Positive involvement of HCO<sub>3</sub><sup>–</sup> in modulation of melon resistance to powdery mildew

Inorganic salts such as KHCO3 are considered as potential powerful weapons for protecting plants from disease challenges, while their effects remain largely unknown on melon (Cucumis melo L.) resistance to powdery mildew caused by Podosphaera xanthii. In this study, the alleviatory effects of KHCO3 were physiologically explored on P. xanthii-infected seedlings of 'Yangjiaomi', an agrestis melon cultivar being susceptible to powdery mildew, by exogenous spraying at 7 d after inoculation (DAI) and 2 d before inoculation (DBI), respectively. The significantly improved resistance to P. xanthii was observed in melon seedlings sprayed with 2.5 g·L−1 KHCO3 (pH 8.54). Further investigation showed that the activities of PAL and PPO, as well as the accumulation of resistance-benefiting secondary metabolites, were stimulated by KHCO3 treatments. Meanwhile, the activities of SOD, POD and CAT were significantly increased, and the contents of O2.−, H2O2 and MDA were dramatically lowered in the KHCO3-sprayed seedlings than those in the H2O-sprayed seedlings. Another four treatments [H2O, KOH (pH 8.54), 1.86g·L−1 KCl and 2.5g·L−1 KHCO3 (pH 8.54)] were carried out for melon seedlings at 2 DBI. The significantly decreased disease index and the stimulated ROS and phenylpropanoid metabolic pathways were only observed for the KHCO3-sprayed group, suggesting the crucial roles of HCO3− involved in the protection of melon seedlings from powdery mildew. Collectively, our results provide new physiological insights into inorganic salt-mediated plant protection and could benefit the green production of melon in the future.