Evaluation Of Drought Resistance Of Alfalfa Based On Leaf Veins And Stomatal Traits

Drought is a key environmental factor limiting the growth and quality of alfalfa（Medicago sativa.L）in semi-arid regions of China.Leaf veins and stomata are the most important pathways for leaf water transport and evapotranspiration,and play an important role in regulating leaf water balance and photosynthesis.However,there are few reports on the characteristics of alfalfa leaf veins and stomata traits in response to drought stress and their effects on hydraulic properties and photosynthesis.Therefore,in this study,38 alfalfa varieties were used as materials in a field plot test,and the diversity of leaf veins and stomata of each variety was analyzed;based on this,two representative varieties with different leaf veins and stomata characteristics were selected:the introduced species WL354HQ（WL354）and the Chinese bred variety Zhonglan No.2 alfalfa（ZL2）for a pot test to study the leaf veins and stomatal traits of the two alfalfa varieties in response to different drought stresses[1].We investigated the differential response of leaf veins and stomatal traits to different drought stresses[80±5%of field water（control,CK）,55±5%of field water（moderate drought stress,MS）and 30±5%of field water（severe drought stress,SS）]to investigate the effects of changes in leaf veins and stomatal traits on leaf hydraulics and photosynthesis of alfalfa,so as to reveal the effects of leaf veins and stomatal traits on the drought resistance of alfalfa.In this way,the effects of leaf veins and stomatal traits on the drought resistance of alfalfa were revealed,and a method was provided for the selection of drought-resistant varieties of alfalfa.The main results were as follows:（1）38 alfalfa varieties differed significantly in leaf veins and stomatal traits.Among them,six leaf vein traits（main vein density,veinlet density,total leaf vein density,leaf vein diameter,mesh area and closed loop structure）and seven stomatal traits（upper surface stomatal density,lower surface stomatal density,total stomatal density,upper surface stomatal length,upper surface stomatal size,lower surface stomatal size and stomatal area per unit leaf area）had high coefficients of variation（CV>0.15）,based on which Based on these 13 traits,seven alfalfa varieties with significant differences in leaf veins and stomatal traits were selected,and the introduced species WL354HQ（WL354）with high density of leaf veins and stomata and small stomatal area,and the Chinese bred species Zhonglan No.2 alfalfa（ZL2）with low density of leaf veins and stomata and large stomatal area were selected as test materials for the subsequent study.（2）With the enhancement of drought stress,leaf area and specific leaf area of the two alfalfa varieties decreased,leaf thickness increased;stomatal density increased and stomatal area decreased;leaf vein density and mesh area increased,and leaf vein diameter and vein spacing decreased.The veinlet density and lower surface stomatal density of the two alfalfa varieties were significantly different,and the veinlet density and lower surface stomatal density of WL354 were significantly higher than those of ZL2 under the same treatment.closed-loop structure,veinlet density,and total vein density were significantly negatively correlated with stomatal traits,while reticulate area,vein diameter,and vein spacing were significantly positively correlated with stomatal traits,and vein density and stomatal density were significantly negatively correlated with leaf area.（3）The response of WL354 and ZL2 to drought stress differed in the degree of leaf water use efficiency(WUE_leaf)and water use efficiency per plant(WUE_plant)of WL354 variety decreased under MS conditions and increased under SS conditions;WUE_leaf and WUE_plant of ZL2 variety increased under MS conditions and decreased under SS conditions.The leaf hydraulic conductivity(K_leaf)and leaf water potential（ψ）decreased and the embolism index(P₅₀)increased in both,with WL354 showing a smaller decrease than ZL2 and significantly higher leaf hydraulic properties index under the same treatment than ZL2.K_leaf was significantly negatively correlated with leaf vein density,closed annulus and stomatal density,and positively correlated with other leaf vein stomatal-related traits;WUE_plant was significantly negatively correlated with leaf vein density,while significantly positively correlated with stomatal density.（4）Drought stress caused a decrease in photosynthetic rate,transpiration rate,stomatal conductance,PSⅡactivity parameters,stem dry weight and leaf dry weight in both alfalfa varieties,and an increase in PSⅡreaction center parameters,which hindered photosynthesis and decreased photosynthetic capacity.Under all treatments,the values of photosynthetic parameters of alfalfa variety WL354 with higher density of leaf veins and stomata were higher than those of alfalfa variety ZL2 with lower density of leaf veins and stomata.photosynthetic parameters were negatively correlated with leaf stomatal density,leaf vein density and closed ring structure,and positively correlated with leaf vein diameter,stomatal area,and stomatal area per unit leaf area;leaf veins and stomatal traits were positively correlated with biomass,but not with chlorophyll fluorescence There was no significant correlation with chlorophyll fluorescence parameters.Nine indicators closely related to the drought resistance of alfalfa were selected from each index by stepwise regression analysis,among which veinlet density and abaxial leaf stomatal density were the key traits affecting the drought resistance of alfalfa.In conclusion,under drought stress,alfalfa leaves maintain water transport,secure water supply and maintain stomatal movement by increasing veinlet density and decreasing stomatal area to maintain the stability of leaf water and photosynthesis.Compared with ZL2,WL354still has higher veinlet density,stomatal density and closed loop structure after drought stress,which can better regulate the balance of water supply and demand to optimize plant growth in order to prevent severe drought stress,reflecting the trade-off between growth and drought resistance,therefore,cultivation practices and selection of crop genotypes with higher veinlet density and stomatal density should be further optimized in production.