Study On The Indication Of Physiological Characteristics Of Soil Water Availability For Typical Forage In The Semi-arid Region

Grassland construction is the main content of ecological construction in the semi-arid region,the growth of grassland was limited by the imbalance of seasonal precipitation in this area.Therefore,exploring the response mechanism of forage to drought and rewatering is important for ecosystem stability and vegetation recovery.This study selected typical forage?sainfoin?Onobrychis viciifolia Scop.?,erect milkvetch?Astragalus adsurgens Pall.?,white sweetclover?Melilotus albus Medic.ex Desr.?,drooping wildryegrassand?Elymus nutans Griseb.?and annual ryegrass?Lolium multiflorum Lam.??in the semi-arid region as the subject.We conducted the continuous drought experiment for sainfoin,erect milkvetch,drooping wildryegrassand and annual ryegrass in 2015.The leaves gas exchanges parameters were monitored in the experiment period.The results in the study could provide important information in determine the zonal soil water availability?SAW?.Experiments were designed to progressive drought and rewatering for sainfoin,white sweetclover,drooping wildryegrassand and annual ryegrass in 2016.We studied the response of forage to soil water variation in growth,leaf water,gas exchanges and chlorophyll fluorescence parameters.The main conclusions of the research are as follows:?1?There was obvious critical effect of photosynthetic parameters?net photosynthetic rate?P_n?,transpiration rate?T_r?,water use efficiency?WUE?,stomatal conductance?g_s?,intercellular CO₂ concentration?C_i?,stomatal limitation value?L_s??on soil water in sainfoin,erect milkvetch,drooping wildryegrassand and annual ryegrass.Based on the response of photosynthetic physiological,for sierozem,the threshold range as the lower limit of SAW on sainfoin,erect milkvetch,drooping wildryegrassand and annual ryegrass was 11.7%-20.2%,15.5%-25.5%,17.2%-27.2%and 24.6%-35.8%,respectively,which express by relative soil water content?RWC?.And the optimal threshold range as SAW on sainfoin,erect milkvetch,drooping wildryegrassand and annual ryegrass was 71.2%-82.8%,72.1%-84.2%,79.3%-85.1%and 76.7%-85.6%,respectively.?2?Under drought condition,the ground morphologic index?plant height,sngle leaf area,total shoot dry matter,root:shoot?of forage presented obvious downtrend,and the underground morphologic index?total root dry matter?was increased.Leaf water physiological indexes?leaf water potential?LWP?and leaf relative water content?LRWC??was decline sharply.P_n,T_r,g_s,WUE of sainfoin and white sweetclover showed a downward trend after the first rise,and that of drooping wildryegrassand and annual ryegrass were showed a continued steady decline trend.Maximal fluorescence?F_m?,maximum photochemical efficiency of photosystem II?F_v/F_m?,potential activity of photosystem II?F_v/F_o?,effective quantum yield of photosystem II?Yield?and photochemical quenching coefficient?qP?were in decline.The changing tendency of initial fluorescence?F_o?and non-photochemical quenching coefficient?NPQ?were showed a contrary tendency.?3?After rewatering the physiological indexes of each forage such as photosynthesis and chlorophyll fluorescenceall returned to normal even appeared a certain degree of super compensation.While there have the difference in needed time for returning to normal,and leguminous grasses?sainfoin and white sweetclover?need 3-4 weeks and gramineae grasses?drooping wildryegrassand and annual ryegrass?need 1-2 week for recovery.?4?Under natural drought,four forage on sorting by drought resistance:sainfoin>white sweetclover>drooping wildryegrass>annual ryegrass.The drought adaptation strategy of sainfoin and white sweetclover belong to the high water potential delay dehydration type and that of drooping wildryegrass andannual ryegrass belong to low water potential to endure dehydration type.