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

Abstract

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.

Keywords: Loess Plateau; apple orchard; soil-plant-atmosphere continuum (SPAC); water potential gradient.