Kaolin treatments on Tonda Giffoni hazelnut (Corylus avellana L.) for the control of heat stress damages

The aim of the study was to verify if a new kaolin formulation, which contains 100% aluminium silicate, can reduce the negative effects of high summer temperatures and heat stress on canopy physiological processes, yield and fruit quality in hazelnut (Corylus avellana L.). Kaolin was applied on young Tonda Giffoni plants three times in 2018 summer at 3% concentration: sprays were carried out when the maximum air temperature exceeded 35 °C for 2–3 consecutive days. Kaolin coating increased light albedo, reducing leaf temperature from 2.9 °C up to 6.9 °C and the consequent heat stress, without significant negative effects on photosynthetic activity (Pn), stomatal conductance (gs) and water use efficiency (WUE). Occasionally kaolin significantly reduced leaf transpiration (E). Under water shortage stomatal resistance increased, independent of leaf temperature, therefore in kaolin treated plants (KL plants) the Pn, gs, E and WUE were less than those in control, whereas in post-harvest period Pn and WUE were significantly higher. At re - watering the KL plants have greater Pn and WUE values and less water loss due to transpiration. Moreover, summer rainfall is not able to wash out the whole kaolin covering, allowing an average foliar coverage of the kaolin sufficient to protect the photochemical efficiency of PSII (Fv/Fm). At harvest, the KL plants showed a higher kernel yield and oil content. Kaolin application is a viable strategy to contrast the increase of anomalous high temperature due to actual global warming and can allow to extend hazelnut growing in areas where summer heat stress are limiting.