Copper deficiency and toxicity in Citrus: integrating nutrient absorption and plant nutritional status

Abstract

Copper deficiency (Cu) has been observed in non-bearing citrus orchards established on low fertility soils and with reduced foliar applications of fungicides. In either no-bearing or bearing orchard trees, the opposite can occur, caused by the toxicity of the element due to excess applications of fungicides to control foliar and fruit diseases such as citrus canker. In this condition, the concentration of Cu in soil can increase and cause damage to the roots, which severity depends on the rootstock variety. It is noteworthy that the Cu demand of citrus is associated to nitrogen (N), being more pronounced with higher N fertilization. Moreover, the supply of calcium (Ca) can minimize effects of excess Cu in the plant. The objectives are to evaluate the absorption of Cu and the response of orange trees on three rootstocks in conditions of low, adequate and excessive metal availability in nutrient solution; study the interaction between Cu absorption and biochemical parameters with nutritional status of citrus based on the supply of N and Ca; evaluate the effects of foliar versus root uptake of Cu under appropriate conditions defined in the second objective; and study the redistribution of Cu from old tissues to new parts of the orange trees. Four studies interrelated will be conducted. The first (I) will evaluate the absorption and redistribution of Cu in orange trees on different rootstocks (Rangpur lime, Swingle citrumelo and Sunkimandarin) in response to the availability of Cu in nutrient solution (0, 2.0, 20.0 and 200.0 mmol/L). From this study, it will be selected a rootstock combination of interest for the development of other experiments. Experiments II and III will evaluate the absorption of Cu (0, 0.5, 2.0 and 8.0 g/plant) supplied via root and the interaction with N (20 and 60 g/plant) and Ca (0.5 and 30 g/plant) to the soil, respectively. The last experiment (IV) will be held concurrently with the previous two, first combining sources of the element [CuSO4 and Cu(OH)2] and the supply of foliar Cu (0, 0.5, 1.0 and 2.0 g/plant). Comparison of foliar and root absorption by plants will be conducted for selected treatments of experiments II and III with those of IV treatments receiving the same rates of Cu (0.5 to 2.0 g/plant). In all experiments, the effects of treatments will be evaluated based on plant growth, nutritional status and changes in the activity of enzymes related to the metabolism of Cu and stress caused by the deficiency or toxicity of the element. The research results will contribute to a better understanding of nutritional disorders caused by Cu in citrus plants, and consequently create subsidies to optimize production where Cu is a limiting factor, and consequently provide increase in agricultural efficiency and cost in citrus production. (AU)