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

Abstract

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 (<italic>Cucumis melo</italic> L.) resistance to powdery mildew caused by <italic>Podosphaera xanthii</italic>. In this study, the alleviatory effects of KHCO₃ were physiologically explored on <italic>P. xanthii</italic>-infected seedlings of 'Yangjiaomi', an <italic>agrestis</italic> 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 <italic>P. xanthii</italic> 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.