The Physiological Mechanism Of Efficient Nitrogen Use In Maize Leaves

Leaf is an important organ of photosynthesis.Leaf area,leaf photosynthesis and leaf longevity have direct effect on biomass formation and grain yield.Leaf nitrogen(N)is an important source of N remobilization for grain N.Therefore,improving leaf N use efficiency(NUE)is vital for improving N use efficiency.We studied how to improve leaf nitrogen use efficiency in vegetative stage and gran filling stage.The main results were as follows:1.In our hydroponic experiment,we aimed to how the response of leaf growth to low N stress in vegetative stage.We found that leaf area was reduced by 22%in low N stress to invest relative more N into root to enlarge root growth.Decreasing of leaf area was determined by decreasing leaf elongation rate in low N stress.Length of division zone was shortening by 40%(thus the number of cell in division zone reduced by 23%)and the cell elongation rate was reducing by 15%in low N stress.The decrease of giberellin maybe results in the decrease of length of division zone.And in low nitrogen treatment,cell division rate is higher than that in high nitrogen treatment.Almost all the different expression genes relative cell cycle were up-regulated in division zone and higher auxin concentration(IAA concentration inceased by 59%in LN)contributed to cell division in low nitrogen treatment.In elongation zone,lower gibberellin concentration resulted in a lower cell elongation rate.The expression of GA20ox4 was down-regulated in LN,which is the key enzymes in GA biosynthesis,resluting in decreasing of GA.In mature zone,the different expression genes mainly focused on photosynthesis.In low nitrogen treatment,the genes involved in photosynthesis were down-regulated.This may be the main reason for the decreased photosynthesis.2.In field study,we analyzed the relationship between photosynthetic N use efficiency(PNUE)and N allocation in maize ear-leaves during the grain-filling stage under low N(no N application)and high N(180 kg N ha-1)in a 2-year field experiment.The grain yield decreased by 32%in low nitrogen stress.Low-N treatment reduced by 38%the specific N content of ear leaves by without effect on photosynthetic rate,thereby increased by 54%PNUE.Under low-N stress,maize plants tended to invest relatively more N into bioenergetics to sustain electron transport.In contrast,N allocated to chlorophyll and light-harvesting proteins was reduced to control excess electron production.Soluble proteins were reduced to shrink the N storage reservoir.We concluded that optimization of N allocation within leaves is a key adaptive mechanism to maximize photosynthetic rate and crop productivity during the grain-filling stage in maize under low-N conditions.3.In this study,we analyzed the relationship between the remobilization of different N components and Pn during grain filling stage under low N(no N application)and high N(180 kg N ha-1)in field experiment.Pn decreased dramatically at 30 days after silking(DAS)and 20 DAS for high N(HN)and low N(LN)treatment,respectively.N began to remobilize at 30 DAS and 10 DAS in HN treatment and LN treatment,respectively.Under HN treatment,N in Rubisco,pyruvate orthophosphate dikinase(PPDK)and thylakoid rapidly decreased at 20 DAS and N in phosphoenolpyruvate carboxylase(PEPc)decreased at 10 DAS.Except for Rubisco,N remobilization in those parameters was earlier in LN treatment than HN treatment.N remobilization resulted in the reducing of Pn.The remobilization efficiency of Rubisco(60%),PEPc(71%),and PPDK(74%)in the leaf was higher than that of thylakoid-N(22%).Low-N greatly increased the remobilization efficiency of all the N components except for PEPc,without changing their remobilization pattern.The ratio of Pn to the N in the PEPc,PPDK and Rubisco kept increase in the whole grain filling stage,while the ratio of Pn to chlorophyll(Chl)and thylakoid decreased.Correlation analysis indicated that Pn was more related to the content of photosynthetic enzymes(PEPc,PPDK and Rubisco)than Chl and thylakoid N.It is concluded that photosynthetic enzymes(include PEPc,PPDK and Rubisco)functioning in carbon reduction serve as a N storage reservoir at early grain filling stage and their degradation is critical in determination Pn during later grain filling stage.In conclusion,the physiological mechanism of within-leaf nitrogen utilization efficiency in maize included:(1)modifying gibberellin and auxin signal pathway,changing expression of genes involved in cell cycle,reducing the number of cell in division zone and cell elongation rate could reducing leaf expansion,thus plant invested relative more N into root to enlarge N uptake in leaf growing stage.(2)In grain filling stage,functional leaf was remobilization to grain.Leaf could invest relative more N into electron transport chain and photophosphorylation to keep high photosynthesis to improve photosynthetic N use efficiency.At leaf senescence stage,photosynthetic enzymes were degraded and remobilized into other organs.