Water status and thermal response of lime trees to irrigation and shade screen

Abstract

Most lime trees are cultivated under open-field conditions. However, in the Mediterranean area some growers use shade screen to protect their orchards against abiotic and biotic stresses, inducing changes in the plant’s surrounding microclimate that could affect its water status. This field experiment focuses on the effect of shade screen (high density polyethylene white net with 76% light transmission) and irrigation on plant and soil water status of Bearss lime trees. Two irrigation treatments were applied during the summer-autumn to open-field and shaded lime trees: irrigated (control) and non-irrigated (stress). Volumetric soil water content (θv), air temperature (Ta) and canopy temperature (Tc) were real-time monitored and several derived thermal indices were calculated. Stem water potential (Ψstem) and leaf gas exchange: net photosynthesis (Pn) and stomatal conductance (gs) were measured on representative days. Withholding irrigation caused a gradual decline in soil and plant water status. θv decreased from the outset in the non-irrigated treatments, showing breaking point connecting the rapid and the slower soil drying processes. Significant differences in Ψstem, and gs values were observed between controls and stressed trees 15 days after stress initiation. During early stages of water stress, lime trees cultivated in open-field showed lower values of soil and plant water status than those grown in shaded conditions. However, at the end of stress period, similar values were noted for both cropping systems, pointing to severe water deficit. Tc and Tc-Ta values reflected the current lime tree water status. The soil water content sensors anticipated the plant water stress situation respect to thermal ones. The higher θv, Ψstem, gs, Pn and Tc values registered in shaded lime trees revealed that shading allowed better soil-plant water status and a 12-day window of opportunity to respond to the imposed soil water deficits. Threshold thermal-based values are proposed for precise irrigation management of lime trees in open-field and shaded conditions.