Exogenous foliar application of fulvic acid alleviate cadmium toxicity in lettuce (Lactuca sativa L.)

It was reported that fulvic acid (FA) has a positive effect on enhancing plant tolerance to various environmental stresses, including salinity stress and drought stress and so on. However, there is little study regarding the effects of FA on plants in response to heavy metal stress. Hence, the objective of this study was to investigate the potential effects of fulvic acid (FA) on cadmium (Cd) toxicity alleviation in lettuce seedlings. Our results showed that application of 0.5 g/L FA significantly mitigate Cd-induced toxic symptoms in lettuce seedlings. Cd stress triggered plant growth inhibition, photosynthetic pigment reduction, destruction of the photosynthesis apparatus, reactive oxygen species (ROS) accumulation, and nutrient elemental imbalance. We observed that FA promoted the growth in lettuce under Cd stress, mainly reflected in those alterations that the increase of biomass, chlorophyll content and photosynthesis capacity and reduction of the Cd content and lipid peroxidation in plant tissue. Foliar spraying of FA significantly alleviated these detrimental symptoms and facilitated nutrient element translocation from root to shoot, particularly the absorption of elements involved in photosynthesis, including iron (Fe), zinc (Zn), and manganese (Mn). In summary, foliar application of FA conferred Cd toxicity tolerance to lettuce by increasing ROS-scavenging capacity, inhibiting Cd uptake and the transport of elemental nutrients to shoots, which in turn protected the photosynthetic apparatus and promoted plant growth.