Effects Of Water And Nitrogen On Maize Leaf Respiration And Related Physiological Processes And The Regulatory Mechanisms

Water and nitrogen（N）are two important factors affecting crop growth and development.The interactions between water and N can affect crop water and nitrogen utilization.Nevertheless,the effects and regulation mechanisms of water and N and their interaction on maize growth and physiological processes,especially leaf respiration,are unclear.In addition,a pre-drought stress at the vegetative growth stage can enhance crop tolerance to subsequent drought stress at the reproductive growth stage.However,under different N treatments,the mechanisms underlying the modulated tolerance to drought stress at the reproductive stages under water stresses at the vegetative stages are still poorly understood.The experiments were carried out in a glasshouse.In order to investigate the effect of water and N on maize leaf respiration and related resphysiological processes and maize growth,the soil water treatments included three levels:moderate drought（W50,50%soil water holding capacity（SWHC））,mild drought（W70,70%SWHC）and well-watered（W90,90%SWHC）.The N fertilization included three levels:low N(N1,1.0 g·pot^-1),middle N(N2.5,2.5 g·pot^-1)and high N(N5,5.0 g·pot^-1)levels.The aim of the other study was to examine that the effects of different water and N treatments at the vegetative stage and reproductive stage on physiological processes and drought resistance of maize.Maize plants were exposed to two soil water regimes including 90%（W2）and 60%（W1）of soil water holding capacity representing well watered,moderately water stressed treatments,respectively,in interaction with two nitrogen fertilization doses consisting of 0.5 g·N·pot^-1（N1）and 2.5 g·N·pot^-1（N2.5）representing deficit N and sufficient N treatments,respectively.The maize growth,leaf gas exchange,the photosynthetic CO₂response curve（A_n/C_i）and light response curve（A_n/Q）,and the root morphology were investigated.The results are as follows:1.The root length and root specific surface area increased by 106.39%～208.82%and 45.81%～105.85%,respectively,under water stress condition,while the root dry weight decreased by 23.94%～36.61%under the moderate drought condition.Increasing N dose,especially the high N treatment,significantly decreased the root length and root specific surface area by 41.85%～54.10%and 18.68%,respectively,and the root dry weight decreased by 33.75%significantly compared with low N treatment with soil water stress.Thus,N fertilization aggravated plant water stress in the root zone,leading to reduced root water potential and increased leaf abscisc acid（ABA）concentration,which regulated the physiological processes of above-ground leaves.The G_s decreased significantly by 32.37%～51.97%under the moderate drought condition,and increasing N supply,especially high N treatment,decreased G_s by 35.81%under the moderate drought condition compared to low N treatment.A_n is not only related to stomatal factor,but also to non-stomatal factors.In the present study,both water and N treatments affected CO₂ and light response curves,and water treatments showed a more prominent effect.Under the same N treatments,the dark respiration rate（R）,the maximum net photosynthetic rate(A_max)and the saturation irradiance(Q_sat)derived from the photosynthetic light response curve,and the initial carboxylation efficiency（a）,the rate of the photorespiration（R_p）,the photosynthetic capacity(A_max)and saturation intercellular CO₂ concentration(C_isat)under high N level derived from the photosynthetic CO₂response curve decreased with the increase of water stress levels,and the former parameters decreased more significantly.Increasing N dose further decreased these parameters under the moderate drought condition,indicating that N fertilization inhibited plant photosynthetic performance,and reduced light and CO₂ use efficiency under the moderate drought treatment.In addition,the decrease in chlorophyll content and leaf area under moderate water stress,especially under high N treatment with moderate water stress,affected A_n.Concequently,these non-stomatal and stomatal factors inhibited ultimately the A_n.It was found that during the whole growth period,the A_n decreased significantly by 41.85%～56.14%under the moderate drought condition,and increasing N supply,especially high N treatment,decreased A_n by 30.71%under the moderate drought condition compared to low N treatment.Thus the photosynthetic carbon assimilation ability decreased,resulting in the reduced root biomass.2.During the