Attenuation of cadmium induced oxidative stress in cucumber seedlings by modulating the photosynthesis and antioxidant machinery through foliar applied glutamic acid

In recent decades, adverse effects of unexpected contaminants on the quality of crops have threatened both the food security and human health. Vegetable production in heavy metal contaminated soils is a serious concern regarding the quality of food. Glutamic acid has been extensively studied as a stress-responsive antioxidant molecule and its function is involved in triggering plant growth during abiotic stress. Therefore, in the present study, the alleviating role of exogenously applied glutamic acid was examined in soil grown cucumbers (Cucumis sativus L.) under four levels of cadmium (0, 5, 10, and 20 mg Cd/kg) and two levels of glutamic acid (0, 10 mM). The results showed that the Cd stress reduced the plant growth and chlorophyll contents in the cucumbers. Significant decreases were more pronounced in the photosynthetic parameters under the Cd stress alone in both cultivars. However, pronounced deleterious effects were observed in the Ashly cultivar as compared to SSC-228 in respect to the plant growth and photosynthetic attributes. However, the exogenous treatment of glutamic acid significantly improved the plant growth and chlorophyll contents of plants under the Cd stress. The glutamic acid also decreased the Cd contents in the cucumber roots and leaves, and further decreased the reactive oxygen species (ROS) which were elevated by the high Cd concentrations. Interestingly, the antioxidant enzyme activities (SOD and POD) increased under the different elevated Cd levels in the leaves of the cucumber plants. However, the CAT and APX activities were reduced with an increasing Cd concentration in the soil in both cultivars. Meanwhile, the exogenously applied glutamic acid exhibited synergic effects and further activated the antioxidant enzyme activities in the cucumber leaves under the Cd stress. In this study, the SSC-228 cultivar was found to be more tolerant to Cd stress as compared to the Ashly cultivar. Furthermore, the findings of this study highlight that a glutamic acid application can play a significant role in enhancing the plant growth and stimulating the biochemical activities in cucumbers under Cd stress.