Foliar application of vanillic and p-hydroxybenzoic acids enhanced drought tolerance and formation of phytoalexin momilactones in rice

In this study, two rice cultivars including a drought tolerant (Q8) and a drought susceptible (Q2) were foliar applied with exogenous vanillic acid (VA) and p-hydroxybenzoic acid (PHBA) to examine their effectiveness on drought-tolerant levels and induction of pigments, antioxidants, phenolics, flavonoids, and phytoalexin momilactones A (MA) and B (MB). Generally, the tolerant level of Q2 was more accelerated than those of Q8. Total contents of phenolics, flavonoids, pigments, and antioxidant activity were positively promoted, although the difference between Q8 and Q2 was negligible. In the quantitative induction of phenolic acids, VA, PHBA, and VA+PHBA showed dose-dependent variable effects, of which Q2 was more influenced than Q8. In all treatments, PHBA appeared to have a more significant role toward drought tolerance than VA. Although MB was found only in non-treated Q8, treatments of VA+PHBA caused formation of both MA and MB, however the induced quantities of MA and MB varied among applied doses and rice cultivars. This research is the first to show that, besides increasing antioxidant activity and total pigments, phenolics, and flavonoids, application of VA and PHBA induced phytoalexins MA and MB to enhance rice drought tolerance, of which MB may play a greater role than MA.