Photosynthetic Responses And Physiological Mechanism To Low Nitrogen In Wheat Seedling(Triticum Aestivum L.)

Nowadays,large amount of N fertilizers were applied to increase cereal production,however,large amount of applied nitrogen(N)is lost to environment through volatilization,leaching,running off and other ways,reducing nitrogen use efficiency and causing a series of environment pollutions.To balance between environmental protection and cereal production,N input must be reduced,while still allowing the increase of cereal production.Wheat seedlings with weak root systems and low growth rates have poor ability to absorb N,which would lead to high N loss at the seedling stage.Thus,reducing the application of basal N fertilisers could be an efficient way to achieve the goal of reducing N input.However,the growth of morden wheat cultivars which was selected under high N conditions would be inhibited when reducing basal N fertilizers.Therefore,improving low N tolerance is the key to solve this problem,and studying the response of wheat seedling to low N and the underlying mechanism could provide theotical base for the breeding of high N efficiency cultivar and better N management.Photosynthesis provides matter and energy for plant growth.As N is an important component of the photosynthetic apparatus,reducing N supply restricts photosynthetic rates.Photosynthesis is a highly interactive and regulated system,consisting of CO2 conduction,electron transport,CO2 carboxylation and photosynthetic product output;affecting one link causes changes to other links.Moreover,photosynthesis is closely related to nitrogen and carbon metabolism,and interacts with each other.Therefore,it is important to clarify the effects of low nitrogen(LN)on photosynthetic characteristics and carbon and nitrogen metabolism in different wheat varieties with different LN-tolerance and the underlying its physiological mechanism.Hydroponic experiments were conducted using two wheat(Triticum aestivum)cultivars,Zaoyangmai(low-N sensitive)and Yangmai158(low-N tolerant),with either 0.25 mM N as a low N treatment(LN)or 5 mM N as a control to systematically study the responses of photosynthesis of wheat seedlings to LN,and discuss the physiological mechanisms underlying the tolerance to LN from the following aspects:Rubisco,electron transport,carbon and nitrogen metabolism.15N labeling method was also used in hydroponic experiments to characterize responses of N allocation and reutilization to low-N in two wheat cultivars.The main results are as follows:1.The net photosynthetic rate(Pn)increased at first period of treatment and then decreased under LN conditions,and the LN-tolerant cultivar had higher Pn.At 5 days after treatment(DAT),the Rubisco carboxylation rate(Vcmax)increased,leading to an increased Pn.At 10 DAT,the LN-sensitive cultivar had significantly lower Vcmax when grown under LN,however,LN did not reduce the maximum RuBP regeneration rate(Jmax)at this time;while at 20 DAT,LN reduced Vcmax,Jmax,potential triose-phosphate utilisation rate(VTPU)and Pn in both cultivars,resulting in decreased plant dry weight.However,the LN-tolerant cultivar maintained higher Pn,especially in upper leaves,resulting in less decrease in plant dry weight.In conclusion,under short-term LN conditions,the LN-tolerant cultivar improved Rubisco carboxylation ability to promote photosynthetic ability;under long-term LN conditions,Rubisco carboxylation ability decreased,reducing photosynthetic ability,and electron transport ability and photosynthetic product output ability also decreased;but the LN-tolerant cultivar could maintain higher photosynthetic ability,especially in upper leaves.2.The LN-tolerant cultivar could maintain Rubisco synthesis and improve Rubisco activation to maintain Rubisco carboxylation ability.At 5 DAT,there was no significant difference in Rubisco contents in the LN-tolerant cultivar between LN and CK,while ATP/ADP ratio,Rubisco activase(Rca)activity and Rubisco activation state increased under LN,which led to increase in Rubisco carboxylation ability.Under long-term LN,leaf Rubisco contents decreased significantly,however the LN-tolerant cultivar maintained higher Rubisco concentrations,especially in upper leaves.Besides,ATP/ADP ratio,Rca activity and Rubisco activation state increased under LN,especially