grain filling stage,maize leaf water potential（LWP）decreased,whereas leaf ABA concentration increased,leading to the significantly decreased A_n and G_s under the moderate water stressed treatments,and there was a larger decrease in A_n than G_s,resulting in the significantly decreased WUE_i across the N treatments.Increasing N dose decreased LWP,but increased ABA under the moderate drought condition compared to low N treatment,which further decreased A_n,G_s and WUE_i.During the grain filling stage,A_n was not only related to the decrease in G_s and photosynthetic light and CO₂ response curves,but also to the damaged ultrastructure in leaf chloroplasts.Under the moderately water stressed treatment,the chloroplasts clung on the cell wall became swollen and were spreading to the direction of cell center,meanwhile,the chloroplast envelope was partially ruptured and the number of chloroplast reduced,and the thylakoids were overly disorganized.Increasing N dose under the moderately water stressed treatment severely damaged the structure of chloroplasts.In addition,R_d significantly decreased under the moderately water stressed treatment,and increasing N supply,especially the high N level,decreased R_d compared with low N treatment with same soil water condition（P>0.05）.R_d was affected by the damage of mitochondria.It was found that under the moderately water stressed treatment,the number of mitochondria decreased,further,the mitochondrial membrane became ruptured,and cristae was broken,disintegrated and disappeared,and the mitochondria was severely impaired with increasing N dose under the condition of the same soil water stress.Under the moderately water stressed treatment,especially under high N supply with the moderately water stressed treatment,A_n decreased significantly.Meanwhile,R_d/A_n increased significantly due to a larger decrease in A_n than R_d,and there was a significant negative linear correlation between R_d/A_n and plant C accumulation.Therefore,the accumulation of assimilate was inhibited,thus affected plant growth including reduced plant heigh and above-ground biomass.3.During the vegetative growth stage,the LWP was highest while the chlorophyll content was lowest in N1BW2 treatment.LWP was intermediate and the chlorophyll content wass high in the treatment of N2BW1.Water treatments during the bell growth period（BW）affected significantly the A_n and R_d.A_n was highest whereas R_d was lowest in N2BW1,in contrast,A_n was lowest while R_d was highest in N1BW2.It was found that neither water nor N treatments affected G_s.Therefore,the WUE_i was highest but R_d/A_n was lowest in N2BW1.Differently,the WUE_i was lowest whereas R_d/A_n was highest in N1BW2.Thus,under water stress,sufficient N supply improved the chlorophyll content and WUE_i,meanwhile,reduced R_d/A_n facilitating the accumulation of assimilate in the maize vegetative growth periods.4.Under the same soil water regimes,the chlorophyll content and WUE_i at bell growth stage,A_n and G_s at sillking growth stage,the initial carboxylation efficiency（a）at reproductive growth stage,leaf dry weight,leaf area and the N content of leaf,stem and above-ground in N2 were higher compared to N1treatment,indicating that sufficient N supply facilitated maize growth affected under the same water condition.In general,the LWP of N1 was higher than that of N2 during the reproductive stage.These indicated that N deficit aggravated the water condition of the plant,affecting the water use efficiency and photosynthetic physiology of the plant,which further inhibited the growth of the plants.Under the same N levels,plants exposed to water stresses at the vegetative stage showed higher tolerance to the drought stress at the reproductive stage by improving leaf water potential,the chlorophyll content,the maximum photosynthetic capacity(A_max)and A_n,and enchancing root length and root area as well as root specific surface area,and reduced plant water use,thus improving the water use efficiency（WUE_p）and increasing N uptake.In addition,plants with imposition of water deficit during vegetative development could reduce R_d/A_n leading to enchanced accumulation of assimilates at the reproductive stage and increased above-ground dry weight.It was shown that the chlorophyll content,leaf area,plant height,A_max,A_n,maize dry weight,plant water use and WUE_p,N content of leaf and above-ground dry weight were higher or even highest in N2BW1SW2,and the root system traits were intermediate in N2BW1SW2.However,all the indexes of maize under water stress were lower even lowest in N1BW2SW1.