Effects of two reflective materials on gas exchange, yield, and fruit quality of sweet orange tree Citrus sinensis (L.) Osb.

This study was conducted in an orange orchard (Navelina ISA 315 cv) located in southern Italy. Leaf temperatures, Photosynthetic Photon Flux Density (PPFD), and UV-B reached high values during many days in summer, causing photosynthesis (An) in the outer layers of the canopy to decrease. This reduction was caused due to depression of stomatal conductance (gs), photoinhibition, and damage to the photosynthetic system. By contrast, An was lower in the inner layer because the PPFD was dramatically reduced from outer to inner layers of the canopy. Reflective materials, either kaolin or calcium carbonate, were sprayed onto the canopy of the trees every month from June to September. The behaviour of the treated trees was compared with that of the untreated trees. The light reflected from the kaolin (TK) and calcium carbonate-treated trees (TCC) reduced the leaf temperature to an optimal value for photosynthesis (below 30 °C), and increased the stomatal conductance (gs). Furthermore, treatments reducing PPFD played an important role in protecting the chlorophyll pigments and increasing the maximum efficiency of photosystem II (treated, ∼0.67; untreated, 0.48). Consequentially, in the outer layer of the canopy An was higher in treated trees (TK, 9.2 μmol CO2 m−2% s -1; TCC, 7.6 μmol CO2 m-2 s -1) than in the control trees (TCN, 5.6 μmol CO2 m-2s -1). In the inner canopy layer, the reflective action of kaolin and calcium carbonate improved light penetration and photosynthesis compared to the untreated trees. The total average yield per tree over two years was 15 % and 25 % higher in TCC and TK, respectively, compared with TCN. The fresh weight, total soluble solids, titratable acid, and the ratio between these quantities were better in the treated groups than in the control, whereas no differences were observed for nutraceutical parameters. Kaolin and calcium carbonate also improved the peel colour of the fruits produced by the treated trees. In conclusion, reflective materials improved the physiological, productive, and qualitative performance of orange trees, and kaolin was the more effective of the two reflective materials. Even considering the cost, the use of these reflective materials can be justified for citrus trees that yield high profits, such as the Navelina ISA 315 used in our experiment.