Abiotic stresses sequentially applied enhance natural resistance and reduce postharvest decay

The feasibility to restrain citrus green mould (Penicillium digitatum Sacc.) during storage, by means of induced natural resistance, was investigated following combined heat and ultravioLet light C (254 nm, UV-C) treatments. Heat treatment (HT) was performed by keeping Citrus fruits in a humid saturated room at 36 degrees C for 36 h while, the UV-C light treatment (254 nm) was carried out by illuminating the fruits with 3 or 6 kJm(-2). Physical treatments were sequential and reversely applied to un-inoculated or to 24 h-old inoculated lemons or oranges. Then, fruits were stored for 30 days at 8 or 5 degrees C with 95% relative humidity (RH) followed by a 6 day simulated marketing period at 20 degrees C and 75% RH. Constitutive (cuticle and epicuticular wax) and induced resistance (scoparone biosynthesis) were monitored and the variation in natural resistance was correlated to the decay development. HT influenced positively the constitutive factors of resistance by reducing or delaying cuticular cracking and by remodelling the epicuticular wax layer. Following HT the synthesis of scoparone took place only in the albedo of wounds and was greater when fruits were inoculated. Following UV-C illumination, scoparone accumulated rapidly in the outer rind (flavedo) but, particularly with 6 kJm(-2), favoured cuticular fracturing during storage. Sequential treatments resulted significantly more effective in controlling decay on either, inoculated and non inoculated fruit. The best control of green mould during storage of lemons and oranges was obtained when HT preceded 6 kJm(-2) UV-C illumination with 52 and 45% reduction of natural decay in fruit stored at 8 degrees C compared to the control, respectively.