Transpiration characteristics and environmental controls of orange orchards in the dry-hot valley region of southwest China

Even though the process of plant water use under individual environmental stress conditions (drought, heat, cold, salinity, etc.) has been widely studied, the processes through which plants cope with multiple stresses (such as water stress combined with hot temperature) remain inconclusive. In the dry-hot valley region of southwest China, soil moisture, temperature, radiation and the associated stresses make complex the process of plant transpiration. To determine environmental controls on the water use of a typical orange tree in the region, sap flow was monitored in the trunks of seven orange trees, meteorological factors and soil water content were recorded between 20 May 2020, and 31 May 2022. The seasons were classified into: rainy (June–October), dry (November–February the other year), and dry-hot (March–May). Results showed that the transpiration of orange trees (Tc) in the study area was 659 mm of water per year. Compared with rainy (2.04 mm d-1) and dry (2.14 mm d-1) seasons, Tc was significantly lower during the dry-hot season (1.38 mm d-1). Rainfall had a significant impact on the transpiration process. Compared with the pre-rainfall value, Tc decreased after light rainfall events (<10 mm), but increased after heavy rainfall events (>10 mm). The variation in Tc increased initially and later gradually decreased with increasing rainfall amount. The minimum rainfall threshold that triggered an increase in Tc was 10.9 mm. Compared with the pre-rainfall days, Tc decreased for almost all types of rainfall event on rainy days. The drop in Tc was driven by the volume, time, and duration of rainfall. There was no direct correlation between Tc and most environmental factors (R2 < 0.40). However, Tc/ET0 (ET0 is grass reference evapotranspiration) was strongly correlated with the environmental factors. Net radiation (R2 = 0.61–0.73), air temperature (R2 = 0.39–0.49), daily maximum air temperature (R2 = 0.49–0.73), vapor pressure deficit (R2 = 0.34–0.45), and ET0 (R2 = 0.64–0.74) were negatively logarithmically correlated with daily Tc/ET0. However, daily Tc/ET0 was positively logarithmically correlated with VWC/ET0; R2 = 0.60–0.83 (where VWC is soil water content) and VWC/Tamax; R2 = 0.32–0.56 (where Tamax is maximum air temperature). Among the studied environmental factors, VWC/ET0 had the highest correlation with Tc/ET0 (R2 as high as 0.83). These results were critical for accurate evaluation of the effects of interaction of water and heat on plant water use. The study is therefore useful for effective water management in arid and semiarid regions. This is particularly so when concurrent hot and dry conditions were considered, likely to occur under future climatic conditions.