The interaction effect of chromium and fulvic acid on the morphological and physiological traits of lettuce (Lactuca sativa L.) in hydroponic condition

In higher plants, chromium (Cr) is not essential to plant growth and function. Exposure to Cr may cause tissue necrosis and limit chlorophyll production. In particular, Cr is usually involved in electron transfer and induce production of reactive oxygen species (ROS) (e.g., hydroxyl radicals and superoxide radicals), resulting in oxidative stresses and damages to plant cells and tissues. Humic acid (HA) and fulvic acid (FA) are the major component of humic substances. FA has low molecular weight and contains more oxygen-rich and carbon-poor functional groups compared with HA. It has been reported that soil and foliar applied FA increased the intensification of seedling growth, seed germination energy, and root weight of wheat plants. However, the aim of this research was to investigate the effect of different chromium concentrations on growth and biochemical parameters and stress indices in lettuce fed with fulvic acid, as a stress modifier in hydroponic system.

In order to investigate effects of fulvic acid application on morphological and physiological and biochemical under chromium stress in red leaf lettuce (Lactuca sativa L.), a pot experiment was conducted as factorial based on completely randomized design with four replications under hydroponic culture in university of Mohaghegh Ardabili in 2021. Experimental treatments were consisted of chromium stress at three levels (0, 3 and 15 mg l-1) and fulvic acid application at two levels (0 and 50 mg l-1). In this research, morphological traits including, shoot and root fresh weight, root length, root to shoot ratio and physiological traits such as total chlorophyll and carotenoid, leaf chromium content, soluble protein, malondialdehyde, relative water content, catalase, hydrogen peroxide were evaluated. 

The results revealed that Application of fulvic acid in plants grown under chromium stress caused an increase in root length and soluble protein compared to plants grown under chromium stress alone. While the addition of fulvic acid to the nutrient solution in chromium stressed plants showed a significant decrease in leaf chromium content, membrane peroxidation and hydrogen peroxide compared with the plants grown under Cr stress. The application of chromium at a concentration of 15 mg L-1compared to the control treatment reduced the root and shoot fresh weight, total chlorophyll and carotenoid and catalase by 20.5, 21.7, 22.3, 19.5 and 26%, respectively. Plants fed with 50 mg L-1 fulvic acid compared to the control treatment, root and shoot fresh weight, root to shoot ratio, total chlorophyll and carotenoid water relative content increased by 16.5, 15.5, 103 and 37%, respectively.

The addition of fulvic acid improved morphology and physiological traits, and abiotic stress indicators in lettuce under Cr stress. Result showed that the alleviation of Cr toxicity might be associated with decreased Cr uptake in roots leading to reduced Cr translocation in the shoot. Further, fulvic acid addition decreased membrane stability and hydrogen peroxide in response to Cr stress.