Studies On Mechanisms Of The Effects Of Nitrogen Supplies On Water Uptake And Photosynthetic Characteristics Of Rice Plants

Nitrogen and water are major limiting factors for rice plant growth and production.Nitrogen nutrition not only can improve the water use efficiency of plants,but also can increase the photosynthesis and the yield of crops.However,excess nitrogen supply will lead to the reduced nitrogen use efficiency,even the decreased yield of crops.Photorespiration occurs in all oxygen producing photosynthetic organisms and it can consume a large amount of carbohydrate produced from photosynthesis.So,photorespiration is generally considered a wasteful process as a negative factor in determining the plant productivity.Reducing photorespiration is a possible strategy for improving yield of crops.However,it is not clear the response mechanism of photorespiration to various nitrogen supplies.In this thesis,we focused on the relationship between nitrogen and water uptake,nitrogen and photorespiration,and the reason for the enhanced photorespiration by high nitrogen supply,in order to provide a scientific basis for improving photosynthetic nitrogen use efficiency of rice plants.In this thesis,hydroponic experiments with two independent rice varieties?cv.'Shangyou63' hybrid indica and cv.'Yangdao6' conventional indica,China?supplied by three various nitrogen levels?LN,MN and HN?were conducted in a greenhouse.The effects of various nitrogen supplies on the rice plant growth,water uptake,Aquaporin?AQP?expression,photosynthetic rate,photorespiration,Rubisco specific factor,nitrogen metabolism and energy balance were mainly investigated.We studied the responsibility for increased photorespiration under high nitrogen supply,and discussed the possible approaches to decrease photorespiration.The major results are list as follows:1.Compared with the low nitrogen treatment,high nitrogen inhibited the root growth of rice plants,such as total root length,total root surface area,and total root volume.While,high nitrogen improved the shoot biomass,the leaf area,the number of tillers,and specific leaf weight.Under high nitrogen condition,the net photosynthetic rate and water use efficiency of' leaves were both increased.There was a significant negative relationship between leaf nitrogen content and nitrogen physiological efficiency or photosynthetic nitrogen use efficiency.The chlorophyll content and SPAD values were increased with the increasing nitrogen supply in both rice cultivars.Although there was no difference in leaf water content among various nitrogen supplies,the leaf water potential was decreased in Shanyou63 supplied with high nitrogen,while there was no change in Yangdao6.2.Both of root water uptake rate and leaf water uptake rate per unit weight were enhanced under high nitrogen supply.The water uptake in all treatments was significantly inhibited by HgCl₂,and the Hg-sensitive AQP activity in root was improved under high nitrogen supply.Compared with the low nitrogen supply,the root xylem sap flow rate,root hydraulic conductance,and shoot hydraulic conductance were significantly increased under high nitrogen supply.The enhanced ability of root water uptake was mainly determined by the reduced formation of root aerenchyma,the reduced root porosity,the root lignin,and the improved expression and activity of AQP.Under high nitrogen condition,the ability of water uptake in two rice cultivars showed differences,which was lower in Shanyou63 than that in Yangdao6.Compared with Shanyou63,Yangdao6 had the enhanced root water uptake per weight,root water uptake per leaf area and root hydraulic conductance under high nitrogen supply.There was no difference of water uptake per leaf area between moderate nitrogen supply and high nitrogen supply in Shanyou63?while there was an increase in Yangdao6 supplied with high nitrogen.3.The light response curves showed that the photorespiration was enhanced with the increasing light intensity,and the maximum value was observed at light saturation condition.The photorespiration rate under high nitrogen was higher than that under low nitrogen supply at each light intensity condition.The CO₂ response curves showed that the photorespiration was relative high at low intercellular CO₂ concentration,and high CO₂concentration inhibited photorespiration.Although the carboxylation efficiency and the quantum efficiency of leaf were both enhanced under high nitrogen supply,the photo synthetic nitrogen use efficiency was decreased.The post illumination burst?PIB?,the activity of glycolate oxidase,the CO₂ compensation point in the absence of mitochondrial respiration??*?,and photorespiration rate?Pr?were increased with the increasing nitrogen supplies,which indicated the enhanced photorespiration under high nitrogen supply.When the proportion of photorespiration rate to net photosynthetic rate under low O₂ condition was calculated,it was not significantly changed by various nitrogen supplies.PIB,?*and Pr all had positive relationships with leaf nitrogen content.When the leaf nitrogen content improved by 100%,the PIB,?*and Pr increased by 40%,13%and 57%,respectively.4.The content of ammonium,nitrate and total nitrogen in rice plant were increased with the increasing nitrogen supplies,and the absorption rate of nitrate had significant positive relationship with water uptake rate.The transport rates of ammonium and nitrate in xylem sap also had significant positive relationship with water uptake rate or xylem sap flow rate.Under high nitrogen condition,the GS activity,NR activity,and the amino acid content of root and leaf were significantly improved.Under the same nitrogen supply,the amino acid content of leaf was significant higher than that of root,especially the glutamate,glycine,alanine,and isoleucine.Significant positive relationships between photorespiration rate and Gly/Ser in leaf were observed.5.Under high nitrogen condition,the soluble protein and Rubisco content were both increased,and the proportion of Rubisco to the soluble protein was also increased,indicating the more nitrogen was devoted to Rubisco.The Rubisco activity per leaf area was enhanced with the increasing nitrogen supplies,while the Rubisco activity state proportion was decreased.The Rubisco content and activity had significant positive relationships with photosynthetic rate and photorespiration rate.High nitrogen increased the mesophyll conductance,the total conductance and the chloroplastic CO₂ concentration,but had no effect on the ratio of CO₂ concentration to O₂ concentration in the chloroplast?Cc/Oc?and the ratio of electron transport rate associated with carboxylation to oxygenation?J_c/J_o?.The most important finding is that high nitrogen had no effect on Rubisco specificity factor and the ratio of oxygenation to carboxylation???.Although there were an increase in light-harvesting protein,but there was no energy excess and the energy balance was not disturbed.Under high nitrogen condition,the nitrate assimilation improved,which was likely to consume more energy,but this proportion in the total energy was too small,which had no effects on the energy distribution between photosynthesis and photorespiration.In all,the energy balance was not the key reason for the enhanced photorespiration under high nitrogen condition.6.The responses of rice plants to various amino acid and organic acid were different.Although there was some extent decrease in PIB,?*and Rubisco oxygenation rate in all treatments,the Glu significantly inhibited the plant growth and reduced photosynthetic rate.The OG and Gly had little effect on the biomass,photosynthesis,and electron transport rate.The Ser not only improved rice plant growth and photosynthesis,but also decreased PIB,?*,Rubisco oxygenation rate and?,indicating that Ser played a possible role in decreasing photorespiration and improving yield of rice plants.Compared with the control,the OG and Ser had little effect on leaf organic acid content,but had significant impact on amino acid content,especially the Ser treatment.The Ser significantly increased the content of glutamate and serine,while decreased the isoleucine and Gly/Ser.Gly/Ser can as a maker for the rate of photorespiration.