Abstract: | The euryhaline charophyte Lamprothamnium papulosum (Wallr.)J. Gr. was adapted to media with decreasing salinities rangingfrom 550 to 0 mosmol kg1. Vegetative plants grown inmedia with osmotic pressures ( 0) in the range of 550 to 130mosmol kg1 maintained a constant turgor pressure (![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) ) at309 + 7 mosmol kg1. The ions K+, Na+ and Cl, werethe predominant solutes in the vacuole. Changes in their concentrationsaccount for the variation in internal osmotic pressure ( 1) with, 0. The divalent ions Mg2+, Ca2+ and were also present in significant amounts, but their concentrationsdid not alter with changes in, 0. In cells subjected to hypo-osmotic shock the regulation of ![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) was incomplete. The turgor pressure increased from 302 to 383mosmol kg1. The first rapid response to the sudden decreasein 0 was a loss of K+ and Cl. In contrast to the decreasein ionic concentrations an accumulation of sucrose occurredwhich could account for the increase of ![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) . The increase in sucroseconcentration started 24 to 48 h after the downshock and reachedits highest value after 3 to 4 weeks. The sucrose concentrationin the vacuole was up to 320 mol m3. During this timethe ionic content continued to decrease but did not counterbalancethe sucrose concentration sufficiently to regain the original![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) . High sucrose levels accompanied by an enhanced ![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) were also observedduring the period of fructification (sexual reproduction: formationof antheridia and oogonia) in Lamprothamnium kept under conditionsof constant salinity. It is concluded that high sucrose content and elevated ![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) arecharacteristic of sexual reproduction in this charophyte. Lamprothamniumis able to tolerate different ![{Delta}](http://jxb.oxfordjournals.org/math/Delta.gif) during various developmentalstages (e.g. vegetative and reproductive phases). Key words: Lamprothamnium papulosum, sucrose, turgor pressure |