Physiological Effects of Kaolin Applications in Well-irrigated and Water-stressed Walnut and Almond Trees

Abstract

• Background and Aims Kaolin applications have been used to mitigate the negative effects of water and heat stress on plant physiology and productivity with variable results, ranging from increased to decreased yields and photosynthetic rates. The mechanisms of action of kaolin applications are not clear: although the increased albedo reduces leaf temperature and the consequent heat stress, it also reduces the light available for photosynthesis, possibly offsetting benefits of lower temperature. The objective of this study was to investigate which of these effects are prevalent and under which conditions. • Methods A 6 % kaolin suspension was applied on well-irrigated and water-stressed walnut (Juglans regia) and almond (Prunus dulcis) trees. Water status (i.e. stem water potential, Ψ_s), gas exchange (i.e. light-saturated CO₂ assimilation rate, A_max; stomatal conductance, g_s), leaf temperature (T_l) and physiological relationships in treated and control trees were then measured and compared. • Key Results In both species, kaolin did not affect the daily course of Ψ_s whereas it reduced A_max by 1–4 μmol CO₂ m^–2 s^–1 throughout the day in all combinations of species and irrigation treatments. Kaolin did not reduce g_s in any situation. Consequently, intercellular CO₂ concentration (C_i) was always greater in treated trees than in controls, suggesting that the reduction of A_max with kaolin was not due to stomatal limitations. Kaolin reduced leaf temperature (T_l) by about 1–3 °C and leaf-to-air vapour pressure difference (VPD_l) by about 0·1–0·7 kPa. A_max was lower at all values of g_s, T_l and VPD_l in kaolin-treated trees. Kaolin affected the photosynthetic response to the photosynthetically active radiation (PAR) in almond leaves: kaolin-coated leaves had similar dark respiration rates and light-saturated photosynthesis, but a higher light compensation point and lower apparent quantum yield, while the photosynthetic light-response curve saturated at higher PAR. When these parameters were used to model the photosynthetic response curve to PAR, it was estimated that the kaolin film allowed 63 % of the incident PAR to reach the leaf. • Conclusions The main effect of kaolin application was the reduction, albeit minor, of photosynthesis, which appeared to be related to the shading of the leaves. The reduction in T_l and VPD_l with kaolin did not suffice to mitigate the adverse effects of heat and water stress on A_max.