Kaolin particle film alleviates adverse effects of light and heat stresses and improves nut and kernel quality in Persian walnut

Spraying with kaolin is becoming a feasible and cheap treatment to alleviate the destructive effects of light and heat stresses. Here, the effects of spraying with kaolin were investigated on leaf temperature, photosynthesis and gas exchange rate, sunburn and quality of nut and kernel, potassium (K) and sodium (Na) concentrations and proline content of the walnut. Three spraying levels of kaolin (0%, 3% and 6%) were used in 5 replicates during two consecutive years. Results indicate that the kaolin spray increases leaf area and chlorophyll content (up to 14.71% and 11.99%, respectively) comparing to the control. The severity of leaf, husk, and kernel sunburn of kaolin treated trees was reduced up to 50% comparing to the control plants. Considering physiological parameters kaolin treatment reduced leaf temperature (4.4 °C and 5.6 °C, respectively in kaolin 3% and 6% compared to the control), but increased photosynthesis rate (Amax), gas exchange (gs), internal CO2 partial pressure (Ci) and the water use efficiency (up to 29.85%, 30%, 10.97% and 19.88%, respectively) comparing to the control. Kaolin application also improved nut and kernel weight, kernel percentage and kernel oil content (up to 8.2%, 16.1%, 7.7% and 11.29%, respectively) comparing to the control. Kaolin treatments increased potassium level and K:Na ratio, but decreased Na and proline content, both in leaf and kernel, in comparison with the control. However, there was no significant difference between the results of applying kaolin at concentrations of 3% and 6% in almost all of the measured characteristics, but improved results were obtained with higher levels of kaolin, except for nuts and kernel quantities. Considering global warming, using kaolin application can be recommended as a suitable and cheap method for the acclimation of walnut to high temperatures and solar radiation.