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

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 average Ψstem 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 the Ψstem:Ψsoil gradient and volumetric soil water content (VWC, %). The Ψstem was more strongly correlated with Ψsoil than Ψair. Moreover, the sensitivity of Ψstem to Ψsoil decreased when Ψsoil 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 Ψair /Ψstem and Ψsoil as Ψsoil decreased. There was a threshold effect for the relationship between Ψair and Ψstem. That is, the Ψstem in a day increased with the increasing of Ψair before the latter reached -69 MPa, after which the Ψstem 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.

Response of sap flow rate of apple trees to environmental factors in Loess Platea of Western Shanxi Province, China.

To clarify the effects of environmental factors on transpiration process of apple trees in rain-fed ecosystem, the dynamics of sap flow in apple trees from Loess Plateau area in western Shanxi Province of China were monitored using the thermal diffusion technique. Meanwhile, environmental factors including meteorological elements and soil moisture content were measured. The results showed that both net solar radiation (Rn) and atmospheric vapor deficit (VPD) were closely correlated with sap flow rate (Js), as the main ones among all the measured environmental factors. At both hourly and daily scales, the cumulative variances of the three principal components of the environmental factor were above 86%. The first principal component contained VPD and Rn, and explained the variance of over 52% (at hour scale) and 63% (at daily scale), which could be classified as the evaporation demand index (EDI) and as the key comprehensive environmental varia-bles affecting tree sap flow. The second principal component mainly included soil moisture content (SWC) and other factors, which were referred to the soil water and heat supply index. The third principal component mainly included wind speed and other factors, and could be classified into the hydrothermal dissipation index. At the scale of hourly or daily, the response of Js to EDI showed a significant exponential growth relationship. At the hourly time scale, Js of apple trees could be accurately modelled based on the first principal component EDI (R2=0.72). At the daily scale, Js of apple trees could be better modelled based on potential evapotranspiration (ET0) (R2=0.88). Our results were of great significance for clarifying the responses of water transport in apple tree to environmental factors, estimating water consumption of apple tree based on meteorological factors, as well as directing orchard water management.