Effects Of Water Stress And Re-watering At Seedlings Stage On Wheat Growth And Its Physiological Mechanisms

Wheat is the most important food crops in the world.Drought is one of the main natural disasters affecting wheat production in China.Furthermore,in China,drought frequently occurs during the seedling stage,affecting the formation of photosynthetic organs and tillers and decreasing the accumulation of material and energy required for growth.There are significant differences among genotypes for drought tolerance in wheat,and differences in growth and physiological characteristics decide the drought resistance.Therefore,in utilization of differences of wheat itself in response to drought stress,to clear on the response characteristics of drought stress in wheat seedling and reveal the internal mechanism,plays an important role in breeding for drought tolerance varieties and develop cultivation measures to improve drought resistance.Hydroponic experiments were conducted using two wheat cultivars,Wangshuibai(drought sensitive)and Luohan 7(drought tolerant)to study the effects of water stress and re-watering on growth and photosynthetic characteristics,reveal the response characteristics and physiological mechanism of root morphology,and analy the metabolic characteristics of carbon and nitrogen and the relationships between with photosynthetic characteristics and root morphology.Finally,the effectes of different nitrogen forms and water stress on root morphology and photosynthesis were studied.The main results are as follows:1.The different responses of plant growth to water deficit and re-watering in wheat seedlingsUnder water stress,for both cultivars,plant dry weight,leaf area and tiller number were significantly decreased.However,the decreases in Luohan 7 were less than those in Wangshuibai.After re-watering,the relative growth rate of shoot and root of short-term stressed Luohan 7 was higher than that of controls,and leaf area had no difference between stressed and controls,while the rate and leaf area of Wangshuibai were lower than those of controls;but for long-term stressed Luohan 7,the decreases in the rate was lower than Wangshuibai,and the tiller number recovered to a control level.Under water stress,net photosynthetic rate(Pn),stomatal conductance(Gs),transpiration rate(Tr),Rubisco content,the maximal photochemical efficiency(Fv/Fm),the actual photochemical efficiency of potosysterm?(?PS?),photochemical quenching coefficient(qP)were significantly decreased,and the decreases in Luohan 7 were less than those in Wangshuibai;the initial fluorescence(Fo)and non photochemical quenching coefficient(qN)increased significantly,and the increases in Luohan 7 were significantly less than those of wangshuibai,indicating that under drought stress Luohan 7 could kept higher photosynthetic capacity,carbon assimilation efficiency and light use efficiency,which was in accordance with the results that the down-regulation of the Rubisco activase B encoding gene(RcaB)and chlorophyll a/b-binding protein gene(cab)in Luohan 7 were less than those in Wangshuibai.Under long-trem drought stress,chlorophyll fluorescence parameters of the 1st and 2nd leaf of Luohan 7 had no significant differences between sterssed and controls,while the parameters of all leaves of Wangshuibai were significantly lower than those in control.After re-watering,Pn was higer than control in short-term stressed Luohan7,but lower in Wangshuibai;with the re-watering time went by,compared to control,there were no differences in Pn in the 1st and 2nd leaf of Luohan 7,while significant decreases in Wanshuibai.Under water stress,superoxide dismutase(SOD)activity increased,and the increases in Luohan 7 were more,in accordance with the up-regulation of the Mn-SOD and Cu/Zn-SOD;peroxidase(POD)activity in Luohan 7 increased,while it increased firstly,then decreased gradually in Wangshuibai.Therefore,no differences were observed in superoxide anion(O2·-)production rate between water stress and control in Luohan 7,while significant increases were found in Wangshuibai,indicating that Luohan 7 could maintain the balance of active oxygen generation and scavenging,so that the increases of MDA content and damage of membrane lipid peroxidation were lower.In summary,drought tolerant cultivar could maintain higher photosynthetic capacity under water stress.After re-watering,the Pn in the drought tolerant cultivar recovered faster and had an obvious compensation effect,which contributed to promote leaf growth and tiller formation.2.Effects of drought stress and re-watering on root morphology of wheat seedlingsUnder water deficit,total root length(TRL)and root surface area(RSA)of SRs and NRs significantly decreased for two cultivars.After re-watering,TRL,and RSA of SRs in Luohan 7 were significantly higher than controls,while significantly lower in Wangshuibai,which inferred that Luohan 7 had a faster growth rate in SRs after re-watering.In addition,for both cultivars,the concentrations of abscisic acid(ABA)increased in leaves,SRs and NRs,while the concentrations of indole-3-acetic acid(IAA)and cytokinin(CTK),IAA/CTK,IAA/ABA and CTK/ABA decreased significantly compared to controls under water deficit,and the decreases in Wangshuibai were much higher than those in Luohan 7.After re-watering,the concentrations of IAA and CTK in SRs and NRs increased significantly,and the concentrations of ABA decreased.After re-wtering,IAA levels in SRs of Luohan 7 were much higher,while CTK concentrations were lower than those of controls and ABA concentrations had no differences with controls,resulting in a higher IAA/CTK and IAA/ABA.Further analysis showed that there was a significant positive correlation between TRL of SRs and IAA/CTK,IAA/ABA and CTK/ABA.Under water deficit,the up-regulation