Nitric oxide regulates spearmint (Mentha spicata L.) responses to phenolic acids: growth, phenolics, and antioxidant capacity

Given the widespread recognition of the health risks and environmental damages caused by synthetic herbicides and fertilizers, it is more than ever necessary to unravel the mechanisms underlying plant responses to naturally derived bioactive substances. Phenolic acids constitute a group of aromatic compounds that, by showing both growth-promoting and allelopathic effects, have gained momentum over the past years. We aimed to investigate the effects of some common benzoic acid derivatives, namely vanillic acid (VA), p-hydroxybenzoic acid (pHBA), gallic acid (GA), and salicylic acid (SA), and the alleviative effects of nitric oxide (NO) on phenolic acid-induced inhibition in growth parameters in spearmint (Mentha spicata L.) shoot and underlying mechanisms. pHBA and VA at 0.5 mM were the most influential phenolic acids in the inhibition of growth and increasing the contents of malondialdehyde (by 119% and 83% respectively) and H2O2 (by 83% and 108% respectively) and decreasing chlorophyll content despite accelerating the activities of catalase and superoxide dismutase. In contrast, 0.5 mM SA and 0.05 mM GA significantly increased shoot fresh weight (by 85.1% and 48.1% respectively) and the accumulation of total phenolics (by 302% and 31% respectively) and anthocyanins (by 79% and 49% respectively) and stimulated phenylalanine ammonia lyase (PAL) activity (by 140.4% and 107.7% respectively). NO at 100 µM was highly effective in the alleviation of pHBA and VA-induced growth reduction and lipid peroxidation. In conclusion, NO improved the negative effects of pHBA and VA by increasing PAL activity and enhancing the contents of antioxidative secondary metabolites.