Effects of enhanced UV radiation and water availability on performance, biomass production and photoprotective mechanisms of Laurus nobilis seedlings

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Climate models predict an increase in ultraviolet (UV) radiation and a reduction in precipitation in the Mediterranean region in the coming decades. High levels of UV radiation and water shortage can both cause photo-oxidative stress in plants. The aim of this study was to investigate the effects of enhanced UV radiation and its interaction with low water availability on seedling performance, biomass production, and photoprotective mechanisms of the sclerophyllous evergreen species Laurus nobilis L. (laurel). To achieve this goal, one-year-old seedlings of L. nobilis were grown outdoors under three UV conditions (ambient UV, enhanced UV-A, and enhanced UV-A. +. UV-B) and under two watering regimes (watered to field capacity and reduced water supply). The results show that plants produced more biomass when exposed to above ambient levels of UV-A or UV-A. +. UV-B radiation, especially under low water availability. This was probably related to a UV-induced increase in leaf relative water content and in leaf water use efficiency under water shortage. Even though our results suggest that UV-A supplementation may play an important role in the stimulation of biomass production, plants grown under enhanced UV-A plots showed higher levels of energy dissipation as heat (measured as NPQ) and a higher de-epoxidation state of the violaxanthin cycle. This suggests a greater excess of light energy under UV-A supplementation, in accordance with the observed reduction in the foliar content of light-absorbing pigments in these plants. Strikingly, the addition of UV-B radiation mitigated these effects. In conclusion, UV enhancement might benefit water status and growth of L. nobilis seedlings, especially under low water availability. The results also indicate the activation of different plant response mechanisms to UV-A and UV-B radiation, which would interact to produce the overall plant response ​
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