Diurnal and seasonal osmotic potential changes in Lotus creticus creticus plants grown under saline stress

Lotus creticus creticus plants growing in a greenhouse were exposed to 0, 70 and 140 mM NaCl for 4 months (September-December). Salinity caused a reduction in total dry weight of Lotus plants treated with 140 mM NaCl, whereas no significant effects on growth were observed with 70 mM NaCl. Predawn leaf water potential and predawn leaf osmotic potential showed constant values in control plants during all the experiment, whereas a decrease of both parameters was observed between September and October for the saline treatments. The relative contribution of passive (dehydration) versus active mechanisms (osmotic adjustment) involved in seasonal leaf osmotic potential changes were determined. Seasonal decreases of the osmotic potential at full turgor in the treated plants showed the capacity for osmotic adjustment by accumulation of Na+ and Cl-, because the accumulation of organic solutes due to salts was not consistent. In plants treated with 70 mM NaCl, the seasonal changes of the osmotic potential were produced by net solute accumulation, because the dehydration contribution was negligible. In plants treated with 140 mM NaCl, the seasonal changes of the osmotic potential were originated by ions accumulation, but also by tissues dehydration. At the end of the salinization period (December), a possible diurnal adaptation in water relations was also considered. The dehydration was the major mechanism involved in diurnal changes of leaf osmotic potential, and only at the highest salinity level some diurnal osmotic adjustment could be observed. In conclusion, the osmotic adjustment in Lotus might be a beneficial trait when the plants are treated with moderate levels of salinity (70 mM NaCl). At higher salinity (140 mM NaCl), the high absorption and accumulation of ions caused important toxic effects and induced leaf tissue dehydration. (C) 1998 Published by Elsevier Science Ireland Ltd. All rights reserved.