Mechanisms of maintenance of cytoplasmic osmotic homeostasis in bryophytes cells under salinity stress

The mechanisms of stabilization of cytoplasmic osmotic homeostasis in the bryophytes cells the participation of H+-ATPase and the intracellular ratio K+/Na+ were investigated as one of the effective means of control over absorption and compartmentation of excess sodium ions in plant cells under salinity at the territory of the tailings storage facility of Stebnyk Mining and Chemical Enterprise “Polymineral” . Samples of Barbula unguiculata Hedw., Didymodon rigidulus Hedw. and Brachythecium cam­pestre (Müll. Hal.) Schimp. mosses were taken from 2 experimental plots in the territory of the tailings storage facility, which significantly differed in substrate salinity levels. The highest activity of ATPase was detected in B. unguiculata and D. rigidulus plants, which grew on the substrate with very high salinity level. This indicates an important role of the plasma membrane H+-pump in the adaptation of bryophytes to salt stress. In B. cam­pestre shoots growing on the outskirts of the tailings storage with less salinity of the substrate, the enzymatic activity was found to be lower. It has been shown that the H+-ATPase activity depends on changes of the structural state of membranes induced by lipoperoxidation processes under stress conditions. The salt-tolerant B. unguiculata and D. rigidulus moss species have a lower content of TBA-active products, which positively affects on the functioning of membrane enzymes and leads to the development of general protective stress reaction. It has been shown that increasing of the H+-ATPase activity in moss cells under salinity conditions provides a removal of excess Na+ ions to outside, that was confirmed by the results of our investigations of cellular localization of sodium. The sequential elution technique for determining cellular localization of cations developed for bryophytes by D. Brown and J. Walls (Brown, Wells, 1995) was used. It made possible to determine the ratio between the amount of sodium and potassium cations localized on the extracellular sites and in the intracellular matrix. In shoots of the studied moss species, a correlation between ATPase activity and the localization of sodium ions on the extracellular sites was found. This indicates an important role of H+-ATPase in eliminating of the excess cytotoxic sodium ions from moss cells under salt stress conditions. It was showed that potassium ions are important osmolytes in the bryophytes cells since under saline stress, an increase in content of potassium and K+/Na+ correlation was observed in the intracellular matrix, which improves the plants tolerance to the influence of excess Na+ ions