The effect of low temperature (LT, 10°C) on modification of plasma membrane (PM) H(+)-ATPase (EC 3.6.3.14) activity in cucumber roots was studied. Plants were grown under LT for 3 or 6 days. Some of the plants after 3 days exposure to LT were transferred to control conditions for another 3 days (post-cold, PC). The activity of PM-H(+)-ATPase was decreased in plants treated for 3 days with LT. However, the activity of PM-H(+)-ATPase was higher in plants treated with LT for a longer time and in PC plants as well. Estimation of transcript levels of cucumber PM-H(+)-ATPase in roots indicates that the action of LT involves the gene expression level. The level of PM-H(+)-ATPase mRNA was markedly decreased in roots exposed to LT for 3 days. Moreover, the increased H(+)-ATPase activity in PM isolated from plants treated for 6 days with LT and from PC plants was positively correlated with higher levels of CsHA transcripts. Western blot analysis with an anti-phosphothreonine antibody showed that modification of the activity of PM-H(+)-ATPase under LT stress did not result from phosphorylation/dephosphorylation of the enzyme protein. However, the stimulation of PM-H(+)-ATPase activity in the case of PC plants could partially have emanated from increased activity of PM NAD(P)H oxidoreductase. In addition, modification of the transcript level of proton pump genes could have resulted from the action of H(2)O(2). In PC plants, an increase in H(2)O(2) level was observed. Moreover, treatment of plants with H(2)O(2) induced expression of PM H(+)-ATPase genes.