Applications of UV-B lighting to enhance phenolic accumulation of sweet basil

Ultraviolet B (UV-B) radiation has been previously reported to induce changes in secondary metabolites such as phenolic compounds by improving, in particularly, the nutritional value of aromatic plants. This study aimed to evaluate the dynamic biological stress response of Ocimum basilicum L. plants exposed to different UV-B doses (8.5, 34, 68, 102 kJ m−2day−1), delivered using three different experimental approaches (acute, sub-acute and sub-chronic), on phenolic compounds accumulation and chlorophyll a fluorescence parameters. Sweet basil plants were grown using a novel “in vivo-vitro system” in a closed-type plant production system that allowed the control of UV-B dose easily and accurately by switching on and off the supplemental UV-B lamps. Results suggest that the dynamic accumulation of phenolic compounds as well as the ability to maintain the structural organization of PSII and thus the leaf functionality were dependent on the biologically effective UV-B dose applied and the exposure-history of the plants. UV-B doses continuously (as a single exposure) applied significantly increased phenolics, but severely affected photosynthetic efficiency. Discontinuous UV-B-exposures either applied within a single day (sub-acute) or within 6 days (sub-chronic) yielded in general at the same dose, slightly amount of total phenolics but showed an improved potential to process light energy through PSII compared to continuous UV-B application. Our data suggest that an improved accumulation of phenolic compounds can be achieved when UV-B radiation is applied for long period, within six days, discontinuously, without affecting the efficiency of photosynthetic apparatus.