Comparative effects of kaolin and calcium carbonate on apple fruit surface temperature and leaf net CO₂ assimilation

Abstract: The use of reflective particles on apple fruits has been suggested as a tool to diminish its thermal charge and thus mitigate stress effects caused by high temperature. The products effectiveness is often expressed in terms of damaged fruit, however it is influenced by the sensitivity of the variety, growing conditions and application method. Therefore, it is necessary to quantify the temperature of the fruits surface (FST) achieved according to the residue deposited to determine the degree of thermal protection for each product. Moreover, the residue deposited in the canopy enhances the albedo on the leaves reduces the availability of incidental light. The goal of this work was to evaluate the efficiency of reflective particles in the reduction of superficial temperature of the fruits and its effect on net CO₂ assimilation rate (A_CO2) in apple trees (Malus domestica, Borkh). The fruits were treated with: one, two and four (1X; 2X and 4X) applications of kaolin (treatment K) or calcium carbonate (treatment C) at 2.5% P/V and untreated fruit as control. The residue effect on A_CO2 was evaluated in individual leaves at 2X concentration. Both products showed a thermic protective effect as compared with control. The protection degree depended upon the concentration. The highest temperature of the control was 49.8 ºC and in these conditions kaolin was significantly more effective than carbonate, the thermic reduction was 1.9 ºC vs. 1.3 ºC at 2X and 2.5 ºC vs 2.1 ºC at 4X for kaolin and carbonate, respectively. At 1X there were no statistical differences between products. In turn A_CO2 is only negatively affected under low intensities of light (< 700 mmoles m² s^-1 of PAR). Higher radiation levels compensate the shading effect over leaves and also the maximun A_CO2 (A_max) was not affected.