Hormonal responses to short-term and long-term water deficit in native Scots pine and Norway spruce trees

Water stress has a continuing major negative impact on global forest performance, which necessitates to understand how plants coordinate multiple physiological responses to adverse conditions. Therefore, we studied the effects of short-term and long-term water deficit on the biosynthesis and signalling of phytohormones, especially abscisic acid (ABA), in current-year needles of Scots pine and Norway spruce trees growing in a natural forest in the Bryansk region of Russia. In spruce, clear physiological effects of long-term water shortage were evident, whereas pine trees were affected more by short-term water deficit. However, both species presented similar pronounced increases in ABA content under long-term water deficit. Long-term ABA accumulation was not associated with changes in stomatal density or osmotic adjustment in either species. The dynamics of jasmonates and salicylic acid were similar; the contents of both decreased throughout the entire short-term water deficit period, and the contents were correlated with the decrease in phenolic compounds in the pine trees. The same decrease was observed for auxin, whereas cytokinins decreased only in more tolerant pine. Possible physiological reasons for the dynamics of plant hormone pools in trees under water deficit are discussed.