Preliminary evaluation of the impact of preharvest UV-C on lettuce: potential for the control of Xanthomonas campestris

Two lettuce (Lactuca sativa) cultivars, the susceptible 'Chief' and the tolerant 'Little Gem', were used to study the effect of ultraviolet-C (UV-C) light on bacterial leaf spot (BLS) development. Potted plants were irradiated every 48 h with UV-C to obtain a cumulative dose of 1.6 kJ m^‑2 over a period of 8 d. Inoculation with Xanthomonas campestris pv. vitians was performed 2 d after the last UV-C treatment. Then, 14 d after inoculation, the severity of BLS was assessed on the lettuce seedlings using a six-point scale (0-6). Lettucenin, a known antimicrobial molecule of lettuce, was extracted from controlled and treated tissues and assayed by ultra-performance liquid chromatography. For the susceptible cultivar 'Chief', the severity of BLS was markedly reduced on the UV-C-treated plants in comparison with the non-UV-C-treated plants. The BLS severity scores for 'Chief' were 2.66 and 5.16 in the UV-C-treated plants and the controls, respectively. Ultraviolet-C treatment did not significantly affect the BLS severity index (<2) in the tolerant cultivar 'Little Gem'. The lettucenin A content in the leaves was significantly higher in the tolerant cultivar than in the susceptible cultivar. Ultrastructural changes induced by UV-C on the surface of lettuce leaves were observed by scanning electron microscopy. These observations revealed the occlusion of the stomata of UV-C-treated leaves by a wax-like matrix that remains to be fully characterized. This occlusion seemed partial in the susceptible cultivar and more complete in the tolerant cultivar. The results of this preliminary work suggest a potential antibacterial role of lettucenin in the natural tolerance of lettuce against X. campestris. However, the beneficial effect of UV-C treatment on BLS severity seems to be related at least in part to ultrastructural modifications that restrict the access of the pathogen to the stomatal cavity.