Sodium carbonate and bicarbonate treatments induce resistance to postharvest green mould on citrus fruit

The aim of this study was to investigate the ability of two salts, sodium carbonate and bicarbonate, to activate defence mechanisms in citrus fruit against postharvest green mould caused by Penicillium digitatum. In particular, once confirmed salt antifungal activity in absence of direct contact with the pathogen, changes in enzymatic activity and expression level of chitinase, ß-1,3-glucanase, peroxidase and phenylalanine ammonia lyase (PAL), and, consequently, in phytoalexin (scoparone, scopoletin, umbelliferone) and sugar (glucose, fructose, sucrose) content in treated oranges were analyzed. Overall, sodium carbonate and bicarbonate proved to increase the activity of ß-1,3-glucanase, peroxidase, and PAL enzymes in orange tissues. Gene expression analyses confirmed PAL up-regulation particularly at 12 h after treatment application. Furthermore, HPLC analyses of peel extracts showed increased amounts of the tested sugars and phytoalexins, as compared to those found in control tissues, with sucrose and scoparone being the most represented. The obtained results suggest that, although salts exert a direct antifungal effect on P. digitatum, they are also able to induce citrus fruit defense mechanisms to postharvest decays. The defense response seems correlated with the up-regulation of phenylpropanoid pathway, which has a role in the adaptation to various stresses. This response could sum up to citrus natural reaction to wounding and pathogen attack, enhancing its protective effect. As a consequence, the fruit might have a better chance for a successful defence against the decay.