Response of barley seedlings to water deficit and enhanced UV-B irradiation acting alone and in combination

Responses of barley seedlings to water deficit (WD) induced by polyethylene glycol (PEG 6000) and ultraviolet (UV-B; 280-320 nm) radiation and their interaction (UV-B + WD) were examined. A decrease in dry matter yield and water content of leaves and roots was observed following application of WD and UV-B + WD, while no changes were found after treating barley plants with UV-B. Proline content was increased in leaves under WD conditions and UV-B + WD. In contrast, UV-B treatment had no effect on the accumulation of proline in leaves of barley plants. Changes in root proline content showed a varied response: WD induced an increase in the level of this amino acid, while UV-B as well as UV-B + WD suppressed root proline content. The lipid peroxidation product malondialdehyde (MDA) was increased in leaves under WD and UV-B + WD stresses. Root MDA content increased in WD-stressed plants, but it decreased in the case of combined application of both stresses. The applied stress factors operated in a variable manner on phenylpropanoid metabolism. Phenylalanine ammonia-lyase (PAL) activity in leaves and roots was stimulated after exposure to WD and application of UV-B + WD stresses, while UV-B stress did not affect its activity. On the other hand, UV-B treatment enhanced the activity of 4:coumarate-CoA ligase (4CL) in leaves and this enhancement was positively correlated with the accumulation of anthocyanins and flavonols. However, the combined application of WD and UV-B reduced the positive effect of UV-B on the accumulation of these compounds and the activity of 4CL. Surprisingly, anthocyanins and flavonols were not detected in roots of examined barley seedlings despite increased 4CL activity. The results suggest that UV-B-induced activation of 4CL as well as accumulation of anthocyanin and flavonols in leaves is beneficial for the response to this stress factor. On the other hand, WD-induced reduction of the effect of UV-B on 4CL activity and the contents of anthocyanin and flavonol might be a cause of membrane damage in UV-B- and WD-stressed plants. In addition, conversely to what could be expected, the UV-B effect was perceived by the water-stressed roots, which exhibited reduced lipid peroxidation (MDA) and proline accumulation in WD-stressed plants exposed to UV-B.