土壤水分对黄土区苹果园土壤-植物-大气连续体(SPAC)中水势梯度的影响

Effect of soil moisture on water potential gradients in the soil-plant-atmosphere continuum(SPAC)of apple orchards in the Loess Plateau,Northwest China

开展土壤-植物-大气连续体(SPAC)中水势变化的联动分析对于揭示植物水分状况对环境变化的响应机制有重要意义。在黄土高原苹果园对果树生长季大气水势(Ψair)、木质部水势(Ψstem)、土壤水势(Ψsoil)进行连续监测与分析。结果表明:苹果树在生长季的日平均Ψstem为-0.57 MPa,在-0.24^-2.0 MPa间变化。苹果树SPAC连续体中水势梯度(Ψsoil:Ψstem:Ψair)平均为1∶9.8∶1155,其中植-土界面的水势梯度(Ψstem:Ψsoil)与土壤体积含水率(VWC)间呈极显著的线性正相关关系,Ψstem与Ψsoil间呈良好的线性关系,且相关性强于Ψstem与Ψair间的相关性。当Ψsoil<-0.08 MPa(VWC=17%,约为田间持水量的0.56倍)时,Ψstem对Ψsoil变化响应的敏感性明显降低,气-植界面的水势梯度(Ψair:Ψstem)与Ψsoil间由无明显关系转变为紧密的线性相关关系(R2=0.93)。Ψair对Ψstem的驱动存在阈值效应,在Ψair降到-69 MPa前,Ψstem日变化幅度随Ψair增加而增加,之后呈下降趋势。土壤含水率降低引起苹果树水势、SPAC各界面上的水势梯度明显下降,且在土壤含水率降至约17%时存在阈值效应。研究结果为理解树木水分状况对土壤、大气干旱的响应机制提供了重要依据。

The variations of water potential gradients through the soil-plant-atmosphere continuum(SPAC)are of great significance to reveal the responses of plant water use to environmental changes.We conducted a continuous experiment to monitor the potentials in the near-canopy atmosphere(Ψair),soil(Ψsoil)and plant xylems(Ψstem)during the growing season in an apple orchard located in the Loess Plateau.The results showed that the averageΨstem during the growing season ranged from-0.24 to-2.0 MPa,with a mean value of-0.57 MPa.The average water potential gradient in soil-plant-atmosphere system was 1:9.8:1155(Ψsoil:Ψstem:Ψair).We found a significant positive linear correlation between theΨstem:Ψsoil gradient and volumetric soil water content(VWC,%).TheΨstem was more strongly correlated withΨsoil thanΨair.Moreover,the sensitivity ofΨstem toΨsoil decreased whenΨsoil was lower than-0.08 MPa which corresponded to VWC=17%,0.56 times of field capacity.This was reflected by the increased linearity betweenΨair/Ψstem andΨsoil asΨsoil decreased.There was a threshold effect for the relationship betweenΨair andΨstem.That is,theΨstem in a day increased with the increasing ofΨair before the latter reached-69 MPa,after which theΨstem decreased.The decline of soil water content caused an obvious decrease in water potential gradient through the SPAC system,and the threshold effect existed when VWC was below 17%.The results provide a basis for understanding the mechanisms of plant water in response to soil and atmospheric drought.