen allocation to Rubisco component and cell wall component,but a trade-off between Rubisco nitrogen component and multiple nitrogen components in cotton.（3）The effects of nitrogen supply on nitrogen allocation at different growth stages of cotton were studied.The results showed that,compared with normal nitrogen supply（HN）,single leaf area,chlorophyll content,single leaf biomass and above-ground biomass of cotton at boll stage were significantly decreased under free nitrogen supply（LN）,while LMA was significantly increased.Compared with HN,N_areaand P_nshowed no significant difference at seedling stage and bud stage,but decreased significantly at boll stage（31.2%and 26.5%,respectively）under LN condition.There was no significant difference in PNUE at seedling stage,bud stage and boll stage.Nitrogen supply also affects nitrogen content of different components.Compared with HN,N_rsignificantly decreased by 9.7%at seedling stage,but had no significant difference at bud stage and boll stage in LN.There was no significant difference in N_p,N_sand N_osat seedling stage and bud stage,but decreased significantly at boll stage（21.4%,30.7%and 83.4%,respectively）.N_bincreased significantly by 48.3%at seedling stage,decreased significantly by 46.5%and110.8%at bud stage and boll stage,respectively.There was no significant difference in N_Lat seedling stage and bud stage,but decreased significantly by 24.5%at boll stage.N_cwdecreased by 87.0%and 45.5%at seedling stage and boll stage,respectively,but there was no significant difference at bud stage.The allocation of nitrogen in different components were studied.It was found that during the growth of cotton from seedling stage to boll stage,nitrogen allocation to cell wall component and photosynthetic apparatus components increased.When HN changed to LN at boll stage,the leaf nitrogen allocation to bioenergetic component decreased and that of Rubisco component increased,while the fraction nitrogen allocated to cell wall component remained unchanged.[main conclusion]（1）The effect of nitrogen content per unit area on the photosynthetic rate of cotton leaves has a threshold value,and when it exceeds 3.75 g·m^-2,it is no longer a limiting factor for the photosynthetic capacity of cotton leaves.The main way to improve the photosynthetic rate and photosynthetic nitrogen use efficiency was to increase the content and allocation proportion of nitrogen components in the photosynthetic apparatus.（2）The nitrogen allocation to Rubisco component,light harvesting system component,bioenergetics component,cell wall component,water-soluble storage component and other component accounted for 25%,7%,14%,14%,24%and 16%.Among them,the nitrogen content of Rubisco,bioenergetics and water-soluble stored were the main nitrogen component factors limiting the increase of photosynthetic rate.There is no trade-off between nitrogen allocation to cell wall component and Rubisco component,but rather between Rubisco and multiple components.（3）Nitrogen supply affects leaf photosynthetic rate at boll stage by regulating leaf nitrogen allocation.Under free nitrogen supply,the insufficient supply of nitrogen content in photosynthetic apparatus,water soluble nitrogen content and water soluble stored nitrogen content may lead to the decrease of photosynthetic rate.