Leaf Water Relations in Lime Trees Grown under Shade Netting and Open-Air

Physiological plant water status indicators are useful for managing precision irrigation in regions with limited water resources. The aim of this work was to evaluate the effect of shade netting on the diurnal and seasonal variations of several plant water status indicators in young lime trees (Citrus latifolia Tan., cv. Bearss), grown at the CEBAS-CSIC experimental station in Murcia, Spain. Stem water potential (Ψ(stem)), leaf gas exchange (net photosynthesis (P(n)) and stomatal conductance (g(s))), and canopy temperature (T(c)) were measured on representative days of winter and summer. The Ψ(stem) daily pattern was quite similar in both seasons under both conditions. However, the circadian rhythm of leaf gas exchange was affected by shade conditions, especially in summer, when shaded leaves showed maximum g(s) values for a longer time, allowing higher net photosynthesis (37%). Canopy temperature behaved similarly in both conditions, nevertheless, lower values were recorded in open-air than in shaded trees in the two seasons. The canopy-to-air temperature difference (T(c) − T(a)), however, was lower in shaded trees during the daylight hours, indicating the higher degree of leaf cooling that was facilitated by high g(s) values. The possibility of continuously recording T(c) makes it (or the proposed canopy thermal index, CTI) a promising index for precise irrigation scheduling. Shade netting was seen to favour gas exchange, suggesting that it may be considered alternative to open-air for use in semi-arid areas threatened by climate change.