Studies On The Mechanisms Of The Effects Of Nitrogen On The Temperature Response Of Photosynthesis In Rice At Two Growth Stages

Nitrogen is one of the essential macronutrients for plant growth and metabolism.Because of the investment of nitrogen to nitrogen to chlorophyll,Rubisco enzymes?ribulose 1,5-bisphosphate carboxylase/oxygenase?and other proteins,nitrogen plays an important role in photosynthesis.In previous studies,the application of nitrogen is demonstrated to be capable to alleviate the negative effect of high temperature stress on photosynthesis,but the underlying mechanisms are far from understood.In addition,crop plants may suffer from temperature stresses at different growth stages,but the researches relating to the difference in the tolerance of photosynthesis to temperature stresses between different growth stages is rare.Therefore,pot experiments were conducted to investigate:?1?the influences of nitrogen and growth stages on leaf photosynthetic rate?A?,stomatal conductance?g_s?,stomatal conductance?g_m?and the relative photosynthetic limitation;?2?the effects of leaf morphological and anatomical traits on photosynthesis,leaf hydraulic conductance,and the response of photosynthesis to temperature.The main results are as follows:1. Leaf nitrogen content significantly increased with nitrogen supply under different growth stages.At the tillering stage,leaf nitrogen content at HN treatment was 1.13±0.08 g m^-2,which was 54.8%higher than that of LN treatment(0.73±0.02g m^-2).At the booting stage,nitrogen content of HN treatment was 1.15±0.02 g m^-2,which was 21.1%higher than that of LN treatment(0.95±0.03 g m^-2).In addition,at HN treatment,there was no significant difference in leaf nitrogen content between the two growth stages.The specific leaf weight in LN treated leaves at the booting stage was 44.1±2.1 g m^-2,which was significantly higher than that in HN treatment(39.9±1.5 g m^-2).In contrast,there was no significant difference in specific leaf weight between HN(39.0±1.9 g m^-2)and LN(40.1±1.2 g m^-2)treatments at the tillering stage.2. At the tillering stage,A,g_s,g_mand other gas exchange parameters showed significant differences between HN and LN treatments.At HN treatment,A was 31.8±4.3?mol m^-2s^-1,g_swas 0.595±0.139 mol m^-2s^-1,g_mwas 0.300±0.073 mol m^-2s^-1;was 0.149±0.027 mol m^-2s^-1.A and g_mwere sensitive to temperature at both nitrogen treatments.At the booting stage,there was no significant difference in gas exchange parameters between HN and LN treatments,although A and g_mwere also sensitive to temperature.3. Temperature had a significant effect on leaf hydraulic conductivity(K_leaf)at both growth stages and nitrogen treatments.K_leafat tillering stage was significantly higher than that in booting stage,and K_leafunder HN treatment was significantly higher than that under LN treatment.However,the temperature response of K_leafwas generally similar between two growth stages and nitrogen treatments.Temperature had no significant effect on leaf water potential(?_leaf)at HN treatment,but?_leafdecreased significantly with increasing temperature at LN treatment.Moreover,the sensitivity of?_leafto temperature was higher at booting stage than at tillering stage.Under different nitrogen levels,the different responses to temperature of?_leafmay be one of the reasons for the different responses to temperature of g_m.4. At both growth stages,S_major?area of xylem conduits in major veins per leaf width?under LN treatment was significantly higher than that under HN treatment,while IVD_major?inter-vein distance between major veins?was significantly lower under LN treatment.At the tillering stage,LT?leaf thickness?under HN treatment was 114.8±8.0?m,which was significantly lower than that under LN treatment?124.4±3.7?m?.At the booting stage,LT of HN treatment was 199.0±16.4?m,which was significantly higher than that of LN treatment?175.2±21.0?m?.S_majorand S_minor?area of xylem conduits in minor veins per leaf width?showed a decreasing trend with the growth stages,while S_mid?area of xylem conduits in middle veins per leaf width?,IVD_majorand IVD_minor?inter-vein distance between minor veins?all showed an increasing trend with the growth stages.At both growth periods,the S_majorof LN treatment was significantly higher than that of HN treatment,while the IVD_majorwas significantly lower under LN treatment.5. Nitrogen treatment significantly affected the development of chloroplasts.During the two growth periods,S_c?the surface area of chloroplasts facing the intercellular air spaces?under HN treatment was significantly higher than that under LN treatment.In addition,the S_cat tillering stage was significantly higher than that at booting stage,indicating that nitrogen treatment and growth stage had significant effects on S_c.However,S_m?the surface area of cell wall facing the intercellular air spaces?showed no significant difference between the two nitrogen treatments,but S_mat tillering stage was significantly higher than that at booting stage.In addition,the mesophyll cell wall thickness?T_w?in HN treatment at tillering stage was about 0.193±0.024?m,which was significantly lower than that of LN treatment?0.226±0.030?m?.But there was no significant difference in T_wbetween the two nitrogen treatments at booting stage,T_wof HN treatment at booting stage was about 0.249±0.042?m,T_wof LN treatment was about 0.245±0.022?m,and the T_wat booting stage is higher than that at tillering stage.6. The temperature response of g_mmay be related to S_cand T_w.The sensitivity of g_mresponse to temperature may increased by in HN treatment.And with the change of growth periods,the up-regulating T_wand down-regulating S_cmay cause the down-regulating sensitivity of g_mto temperature.