Relationship Between Rice Water Status And Heat Resistance And Its Mechanisms

Global climate change is increasing ambient temperature and the occurrence of extreme high temperatures,both of which seriously threaten global food security.High-temperature stress(HT)severely affects the growth and development,physiological and biochemical processes,and yield formation of rice,which are associated with plant water status.Water deficit not only affects plant growth and development,but also affects water status of floral organs by altering water physiological processes,thereby affecting yield formation.An increase in water uptake and transport is beneficial to improving the adaptability and resistance to HT in plants.Therefore,the objective of our study is to explore the response of rice plant water status to HT and reveal the underlying mechanism.The study may provide theoretical basis for improving heat tolerance and alleviating heat injury,which is of great significance to ensure the sustainable development of agriculture under the conditions of global warming.Based on those purposes,four experiments were performed under temperature-controlled conditions using two types of genotypes with varying tolerance to HT.(1)Rice plants were exposed to HT under three soil water treatments during flowering stage,spikelet fertility,anther dehiscence,pollen fertility,and water content and water potential of spikelet were measured to explore the relationship between the water status of reproductive organs and spikelet fertility in rice under HT.(2)The physiological mechanism of these processes was investigated in the context of transpirational coolling,osmotic adjustment,and aquaporin expression in reproductive organs under HT.(3)With different elevated air and root temperature treatments,the water uptake rate and water uptake pathways were investigated in rice seedlings to explore the relationship between root water uptake and heat resistance under HT.(4)Under high temperature and drought stresses,morphological and anatomical characteristics,hydraulic conductance and transpiration rate,and aquaporin expression in root and leaf of rice seedlings were investigated to reveal the effects of water uptake and transport to HT and underlying mechanisms under different water conditions.The main results are as follows:1.High temperature treatment reduced grain yield of rice.The decline of grain yield is mainly determined by the decrease of spikelet fertility in rice under HT.In comparison with the control treatment,HT reduced spikelet fertility by 22.5%,anther dehiscence by46.6%and pollen fertility by 26.0%;HT reduced pollen grain diameter by 3.8%,anther water potential by 11.7%,and panicle water potential by 13.3%;the reduction degree of those traits under severe drought conditions were greater than that under well-watered conditions.Correlation analyses showed that spikelet fertility was positively and significantly correlated with anther dehiscence and pollen fertility,respectively;anther dehiscence and pollen fertility were positively and significantly correlated with pollen grain diameter,anther water content,anther water potential,and panicle water potential,respectively.These results showed that the decreased anther dehiscence and pollen fertility may be attributed partly to poor anther water status and poor pollen swelling under HT,leading to the decrease of spikelet fertility.2.High temperature treatment reduced anther dehiscence rate,pollen fertility,and spikelet fertility,and the heat-tolerant genotypes(SY63 and N22)had smaller decreases compared with the heat-sensitive genotypes(LYP9 and TXZ25).In comparison with the control treatment,HT significantly increased spikelet transpiration rate,and this change was accompanied by a significant decrease in the internal temperature of the spikelets.In comparison with the heat-sensitive genotypes,the heat-tolerant genotypes showed higher spiklet transpiration and lower spikelet internal temperature.In addition,HT decreased anther osmotic potential via increased anther soluble-sugar concentration.Different aquaporin genes showed different expression profiles under HT,and the expression levels of PIPs for plasma membrane intrinsic proteins and TIPs for tonoplast intrinsic proteins increased in anthers but decreased in glumes.Correlation analyses showed that anther dehiscence and pollen(spikelet)fertility were tightly associated with anther water status,and the expression levels of almost anther aquaporin genes were significantly correlated with anther dehiscence under HT.Thus,the increased spikelet transpiration rate and decreased internal spikelet temperature alleviated the adverse effects of HT on pollen fertility,anther dehiscence,and spikelet fertility through increasing total expression of aquaporins and soluble sugar content,especially in heat-resistant genotypes.3.In comparison with the control treatment(Ta1Tr1),three high temperature treatments(high root temperature treatment,Ta1Tr2;high air temperature treatment,Ta2Tr1;high air and root temperature treatment,Ta2Tr2)significantly increased total water uptake rate(WUR_t),water uptake rate via apoplastic pathway(WUR_a),and water uptake rate via cell-to-cell pathway(WUR_AQP).High temperature treatments had varying effects on the contributions of WUR_a and WUR_AQP to WUR_t.The proportion of WUR_a to WUR_t were 75%,56%,67%,and 40%under Ta1Tr1,Ta1Tr2,Ta2Tr1 and Ta2Tr2,respectively;the proportion of WUR_AQP to WUR_t were 25%,44%,33%,and 60%under Ta1Tr1,Ta1Tr2,Ta2Tr1 and Ta2Tr2,respectively.The results indicated that apoplastic pathway was the main pathway for water uptake under control treatment,however,the cell-to-cell pathway became the dominant pathway under high air and root temperature treatment.In comparison with Ta1Tr1,Ta1Tr2 had no significant effect on transpiration rate,Ta2Tr1 and Ta2Tr2 significantly increased transpiration rate.Transpiration rate and water uptake rate were the highest under simultaneously high air and root temperature treatment among the four treatments,which were almost 2-3 fold higher than those under control treatment.Moreover,total expression of PIPs and TIPs in root and leaf significantly increased under high temperature treatments.Leaf total expression of PIPs and TIPs were positively and significantly correlated with transpiration rate.Thus,high air and root temperatures collectively regulated water uptake and transpiration,the cell-to-cell pathway became the dominant pathway when the air and root temperatures were elevated;the increase of water uptake was attributed to the increase of expression level of aquaporins.4.High temperature treatment increased WUR_t,WUR_a,WUR_AQP and root hydraulic conductivity(Lp_r)under well-waterd and drought conditions,and WURt was positively associated with Lp_r.HT reduced root diameter and cortical cell thickness,which were positively and significantly correlated with Lp_r and WUR_a,respectively,respectively.HT decreased the contribution of WURa to WURt but increased the contribution of WUR_AQPto WUR_t.HT increased aerenchyma formation and sclerenchyma cell diameter,which were negatively and significantly correlated with Lp_r and WUR_a,respectively.These results showed that the changes of root anatomical characteristics affected water uptake and the contributions of WUR_a and WUR_AQP to WUR_t.HT increased the total expression of PIPs and TIPs in roots,the contribution of WUE_AQP to WUR_tincreased from 6.5%under control to 52%under HT under well-watered conditions and from 2.2%under control to 38.6%under drought conditions;Lp_r was positively correlated with the expression levels of PIPs and TIPs in root,indicating that Lp_r was regulated by the expression level of aquaporins.HT increased leaf hydraulic conductance(K_leaf),transpiration rate and the leaf total expression of PIPs and TIPs,and K_leaf and transpiration rate were positively correlated with the expression level of PIPs and TIPs,suggesting that the increases of K_leaf and transpiration rate were attributed to the increases of leaf total aquaporin expression.Thus,increased aquaporin gene expression may contribute to the increases in WUR_t,Lp_r and K_leaf.In summary,in comparison with the heat-sensitive genotypes,the heat-tolerant genotypes showed that stronger capacity of water uptake and transport,higher spikelet transpiration rate,osmotic adjustment ability and aquaporin expression,indicating those are the physiological basis of its HT resistance.