AbstractConcentrations of ions and sucrose in the vacuolar sap ofChara canescensgrowing in an oligohaline lake (1.5 ‰) were estimated over the main growth period of the plants. During fructification vacuolar sap contained a mean of 41 mol m−3(range 10.2–61.8) sucrose. The mean turgor pressure was 239 mosmol kg−1(range 219–264).In long‐ and short‐term experiments these plants were subjected to increasing salinities up to 22 ‰. When salinity was increased from 1.5 to 4.4 ‰ turgor pressure was restored to only 80 % of the initial value. This reduced level of turgor pressure was maintained up to a salinity of 22 ‰. The increase in vacuolar osmotic potential was due to the monovalent ions Na+, K+and Cl−. The relative amounts of Na+and K+participating in the regulation process were dependent on external salinity.The regulatory mechanisms observed in the brackish water speciesCh. canescensare compared with those reported from freshwater a