UV-B radiation modifies the acclimation processes to drought or cadmium in wheat

Under natural conditions plants are often subjected to multiple stress factors. The main aim of the present work was to reveal how UV-B radiation affects acclimation to other abiotic stressors. Wheat seedlings grown under normal light conditions or normal light supplemented with UV-B radiation were exposed to drought or Cd stress and were screened for changes in the contents of salicylic acid and its putative precursor ortho-hydroxy-cinnamic acid, and in the activity of the key synthesis enzyme, phenylalanine ammonia lyase. Certain other protective mechanisms, such as antioxidant enzyme activities and polyamines, were also investigated. PEG treatment under UV-B radiation did not cause wilting, but resulted in more pronounced salicylic acid accumulation, which may provide protection against drought stress in wheat plants. In contrast, the high level of salicylic acid accumulation in Cd-treated plants was not further enhanced by UV-B stress, but resulted in pronounced oxidative stress and the activation of antioxidant systems and polyamine synthesis. Changes in the levels of phenolic compounds are accompanied by increased phenylalanine ammonia lyase activity in the roots, but not in the leaves. The similar pattern observed for stress-induced changes in salicylic acid and ortho-hydroxy-cinnamic acid contents suggested that salicylic acid may play a decisive role via ortho-hydroxy-cinnamic acid. The results indicated that UV-B radiation might have either a positive or negative impact under the same conditions in wheat, depending on the type of secondary abiotic stress factor. The protective or damaging effects observed may be related to changes in the levels of phenolic compounds