Aspectos fisiológicos e nutricionais em mudas de citros infectadas por HLB e submetidas a diferentes disponibilidades hídricas

Abstract

Citriculture contributes with the largest volume of Brazilian fruit production, obtained from the high productivity of the orchards, as a result of the adoption of technologies such as planting densification, soil fertilization, and irrigation. However, the increase in the incidence of phytosanitary problems, especially huanglongbing (HLB), puts pressure on production costs and leads to a reduction in the sustainability of the sector. The causal agent of HLB is a phloem-restricted bacterium (Candidatus Liberibacter) that impairs the absorption of water and nutrients. Despite the existence of recommendations aimed at reducing plant susceptibility to HLB with better irrigation and soil fertilization management, little is known about the relationship between water availability and physiological and nutritional responses in citrus plants infected with HLB. This study aims to understand the physiological, nutritional and biochemical changes observed in early stages of HLB infection in 'Valência' orange (C. sinensis) plants grafted onto 'Citrumelo Swingle' [Citrus paradisi X Poncirus trifoliata (L.)], subjected to different water supply conditions. The experiment was carried out in pots in a greenhouse and the experimental design used was completely randomized (DIC), in a split-plot scheme, with two infection conditions (HLB+ and HLB-) and two water supply conditions [full irrigation (FI) and deficit irrigation (DI)]. At 75 days after transplanting, the following parameters were evaluated: net photosynthetic rate, transpiration, stomatal conductance, intrinsic efficiency in water use, leaf area and mass, bacterial title in leaves and roots, starch and soluble sugar contents, contents of nutrients and dry mass of leaves, branches, and roots. The results were analyzed by Student's t test at 5% probability. Plants infected with HLB and subjected to irrigation deficit showed lower starch accumulation, higher content of soluble sugars in leaves, higher water use efficiency and higher nutrient content in the shoot. These responses in plants under water deficit were associated with a decrease in bacterial population in previously infected plant parts, as well as a deceleration of disease progress to new canopy parts that developed after infection. The decrease in the bacterial population under more restricted conditions of water supply is important information for future management practices with species that may be tolerant to HLB, given the series of physiological, biochemical and nutritional responses under irrigation deficit.