in the LN-tolerant cultivar.In conclusion,the LN-tolerant cultivar maintained higher Rubisco contents in upper leaves and enhanced Rubisco activation to sustain Rubisco carboxylation ability.3.The LN-tolerant cultivar could dismiss over-excited energy through photorespiratory and Mehler pathways to maintain electron transport and avoid photodamage.Under LN,qL decreased,indicating the PSⅡ was closed,leading to the decreases in ΦPSⅡ and Jt.However,compared with the LN-sensitive cultivar,the LN-tolerant cultivar maintained higher qL,ΦPSⅡ,Jt under LN.Under LN,the LN-tolerant cultivar had lower ratio and higher Je(PCR)and Ja while lower NPQ and ΦNPQ compared with the LN-sensitive cultivar.Moreover,Fv/Fm decreased in the LN-sensitive cultivar under LN,indicating the occurrence of photodamage,while it did not change in the LN-tolerant cultivar.In conclusion,the close of PSⅡ reduced electron transport under low-N conditions,but the LN-tolerant cultivar consumed excess excitation energy through photorespiratory and Mehler pathways to maintain the open of PSⅡ,sustain electron transport and avoid photodamage.4.The LN-tolerant cultivar could maintain higher carbon metabolism to avoid feedback inhibition on photosynthesis.Under LN,oxaloacetic acid,malic acid,citric acid and other organic acids,and metabolism-related enzymes activities,the activities of phosphoenolpyruvate carboxylase(PEPC),isocitrate dehydrogenase(ICDH)and pyruvate kinase(PK)decreased.However,the decrease in organic acid metabolism-related enzymes activities and organic acids concentration was less in the LN-tolerant cultivar.The decrease of organic acid metabolism under LN led to the accumulation of sucrose and soluble sugars in leaves,suppressing the sucrose phosphatase(SPS)activity.However,the increase of sugar concentrations and the decrease of SPS activity were less significant in the LN-tolerant cultivar.In conclusion,under LN conditions,the LN-tolerant cultivar could maintain organic acid metabolism,reduced the accumulation of carbohydrates in leaves,and avoided feedback inhibition on photosynthesis.5.The LN-tolerant cultivar could regulate inner nitrogen distribution and utilization to maintain nitrogen supply in upper leaves.Leaf N concentration and soluble concentration decreased significantly under LN;however the LN-tolerant cultivar maintained higher N concentration and soluble concentration,especially in upper leaves.At 5 DAT,NO3-concentration and NO3-/soluble protein ratio decreased,especially in the LN-tolerant cultivar;and soluble protein concentration was not decreased,indicating the LN-tolerant cultivar had higher ability to assimilate NO3-to protein.To characterize responses of N allocation to LN,N was labeled with 15N after LN treatment.15N partitioning increased in newer leaves while decreased in older leaves of the LN-tolerant cultivar under LN compared with CK,which was not significant in the LN-sensitive cultivar.To characterize the responses of N remobilization and loss to LN,N was labeled with 15N before LN treatment.15N content decreased in old leaves and increased in new leaves under LN conditions compared with CK,especially in the LN-tolerant cultivar,which is associated with up-regulation of GS1 in old leaves.Besides,15N loss reduced under LN compared with CK,which was associated with up-regulation of GS2,and the extent was higher in the LN-tolerant cultivar.In conclusion,the LN-tolerant cultivar improved vacuole-stored NO3-utilization,N allocation and N reutilization to optimize nitrogenous compounds supply in newer leaves under LN conditions.In conclusion,under short-term LN conditions,the LN-tolerant cultivar assimilated vacuole-stored NO3-to produce Rubisco and improved Rubisco activation to promote Rubisco carboxylation ability.Under long-term LN conditions,the LN-tolerant cultivar regulated nitrogen allocation and utilization to maintain Rubisco synthesis in upper leaves,and improved Rubisco activation to maintain Rubisco carboxylation ability;consumed excess excitation energy through photorespiratory and Mehler pathways to maintain the open of PSII and sustain electron transport;and improved carbon metabolism to avoid feedback inhibition on photosynthesis.As a result,the LN-tolerant cultivar maintained higher photosynthetic ability and plant growth under LN conditions.