Kaolin Spray Improves Growth, Physiological Functions, Yield, and Nut Quality of 'Tardy Nonpareil' Almond Under Deficit Irrigation Regimens

This research was planned to investigate the interaction of deficit irrigation (50% of full crop water needs representing severe water stress, 75% of full water needs representing mild water stress, and 100% as control) and kaolin foliar spray (0% [control], 3%, and 6%) on shoot growth, tree yield, tree physiology, and nut quality of 'Tardy Nonpareil' almond over two consecutive growing seasons. Results indicated that deficit irrigation, particularly at 50% of full irrigation, negatively affected shoot growth, yield (much more severely in the second season), kernel weight, photosynthesis rate, and stomatal conductance, as well as leaf and kernel nutrient contents. Kaolin spray improved shoot growth up to 14.7%, 31.1%, and 23.1%, respectively, for the 100%, 75%, and 50% irrigation regimens. Kaolin foliar application also prevented premature leaf abscission in trees under water stress (by 5% and 9%, respectively, for mild and severe water stress). Water use efficiency and tree yield were significantly enhanced in both years in trees receiving 75% of full irrigation and spraying with kaolin; however, kaolin treatment failed to improve yield or water use efficiency in severely stressed trees. Foliar spray of kaolin reduced leaf temperature by up to 3.25 degrees C in both deficit irrigation regimens, and it increased the photosynthesis rate and stomata conductance in trees receiving mild deficit irrigation. The highest and lowest photosynthesis rates (12.34 and 6.56 mu mol m(-2) s(-1), respectively) were obtained with 75% irrigation plus 6% kaolin and with 50% irrigation without kaolin, respectively. Overall, foliar kaolin spray efficiently improved shoot growth, tree yield, nut attributes, and physiological parameters in trees receiving 75% of full irrigation (with no significant difference between the 3% and 6% concentrations in many cases), but it failed to mitigate adverse effects of severe water deficit.