Effect Of Soil Moisture On Photosynthetic Physiological And Biochemical Characterisitcs Of Three Afforestation Tree Species

Water is the main factor which limited plant growth, distribution and ecologicalrestoration. The main features of water resources is that the distribution is uneven in time andspace in our country. Especially in northern China semi-arid loess hilly area, water scarcity issevere and ecology is fragile. In this paper according to the main features of China’s arid andsemi-arid drought and water shortage, we used Prunus sibirica、Hippophae rhamnoides andPinus tabulaeformis as the test samples, using potted experiment and natural drought method,light response process and CO₂response process of gas exchange parameters were determined,as well as response process and change rules of eight physiological and biochemical indexeswere determined.Light response process of gas exchange parameters, such as net photosynthetic rate（Pn）、transpiration rate（Tr）、water use efficiency （WUE）、intercellular CO₂concentration（Ci）、stomatal conductance（Gs）in three tree species and the mechanisms of thephotosynthetic physiological light response process were revealed in the process of graduallyintensified soil drought, the soil moisture critical point of the photosynthetic characteristicparameters changed significantly and the quantitative relationship of thus change and the soilmoisture were clear, the soil moisture range of the three trees to maintain higher Pnand WUEwere determined.CO₂response process of the three trees and the response of characteristic parameters tosoil water and the mechanisms of the characteristic parameters changed significantly wererevealed, the quantitative relationship of photosynthetic CO₂response process and itscharacteristic parameters were determined in the process of gradually intensified soil drought.The light response process and CO₂response process and the photosyntheticphysiological parameters of the three tree species were fitted by rectangular hyperbolic model,non-rectangular hyperbolic model, exponential equation and modified rectangular hyperbolicmodel, the applicability of different models to fit the photosynthesis process of the three treespecies was discussed in different soil moisture conditions.The variation and the difference of eight physiological and biochemical indexes of thethree tree species were determined in the process of intensified soil drought, droughtresistance of the three tree species and the quantitative relationship of the physiological and biochemical indices and the soil moisture were explored. Through the study of the variationand response characteristics of the light response process and CO₂response process and thephysiological and biochemical indices in the three tree species, a theoretical basis could beprovided to understand the hydrological and ecological characteristics and drought resistancemechanism deeply, to guide species configuration, reasonable cultivationin and systemconstruction in arid areas; at the same time the test evidence could be provide to evaluate theapplicability of different photosynthetic models. The main results are as follows:（1）Leaf gas exchange parameters of the three tree species had obvious thresholdresponses to soil moisture and light intensityAccording to the analysis results of the relation between gas exchange parameters suchas net photosynthetic rate、transpiration rate、water use efficiency and the soil moisture,suitable soil moisture conditions, the biggest soil moisture deficit and suitable light intensityrange of the three tree species to improve photosynthetic productivity and water use efficiencywere clear initially.①The suitable soil moisture ranges for P.sibirica、H. rhamnoides、P. tabulaeformis were52.3%～84.8％、50.2%～84.6%、44.3%～83.6%, respectively. Suitable light intensity rangesfor them were500～1300、300～1500、300～1500μmol·m-2·s-1, respectively. The lower thebiggest soil moisture deficit value, the stronger drought tolerance ability under drought stress.From the biggest soil moisture deficit value we could see that their drought resistance fromhigh to low were in the order of P. tabulaeformis、H. rhamnoides、P. sibirica.②The apparent quantum yield（Φ）、dark respiration rate（Rd）and light compensationpoint（LCP）of the three species were affected significantly by soil moisture contents, they allhad clear threshold response characteristics to the soil moisture. The order from high to low ofthe ability to utilize low light of the three species were P. tabulaeformis、H. rhamnoides、P.sibirica.（2）Soil moisture significantly affected leaf photosynthetic CO₂response process and thephysiological parametersThe suitable soil moisture ranges of photosynthetic CO₂response for P. sibirica、H.rhamnoides、P. tabulaeformis were in the ranges of46.3%～81.9%、42.8%～83.5%and35.3%～84.2%, respectively. When RWC exceeded these ranges, photosynthetic capacity（Pnmax）、CE and CO₂saturation point（CSP）significantly reduced when the soil moistureincreased or reduced. The order of the ability to assimilate low CO₂of the three species from high to low were P. tabulaeformis、H. rhamnoides、P. sibirica.（3）Model simulation of photosynthetic light response process and CO₂response processand their characteristic parameters①Effect of light response data fitted by rectangular hyperbola model、non-rectangularhyperbola model、exponential equation and modified rectangular hyperbola model hadsignificant differences under different soil moisture conditions. The photosynthetic lightprocess and its characteristic parameters such as the apparent quantum yield（Φ）、darkrespiration rate（Rd）and light compensation point （LCP）were well fitted by the4modelswhen RWC of P. sibirica were from52.3%to84.8%、that of H. rhamnoides were from50.2%to84.6%、that of P. tabulaeformis were from44.3%to83.6%，and the values fitted bynon-rectangular hyperbola model were the most close to measured values. Only the modifiedrectangular hyperbola model could well fit the light response process and its characteristicparameters when RWC were ouside of the above ranges.②The photosynthetic CO₂response process and its characteristic parameters CE、CO₂compensation point （CCP） and light respiration rate （Rp） were well fitted by rectangularhyperbola model、non-rectangular hyperbola model、exponential equation and modifiedrectangular hyperbola model when RWC of P. sibirica were from46.3%to81.9%、that of H.rhamnoides were from42.8%to83.5%、that of P. tabulaeformis were from35.3%to84.2%，the fitting precision was that non-rectangular hyperbola model> modified rectangularhyperbola model> exponential equation> rectangular hyperbola mode. Only the modifiedrectangular hyperbola model could well fit the photosynthetic CO₂response process and itscharacteristic parameters when the soil moisture content exceeded the above ranges.（4）Changes of physiological and biochemical indexes in the three species and thequantitative relationship of the changes and the soil moisture under different soil moistureconditionsLeaf relative water content （LRWC） and the chlorophyll （Chl） content decreased whilethe cell membrane permeability increased gradually in the three tree species following therelative water content （RWC） of soil reduced. The changes of the tested responses in P.sibirica and H. rhamnoides were bigger than those in P. tabulaeformis．LRWC, Chl contentand the cell membrane relative permeability changed significantly when the soil RWC werelower than61.3%,52.3%, and54.3%for P. tabulaeformis, P. sibirica and H. rhamnoides,respectively．The content of malondidehyde （MDA） increased and then declined, the decrease of MDA content under light drought stress condition was related with the protective enzymeactivity increase, and the decrease under severe drought stress condition was related withMDA itself degradation．The activity of SOD and POD in the three tree species increased firstand then decreased following the soil water reduced．The response time and the defensive roleof SOD and POD to drought stress were different. SOD played an more important role in theearly drought stress and POD played a bigger role in the late drought stress when droughtstress was intensified．The soil RWC required to maintain higher enzyme activity were34.6%⁸5.6%,52.3%⁸7.2%and39.9%⁸7.7%for P. tabulaeformis, P. sibirica, and H.rhamnoides, respectively. The Pro content and the Ss content increased following the soilwater reduced, with the higher level in P. tabulaeformis. The Pro increased faster than Ss. Sowe conclude that the three tree species enhance their tolerance and adaptability to droughtstress by osmotic adjustment and protective enzyme system．The comprehensive evaluationwas made about the drought-resistant capability of the three tree species by using the subjectfunction method. The order was P. tabulaeformis＞H. rhamnoides＞P. sibirica．...