Protetores solares para minimizar os efeitos dos estresses climáticos sobre a fotossíntese em citros

Abstract

Extreme weather episodes are becoming more and more common, and are characterized by high light radiation and high air temperature, resulting in stress and a large impact on citrus development, quality and production. Citriculture is one of the Brazilian activities with the most relevant segments in world agribusiness and presents expressive numbers that reflect the great economic and social importance throughout the country's economy. Under stress conditions, plants have limited metabolic activities, causing damage to flowering and fruiting of plants due to increased production of reactive oxygen species (ROS) that damage metabolism and plant structure. The foliar application of products that act similarly to a sunscreen can prevent photo-oxidative damage, as the reduction in leaf temperature allows for better stomatal opening and, consequently, an increase in CO2 assimilation. The present study aims to evaluate the efficiency of using sunscreens to reduce the effects of stress on photosynthesis in citrus. The experiment was conducted in a citrus orchard of sweet orange plants cv. 'Valencia' [Citrus sinensis (L.) Osb.] grafted on 'Swingle' citrumelo [C. paradise Macf. x Poncirus trifoliata (L.) Raf.] at the Sylvio Moreira Citriculture Center (IAC), municipality of Cordeirópolis, SP. The treatments were, T1: control, T2: powdered kaolin [wettable powder 1 µm (30 g L-1)] and T3: powdered calcium oxide-hydroxide carbonate [wettable powder, <1 µm (30 g L-1)], with six repetitions per treatment . The following variables were evaluated at 9:00 am and 12:00 pm: leaf temperature with the help of a thermometer (Fluke 59 Max, Fluke Co., Everett, USA), hydrogen peroxide and lipid peroxidation, gas exchange measurements and fluorescence of the chlorophyll a, with the help of IRGA (Li-6800, Inc. Lincoln, USA) thus defining stomatal conductance, transpiration, internal CO2 concentration, CO2 assimilation, rubisco carboxylation, electron transfer rate, photosystem II efficiency and the energy used for photochemistry. The results were analyzed using the Tukey test at 5% probability. There was no statistically significant difference for any of the variables at the time of 9:00 am, whereas for the time of 12:00pm there was a difference for the variables E, Anet, gsw and A/Ci and Fv/Fm with application of kaolin. There was also a difference in leaf temperature for plants sprayed with kaolin, which reduced leaf temperature by about 3°C and plants sprayed with calcium carbonate have a tendency to lower their temperature when compared to control plants. There was no production of hydrogen peroxide for any of the treatments and there was only an increase in MDA for treated plants with calcium carbonate. We can conclude that the foliar application of kaolin (30 g L-1) in 'Valencia' orange plants is able to change photosynthetic parameters, providing positive effects for gas exchange and reduction in leaf temperature.