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

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(Ψ_air)、木质部水势(Ψ_stem)、土壤水势(Ψ_soil)进行连续监测与分析。结果表明: 苹果树在生长季的日平均Ψ_stem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(Ψ_soilΨ_stemΨ_air)平均为1∶9.8∶1155,其中植-土界面的水势梯度(Ψ_stemΨ_soil)与土壤体积含水率(VWC)间呈极显著的线性正相关关系,Ψ_stem与Ψ_soil间呈良好的线性关系,且相关性强于Ψ_stem与Ψ_air间的相关性。当Ψ_soil<-0.08 MPa(VWC=17%,约为田间持水量的0.56倍)时,Ψ_stem对Ψ_soil变化响应的敏感性明显降低,气-植界面的水势梯度(Ψ_airΨ_stem)与Ψ_soil间由无明显关系转变为紧密的线性相关关系(R²=0.93)。Ψ_air对Ψ_stem的驱动存在阈值效应,在Ψ_air降到-69 MPa前,Ψ_stem日变化幅度随Ψ_air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。

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

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 (Ψ_air), soil (Ψ_soil) and plant xylems (Ψ_stem) during the growing season in an apple orchard located in the Loess Plateau. The results showed that the average Ψ_stem 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 (Ψ_soil:Ψ_stem:Ψ_air). We found a significant positive linear correlation between the Ψ_stem:Ψ_soil gradient and volumetric soil water content (VWC, %). The Ψ_stem was more strongly correlated with Ψ_soil than Ψ_air. Moreover, the sensitivity of Ψ_stem to Ψ_soil decreased when Ψ_soil was lower than -0.08 MPa which corresponded to VWC=17%, 0.56 times of field capacity. This was reflected by the increased linearity between Ψ_air /Ψ_stem and Ψ_soil as Ψ_soil decreased. There was a threshold effect for the relationship between Ψ_air and Ψ_stem. That is, the Ψ_stem in a day increased with the increasing of Ψ_air before the latter reached -69 MPa, after which the Ψ_stem 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.