of TaAQP2,TaAQP3 and TaAQP4 in Luohan 7 were higher than those in Wangshuibai,suggesting that the capacity of water absorption and transport of roots in Luohan 7 was better.After re-watering,TaAQP4 and TaAQP6 were up-regulated significantly,indicating that re-watering accelerated root water uptake transport of Luohan7 to promote the root and shoot growth.We concluded that a faster growth rate of SRs in response to re-watering,regulated by IAA/CTK and IAA/ABA and the expression of aquaporin encoding genes in Luohan 7,contributed to the promotion of recovery of the whole root systerm.3.Effects of drought stress and re-watering on carbon and nitrogen metabolism of wheat seedlingsUnder water stress,the increases in soluble sugar and sucrose content in leaves and decreases in roots of Luohan 7 were significantly lower than those of wangshuibai;in Luohan 7 the increases in new leaves were significantly lower than those in old leaves,while in Wangshuibai were significantly higher than those in old leaves.After re-watering,soluble sugar and sucrose content in 1st leaf of Luohan 7 was significantly lower than the control,while were significantly higher in roots.Water stress had no effects on sucrose phosphate synthase(SPS)activity and sucrose synthase(SS)activity in Luohan 7,while decreased them in Wangshuibai.Under water stress,contents of free amino acid and nitrogen were decreased;however,the decreases in Luohan 7 were less than those in Wangshuibai;after re-watering,they recovered fater in Luohan 7 and compared to Wangshuibai,the 1st leaf of Luohan 7 had stronger adaptation and recovery capability.Under water stress,nitrate reductase(NR)activity,glutamine synthetase(GS)activity,NO3--N and Rubisco content in leaves were decreased;however,the decreases in Luohan 7were less than those in Wangshuibai,suggesting that the rate of nitrogen assimilation and CO2 carboxylation efficiency in Luohan 7 were higher.Under water stress,the increases in root NR and GS activity were higher and the decreases in root NO3--N content were lower in Luohan than those in Wangshuibai,suggesting that the root of Luohan 7 had stronger ability of nitrogen uptake and assimilation.Under water stress,the increases in the ratio of soluble sugar to nitrogen in leaves of Luohan 7 were significantly lower than those of Wangshuibai,and in 1st leaf were significantly lower than those in 2nd and 3rd leaf.After re-watering,the ratio in Luohan 7 was the same as the control,while still higher in 1st leaf of Wangshuibai.Root sucrose content and root IAA content and total root length growth rate were positively correlated;root nitrogen content and total root length,root IAA/CTK,IAA/ABA and CTK/ABA were significantly positive correlation.In summary,Luohan 7had more adaptability and could provide carbon and nitrogen for plant growth.Moreover,the new leaf of Luohan 7 had better capcity of assimilate transport,alleviating the inhibition of Pn in new leaf photosynthesis and promoting the plant growth.4.Mechanisms of nitrogen forms and drought stress on plant growth of wheat seedlingsUnder water deficit,plant dry weight and leaf area were decreased,and the decreases in NAN supplied plants were significantly less than in NN supply.Under water deficit,the decreasea in total root length(TRL)and root surface area(RSA)in NAN supplied plants were less than those in NN supply,suggesting that under drought deficit,NAN supplied plants have greater water absorption area and root activity.Under water deficit,the decreases in net photosynthetic rate(Pn)and carboxylation efficiency(CE)in NAN supplied plants were much less than in NN supply.Under water deficit,there were no significantly differences in the effective quantum yield of PS?photochemistry(?PS?)and the maximum photochemical efficiency after full dark adaptation(Fv/Fm)in Luohan 7under NAN nutrition compared to control,and the decreases in Wangshuibai under NAN nutrition were less than those under NN nutrition;and the increases in SOD and POD activity of NAN supplied plants were higher than those of NN and AN nutrition.Under water deficit,the decreases in the chlorophyll content,Rubisco content and activity and nitrogen accumulation in NN supplied plants were much higher than those in NAN supplied plants.In addition,under water deficit,Rubisco content/nitrogen accumulation in NAN supplied plants was increased significantly,suggesting that NAN nutrition improved the allocation ratio of nitrogen to Rubisco.Furthermore,under water deficit,the increases in sucrose content in leaves and decreases in roots in NAN supplied plants were less than those in NN supplied plants.Collectively,under water deficit,compared to the NN supplied plants,higher photosynthetic capacity and metabolic activity were observed in NAN supplied plants,enhancing the drought tolerance of wheat seedlings.In conclusion,the major differences between drought tolerant and sensitive cultivar were in photosynthetic capacity,root morphology and activity of carbon and nitrogen metabolisms.Under drought stress,drought resistant varieties had a stable photosynthetic system,high carbon use efficiency and activities of antioxidant enzymes,to maintain a high photosynthetic activity and antioxidant capacity and alleviate the decrease of dry matter accumulation.After re-watering,a faster growth rate of seminal roots of drought tolerant cultivar by increasing IAA/CTK and IAA/AB A to obtain more carbon and nitrogen meet to water and nutrient supply of the shoot,which contributed to the recovery of photosynthetic capacity and leaf growth.Under water stress,plant growth could also be influenced by nitrogen form,and the mixture of NO3-and NH4+ nutrition increased photosynthetic capacity of leaves and promoted root growth,resulting in an enhanced drought tolerance of wheat seedlings.