Effects of Enhanced UV-B Radiation and Drought Stress on Photosynthetic Performance of Lettuce (Lactuca sativa L. Romaine) Plants

Aims: The aim of the present study was to investigate some biochemical changes in field grown lettuce (Lactuca sativa L. cv Romaine) plants in terms of importance of the accumulation of anthocyanins, flavonoids and photosynthetic pigments as well as photosynthetic limitations which changed during exposure of plants to drought stress and UV-B radiation in order to circumvent the deleterious effects of these Stresses.
Place and Duration of Study: The experiment was conducted under filed conditions from November 2012 to January 2013, at the Agricultural Research Center, KAU.
Methodology: The experimental design was a factorial arrangement in randomized complete blocks with four replicates. The first factor was UV-B (300 nm). The second factor was irrigation regime (complete irrigation to field capacity and limited irrigation. Gas exchange measurements were carried out using a LI-6200 portable IRGA. Chlorophyll fluorescence of Fv/Fm was measured by PAM 2000 fluorometer. Biochemical analyses and antioxidant enzymes assays were performed according to the appropriate methods.
Results: Exposure of lettuce plants to enhanced UV-B radiation and drought stresses (DS) negatively and significantly affected the process of photosynthesis including CO2 assimilation (PN), stomatal conductance to water vapour (gs) and transpiration rate (E). However, the amplitude of the effects of both stressors was dependent on the interactions. This resulted in alleviation of the negative effect of drought on photosynthesis and transpiration by UV-B radiation as the water stress intensified. Intercellular CO2 (Ci) concentration was only reduced due to water stress compared to control plants. The maximum efficiency of photosystem II (Fv/Fm) was not affected by UV-B radiation stress but reduced by drought. There was an increase in the activities of some antioxidant due to both stresses when applied singly and in combination UV-B irradiation increased the contents of the UV-B absorbing compounds (carotenoids, soluble phenols, anthocyanins), while drought stress caused a notable increase in free proline content. Increase in content of Proline may be the drought-induced factor which plays a protective role in response to UV-B.
Conclusion: UV-B radiation provoked in general more severe damage, evaluated as changes in the amounts of stress markers, than DS, when applied separately. Under multiple stress conditions, each of the stress factors seems to bring out some adaptive effects to reduce the damage experienced by plants caused by the other one. DS can induce accumulation of UV-B absorbing compounds (flavonoids, carotenoids and soluble phenols), which is likely to offer some increased protection from UV-B.