Examining Organic Acid Root Exudate Content and Function for Leafy Vegetables Under Water-Stressed Conditions

At the plants’ exposal to abiotic stress, organic acids, including citric acid, are exuded through their roots. Previous studies have suggested that the exogenous application of citric acid increases antioxidant activity within the plant. Thus, we postulated that organic acids released into the surroundings during times of environmental stress may function as signaling molecules to increase antioxidant enzyme activity. To gain further insight into this phenomenon, we identified individual organic acids exuded from the roots of leafy vegetables under drought stress. We then analyzed enzyme activity and the root/shoot lengths of seedlings after treatment with the types of organic acids found to be exuded from the studied leafy vegetables, including acetic, citric, lactic, and tartaric acids. There was a significant increase in catalase and ascorbate peroxidase enzyme activity in Napa cabbage (Brassica rapa var. pekinensis) after exogenous citric acid application. Root lengths of cabbage (Brassica oleracea var. capitata) and Napa cabbage seedlings were significantly longer in citric and lactic acids pretreated seedlings compared to those of the control. The above results support the conclusion that exogenous application of citric acid alleviates drought stress. However, there is insufficient evidence to prove that organic acids act as signaling molecules to prime neighboring plants for upcoming stress.