Adaptation of the Euryhaline CharophyteLamprothamnium papulosumto Brackish and Freshwater: Turgor Pressure and Vacuolar Solute Concentrations During Steady-State Culture and After Hypo-osmotic Treatment

Abstract

The euryhaline charophyte Lamprothamnium papulosum (Wallr.) J. Gr. was adapted to media with decreasing salinities ranging from 550 to ∼0 mosmol kg−1. Vegetative plants grown in media with osmotic pressures (φ0) in the range of 550 to 130 mosmol kg−1 maintained a constant turgor pressure (Δφ) at 309 + 7 mosmol kg−1. The ions K+, Na+ and Cl−, were the predominant solutes in the vacuole. Changes in their concentrations account for the variation in internal osmotic pressure (φ1) with, φ0. The divalent ions Mg2+, Ca2+ and SO42- were also present in significant amounts, but their concentrations did not alter with changes in, φ0. In cells subjected to hypo-osmotic shock the regulation of Δφ was incomplete. The turgor pressure increased from 302 to 383 mosmol kg−1. The first rapid response to the sudden decrease in φ0 was a loss of K+ and Cl−. In contrast to the decrease in ionic concentrations an accumulation of sucrose occurred which could account for the increase of Δφ. The increase in sucrose concentration started 24 to 48 h after the downshock and reached its highest value after 3 to 4 weeks. The sucrose concentration in the vacuole was up to 320 mol m−3. During this time the ionic content continued to decrease but did not counterbalance the sucrose concentration sufficiently to regain the original Δφ. High sucrose levels accompanied by an enhanced Δφ were also observed during the period of fructification (sexual reproduction: formation of antheridia and oogonia) in Lamprothamnium kept under conditions of constant salinity. It is concluded that high sucrose content and elevated Δφ are characteristic of sexual reproduction in this charophyte. Lamprothamnium is able to tolerate different Δφ during various developmental stages (e.g. vegetative and reproductive phases).