Physiological responses of young cacao trees to soil water deficiency as affected by Pb pollution and Fe availability

Abstract

Plants are subject to various abiotic stresses such as water shortage and exposure to heavy metals, such as Pb in soil. These types of stress trigger a series of plant responses, ranging from a decrease in leaf gas exchange, an increase in lipid peroxidation, and even changes in the chemical composition of leaves and roots. As an essential micronutrient, Fe is involved in several physiological and biochemical processes in plants, such as photosynthetic activity, directly participating in chlorophyll synthesis and electron transport, being necessary for the maintenance of the structure and functioning of chloroplasts. The main objective of this work was to evaluate the action of Fe in mitigating Pb toxicity in young plants of the CCN 51 cacao genotype, subjected to water deficit in the soil, through photosynthetic responses and analysis of the chemical composition of different parts of the plant. Plants were grown with Pb (2 mmol Pb kg-1 soil) and different equimolar doses of Pb+Fe in the soil (2+0.5, 2+1, 2+1.5 and 2+2 mmol kg−1 soil), maintaining constant the Pb dose, whose soil was subjected to water deficit, with gradual reduction of water content, or its moisture was maintained near to field capacity, making a total of 12 treatments, together with the control treatment (without addition of Pb and Fe in soil). It was observed that the greater Fe absorbing, by the root system, mitigated the Pb toxicity. Fe application, in adequate dose in soil (2 mmol Pb kg-1 soil + 0.5 mmol Fe kg−1 soil), mitigated the toxic effects caused by Pb and water deficit in the plants, through the greater Fe taking up and translocation for the shoot in Pb detriment. Greater Pb translocation and accumulation in the leaves caused damage to leaf gas exchange and to the accumulation of carbohydrates in leaves. Fe and Pb applied in soil competed with each other for root absorbing regardless of soil water conditions.