The Study On Mechanism Of Nitrogen Nutrition Regulating Rice Plants Water Balance And Photosynthesis

In China,we are facing water shortage,and development of the country bases on agricultural production,which consuming 80%water of the country use.In agriculture water consumption,70%water was used in rice paddy fields,but the water use efficiency was very low,in comparison with more than 80%wasted,only 20%water was used for grain production.To save water resource,it is meaningful to grow rice plants in upland and under control water supply condition.In dry land conditions,the nitrogen forms would transfer from ammonium into nitrate in soils,while in our previous studies,it was demonstrated that ammonium could enhance rice seedlings drought tolerance,compared with nitrate nutrition.It still remains unclear how nitrogen forms and drought stress affect roots water uptake,in which process AQP might involved regulation.So how AQP affect rice seedling drought tolerance,associating with nitrogen forms?Additionally,it was demonstrated that nitrogen use efficiency（NUE）decreased significantly when increasing nitrogen use level,and present NUE was only 35%-40%.Our previous studies focused on clarifying the mechanism of PNUE（CO₂ assimilation rate/leaf nitrogen content）decrease under high nitrogen supply,in which have shown related to CO₂ diffusion resistance in leaves mesophyll cells.AQP was shown to affect C02 diffusion in leaves,but it remains unclear how different nitrogen levels affect AQP in rice plants,and how AQP affect plants water balance and CO₂ assimilation.In this study,based on the function of AQP,it was demonstrated that how AQP affects rice seedlings drought tolerance and how AQP affects CO₂ assimilation.With supplying different nitrogen forms and PEG6000 simulated drought stress,we studied rice growth and development,root water uptake ability,roots AQP expression and activity,and also phytohormone（ethylene and ABA）.It was discussed that the relationship between AQP and root radial water transport.In addition,with supply different nitrogen levels,we analyzed rice plants CO₂ assimilation efficiency,AQP expression and activity in roots and leaves,and the relationships between AQP and chloroplast development,we discussed how AQP affected CO₂ diffusion in rice plants.The results including:1,Compared with OsPIP1s,OsPIP2s showed obviously water transport function when expressed in heterologous expression system.In oocytes expression system,the cells water permeability increased significantly when expressing OsPIP2;1,while not for the OsPIP1;1 and OsPIP1;2.In yeast expression system,all four OsPIP2s genes increased yeast cell water permeability from stopped-flow measurement,especially for OsPIP2;1,the Pf was 0.45 cm/s.When compared with OsPIP2;5,the Pf of OsPIP2;1 was six-fold higher.Three OsPIP1s genes showed lower Pf than OsPIP2s.2,OsPIP2;1 could affect rice plants shoots and roots growth.In OsPIP2;1RNAi rice plants,roots showed significantly inhibition,including total length,surface area,volume and tips number,as soon as lateral roots also showed decrease of lateral numbers and length comparing with WT.OsPIP2;1 affected shoots biomass and spike formation.Roots hydraulic conductivity decreased significantly in OsPIP2;1RNAi rice plants than WT,when compared with WT the conductivity decreased by 80%in OsPIP2;1RNAi rice plants.The drought tolerance decreased in OsPIP2;1RNAi rice plants,which showed leaf rolling and shoots biomass decreased under drought stress condition.3,Under drought stress condition,roots AQP expression and activity were increased by ammonium nutrition supply,which resulted of roots hydraulic conductivity increase and water uptake ability increase.Under drought stress,ammonium nutrition could sustain shoots growth and roots growth and development.Drought stress induced roots AQP expression and activity up-regulated and hydraulic conductivity increased in rice plants supply with ammonium;roots showed higher ABA content in rice plants supply with ammonium than nitate in both water conditions.Under drought stress condition,more ethylene accumulated in roots when supply with nitrate,which would inhibit AQP expression and activity,also inhibit root growth.4,OsPIP1;1 was involved in regulating CO₂ diffusion in rice leaves.The shoots growth and spike development were inhibited in ospip1;1 mutant rice plants.Compared with wild type,leaf CO₂ assimilation rate decreased significantly in mutant,especially under high measuring irradiance.In comparison with wild type,mesophyll conductance（gm）and chloroplastic CO₂ concentration（Cc）decreased significantly in ospip1;1 mutant plants,gm and Cc were decreased by 50%and 30%.There was no difference of roots hydraulic conductivity between WT and ospip1;1 mutant.5,In high N treatment,PNUE decrease was related to CO₂ diffusion resistance increase,in which process AQP played vital roles.With N levels supply increasing,shoots biomass and nitrogen content increased significantly.In comparison with low N treatment,the photosynthetic rate and mesophyll conductance increased signigicantly in high N treatment.While,PNUE decreased by high N supply.The theoretical Cc was higher than normal Cc,especially under high N supply,and the difference of Cc increased from 84 to 137μmol.mol-1.RT-PCR results indicated the expression of OsPIP1s and OsPIP2s were lower in high N supply than in low N,which showed positive relationships with total CO₂ transport ability.6,It was observed that the different response of AQP between in roots and leaves affecting by different nitrogen supply levels.Roots AQP expression and activity were increased by high N supply.Leaves AQP expression and activity were decreased by high N supply;under high N supply,leaves water potential was lower than low N treatment.Under high N supply,the water balance was breaked between roots and shoots,which caused shoots growth insufficient water supply.Compared with wild type rice plants,chloroplast volume showed increase in OsPIP2;1RNAi rice plants,indicating chloroplast development was affected by AQP and furthermore affected photosynthetic CO₂ assimilation.In conclusion,AQP plays vital roles in root water uptake and CO₂ diffusion in leaves.Under drought stress condition,ammonium nutrition increased roots AQP genes expression and activity,which enhanced roots water uptake ability and sustained shoots growth.In leaves,PNUE was decreased by high nitrogen supply,and CO₂ diffusion resistance increased,in which process AQP regulated.Different nitrogen supply could affect shoots and roots water balance,through regulating AQP expression and activity,and furthermore affected carbon fixation efficiency per nitrogen content.