Pre-harvest hormetic doses of UV-C radiation can decrease susceptibility of lettuce leaves (Lactuca sativa L.) to Botrytis cinerea L.

Post-harvest applications of UV-C radiation have proven very efficient in reducing the development of post-harvest diseases in many species including lettuce (Lactuca sativa L.). Several studies suggest that UV-C radiation is effective not only because of its disinfecting effect but also because it may stimulate plant defenses. Pre-harvest treatment with UV-C radiation may thus offer an interesting potential for lettuce protection, provided that application doses are effective while excluding any harmful effects on the plants. Here we provide evidence that 0.85 kJ m−2 and 1.70 kJ m−2 represent doses of UV-C radiation that are not deleterious for lettuce plants. We used several criteria to evaluated the effect of UV-C radiation on the plant, including histological observations; the concentration of malondialdehyde, an indicator of membrane integrity, as well as parameters derived from measurements of chlorophyll fluorescence, such as maximal efficiency of photosystem II (Fv/Fm) and the Performance Index of Strasser. We observed that a single dose of 0.85 kJ m−2 slightly increased plant resistance to grey mould (Botrytis cinerea L.) while a single dose of 1.70 kJ m−2 had the opposite effect. When a 0.85 kJ m−2 dose was applied 4 times, at two-day intervals, there was an increase in the total phenol content of leaves, and in PAL, CAT, and MDAHR activities. Leaves inoculated 2 days after the latter UV-C treatment showed significantly decreased sensitivity (−30%) when compared to the control