Nitrogen Utilization And Its Adaptive Mechanism Of Rice (Oryza Sativa L.) Under Water Stress

The increasing drought risk caused by shortage of agricultural water resources and climatic anomaly and the decreasing nitrogen?N?-use efficiency due to unreasonable fertilization are two main problems in rice production.Optimal allocation of N in rice plants is essential for efficient N-use and water stress acclimation.However,the studies on the mechanisms that water regulates N utilization and the relationship with water stress acclimation are not fully studied in rice.In this study,with rice cultivars‘Yongyou538'?inter-subspecific hybrid?,‘Jiayou 5'?japonica hybrid?and‘Zhongzheyou 1'?indica hybrid?as materials,hydrauponical experiments and pot experiments were conducted to:?1?study the effects of water deficit stress on internal N utilization in rice;?2?study the role of leaf N allocation in coordinating N use efficiency and waer stress acclimation;?3?analyze the effects of soil drying on leaf N metabolism and its relationship with water stress acclimation in rice;and?4?reveal the mechanisms underlying N-mediated the acclimation of rice plants to water stress,by comparing the effects of water deficit stress on carbon and N metabolism pathways in rice at different N levels.The results were as follows.1.Water stress had no significant effect on N utilization index?NUI?in‘Yongyou538'under high N,but significantly decreased NUI in‘Zhongzheyou 1'.Roots played an important role in regulating N utilization in rice under water deficit stress,and NUI positively correlated with the proportion of N allocated in the roots.The results of gene expression and ¹⁵N tracing revealed that water stress reduced N uptake in‘Yongyou 538',promoting N remobilization from shoot to roots and decreasing the N concentration in shoot.Simultaneously,water stress decreased the glycolysis and promoted the accumulation of soluble sugar and carbon in roots of‘Yongyou 538'.In contrast,‘Zhongzheyou 1'maintained high N uptake activity under water stress,while N assimilation was reduced.The accumulation of NH₄⁺and free amino acids in roots was significantly increased under water stress,which impeded N remobilization from shoot to roots.Additionally,water stress increased sugar catabolism and reduced biomass accumulation in roots.The results indicate that properly reduction of N uptake and root glycolysis could promote the efficient utilization of internal N in rice plants under water deficit stress condition.2.Leaf N allocated in the photosynthetic apparatus was significantly positive correlated with photosynthetic N-use efficiency?PNUE?.N allocation in the carboxylation system and bioenergetics components of the photosynthetic apparatus were the primary two limiting factors of PNUE under the conditions of high N and water deficit stress.The coordination of leaf N allocation between photosynthetic and non-photosynthetic apparatus,and among the components of the photosynthetic apparatus is important for the trade-off between N utilization and acclimation of water deficit stress in rice.Under low N conditions,the plants maintained the stability of PNUE under water stress by reducing N allocation in the light-harvesting system and increasing soluble protein and free amino acids,or reducing N allocation in the cell wall.Under high N conditions,however,the plants sacrificed PNUE to adapt to water stress by increasing N distribution in non-photosynthetic apparatus.3.N metabolism plays diverse roles in the photosynthetic acclimation of rice plants to soil drying.Soil drying increased nitrate reducing in‘Zhongzheyou 1',but the activities of glutamate dehydrogenase?GDH?,glutamic-oxaloacetic transaminase?GOT?and glutamic-pyruvic transaminase?GPT?were less affected.Nitrate reducing cooperated with non-photochemical quenching?NPQ?increased the dissipation of excessive energy,protecting the photosynthesis of‘Zhongzheyou 1'from soil drying.The accumulation of nitrate was significantly increased in‘Jiayou 5'under soil drying conditions,although the activity of nitrate reductase was increased.The activities of GDH,GOT and GPT were typically increased under soil drying.Amino acids and soluble sugar were significantly increased under mild and moderate soil drying,respectively.The accumulation of nitrate and amino acid under soil drying serves as osmolytes in‘Jiayou 5'.4.Compared with low N treatments,high N treatments alleviated the inhibition rate of water stress on photosynthetic rate by 54.1%in rice.Under water stress,the photorespiratory serine metabolism was increased,which enhanced glutathione biosynthesis and the ascorbate-glutathione cycle,alleviating the oxidative stress injury of plants under water stress.High N treatment up-regulated GS2 under water stress and increased the activities of aspartate aminotransferase and glycine:glyoxylate aminotransferase?GGAT?and phosphoenolpyruvate carboxylase?PEPC?-mediated anaplerotic reaction,which increased the supply of a carbon skeleton,2-oxoglutarate,increasing the re-assimilation of NH₃ released by photorespiration.High N treatment also increased the synthesis of sugar and starch and the regeneration of RuBP under water stress.Under low N conditions,water stress-induced protein degradation and down-regulation of NH₃ assimilation,as well as the failure of the ascorbate-glutathione cycle due to reduced glutathione biosynthesis,perturbed the function of chloroplasts.The results reveal that high N supply preserves the biochemistry of photosynthesis through coordinated regulation of the C and N metabolism to facilitate the acclimation of rice photosynthesis to water deficit stress.In conclusion,there is a balance between N utilization and water stress acclimation in rice.Properly reducing N uptake and assimilates consumption are the physiological basises for rice to maintain efficient internal N utilization under water stress conditions.Sacrificing N-use efficiency to enhance stress resistance is one of the important strategies for rice to adapt to water stress.Maintaining or improving N metabolism level under water stress is important for improving rice stress-resistance.This study provides a new way of thinking for the studies of regulation of N utilization and stress-resistance in rice.The results provide a basis for further understanding the regulatory mechanism of N optimal allocation and efficient utilization in rice and its relationship with stress resistance,and are important for theoretical guidance of improving N utilization efficiency in rice and achieving N optimal management in paddy.