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 there was confirmed salt antifungal activity in the absence of direct contact with the pathogen, changes in enzyme activity and expression levels of chitinase, β-1,3-glucanase, peroxidase and phenylalanine ammonia lyase (PAL), and phytoalexin (scoparone, scopoletin, umbelliferone) and sugar (glucose, fructose, sucrose) contents in treated oranges were analyzed. Overall, sodium carbonate and bicarbonate increases the activity of β-1,3-glucanase, peroxidase, and PAL enzymes in orange tissues. Gene expression analyses confirmed PAL up-regulation particularly 12h after treatment application. HPLC analyses of peel extracts showed increased amounts of the sugars and phytoalexins, compared to control tissues, with sucrose and scoparone being the most represented. The results suggest that, although salts exert a direct antifungal effect on P. digitatum, they are also able to induce citrus fruit defence mechanisms to postharvest decay. The defence response seems correlated with the up-regulation of the phenylpropanoid pathway, which has a role in the adaptation to various stresses. This response could result in natural reaction to wounding and pathogen attack in citrus, enhancing its protective effect. As a consequence, the fruit might have a better chance of successful defence against the decay.