Phenolics Signatures in Response to Salinity Stress Provide Novel Insights into Physiological Basis of Salt Tolerance in Mango (Mangifera indica L.)

The effect of salt stress was evaluated on putative mutant seedlings of three polyembryonic mango genotypes viz., Bappakkai, Nekkare, and Kurukkan. Imposition of salinity stress resulted in a decrease in chlorophyll content, relative water content, and gas exchange parameters while enhancing the levels of stress markers like Na⁺/K⁺ ratio, total phenols, and proline. In total, thirteen phenolic acid compounds were identified including eight hydroxybenzoic acids and five hydroxycinnamic acids wherein hydroxybenzoic acid (majorly gallic acid) comprised more than 99% of total phenolic acids. In all the three genotypes, the concentration of protocatechuic acid, 2, 4-Dihydroxy benzoic acid, gallic acid, chlorogenic acid, and t-cinnamic acid increased with increasing level of salt stress indicating their potential role in mango salt tolerance. Bappakkai recorded higher K⁺, highest fold increase in proline content (+ 7.27 fold), highest percent increase in chlorogenic acid (+ 510%), protocatechuic acid (+ 750%), and ferulic acid along with lower Na⁺/K⁺ ratio and lower reduction in the levels of caffeic and sinapic acid at higher level of salt stress suggesting that putative mutants of Bappakkai were better at tolerating salt stress as compared to the other two genotypes. Exogenous application of ferulic acid (FA) to Nekkare putative mutants increased the activity of enzymatic antioxidants, superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX). Further, quantitative real-time PCR analysis also revealed higher expression levels of SOD, CAT, and APX in FA-treated Nekkare mutants under salinity stress. The results of this study besides identifying distinct phenolic acid signatures in response to stresses like gamma irradiation and salinity, also confirm the potential of physical mutagenesis in breeding salt-tolerant rootstocks in mango.