Effects Of Aquaporin Gene(PtPIP1;3)on Mesophyll Conductance Of Poplar Under Drought And Shading Conditions

In recent years,mesophyll conductance（gm）has become the indispensable factor in the process of studying photosynthesis.Stomas were forced to close under drought stress and the reduction of gm limited plant photosynthesis.Also,gm has become another important limitation factor of photosynthesis besides stomatal conductance（gs）and biochemical capacity.Aquaporins（AQP）can facilitate the transportation of some small molecules（e.g.CO2）in the mesophyll,besides transport H2O.Therefore,the more expression of AQP,the more beneficial for CO2 to enter into the caoboxylation sites,which improves the plant photosynthesis.Most studies on AQP and gm were focused on the crop plants,the growth cycles of which are short.Few studies were focused on fast growing species like poplars,especially researches of the different AQP expression levels of transgenic poplars under different stress conditons on photosynthesis and adaptation stragegies.Researches in this area will be useful to understand the contribution to photosynthesis and the adaptation strategies of plant.This will offer thoughts and experimental foundation for the study of the relationship of AQP and gm under different conditions.Pt PIP1;3 overexpression and suppressed expression of one year old hybrid poplar 84K（Populus alba×P.glandulosa）was main research object to analyze the changes of photosynthetic capacity and gm under short-and long-term drought stesss;to analyze the influence of shading on plant growth,biomass allocation and leaf anatomical structure of different gene types poplars to analyze the contribution of aquaporin to photosynthesis.Main results are as follows:1.In this experiment,photosynthetic parameters were measured to analyze the main limiting factors of photosynthesis of plants in the process of drought and rehydration.In this study,on the sixth day of drought gs dropped to 0.05 mol CO2·m^-2·s^-1 by analyzing the photosynthetic capacity of poplar under short-term drought stress in the greenhouse.Results showed that photochemical reaction process and photochemical quenching process declined significantly,but the decline extent was less than gs and gm.During this process photosynthesis was mainly limited by gs and gm.After rehydration the mesophyll conductance recovered quickly.Whereas the carboxylation reaction and photochemical quenching process did not store to the normal level.At this time photosynthesis was mainly restricted by photochemical quenching or carboxylation reaction.During the drought and rehydration treatments,the responses of stomatal conductance（gs）and mesophyll conductance（gm）were not synchronous.2.In this experiment,photosynthetic parameters and chlorophyll fluorescence parameters of PB13 and WT were measured under short-term drought stress to analyze the role of aquaporin in the occurrence and relief of stress.After appropriate extension of drought stress time,we found that after 6 days of drought net photosynthetic rate（Pn）and stomatal conductance（gs）of overexpression of PIP1 poplar decliend more quickly than that of wild type plant（WT）.Once exposed to the drought stress PB13 reduced gm and gs quickly.On the tenth day of drought,P_n declined to 2μmol CO₂·m^-2·s^-1,g_s declined to 0.05 mol CO₂·m^-2·s^-1 and gm reached less than 10%of control plants.Photosynthesis was limited by gm.After one day rewatering gm recovered quickly,during which plant photosynthesis was limited by biochemistry capacity.After three days of rewatering,the photosynthesis parameters and chlorophyll fluorescence parameters recovered more quickly than those of WT.Biochemical process and gm were the main limitation factors of photosynthesis.After rewatering the photosynthetic recovery rate of PB13 was faster than that of WT,which means aquaporin PIP1was helpful for the recovery of photosynthesis after drought stress.In the greenhouse,the overexpression of aquaporin（PIP1;3）gene improved the mesophyll conductance（gm）and net photosynthetic rate（Pn）,which was helpful for the photosynthesis rapid recovery of poplar after drought stress.3.By comparing the photosynthetic parameters,mesophyll conductance and biomass allocation of PB13,PR13 and WT growing in the field,the effects of aquaporin on CO2conductance and growth under relative long-term stress were studied.In the field conditon,the overexpression of gene PIP1;3 improved the gm,while not significantly in Pn.Besides it was benificial to water homeostasis in plant.Compared with WT,PB13 showed stronger drought tolence under long-time severe drought stress in less declined extent of photosynthesis,gm and growth.On the contrary under normal water supply condition,the supression expression of PIP1;3（PR13）caused photosynthesisi and gm declined,especially growth was influenced significantly.Weaker drought tolerance existed in PR13 under long-term drought stress,gm of which dropped to 2%of control,especially the photosynthesis and growth.It is noteworthing that,under moderate drought stress,the root/shoot ratio of PR13 increased significantly,suggesting that aquaporin participated in the regulation of photosynthetic performance.Because of the inhibition of gene expression,CO2 absorption was influenced.The aboveground part of PR13 was more inhibited.Under moderate and severe drought stress gm was the main limitation of photosynthesis,while biochemistry limitation was the main limitation factor of PR13.Supression expression of aquaporin restrained plant growth,declined the drought tolerance,even changed the biomass allocation.These changes could not compensate the influence caused by the surpression.Overexpression of aquaporin would be beneficial to plant adaptation to long-term drought stress.4.In this study,variations in mesophyll conductance and leaf anatomical sturcture of transgenic and wild-type poplars were measured to analyzed the regulation of aquaporin on mesophyll conductance under shading conditions.After shading gm of PB13 declined most,only reached 18%of control.Shading caused the anatomical structure of PR13 changing most and PB13 least,which means the changes in structure can not explain the difference of gm aftering shading.Under full light PR13 has the thickest blade,least leaf area,lagest specific leaf weight,which might compensate low photosynthetic compacity by thickening leaf to resist photoinhibition under high temperature.This kind of structure restricted plant photosynthesis under normal light conditon causing lower gm than PB13 and WT.Especially under shading PR13 showed although structural adjustment like the thinnest blade,smallest specific leaf weight,smallest leaf area and least content of Cc caused the supression on PR13 in growth.Aquaporin was involved in the regulation of gm under low light condition.