INFLUENCE OF LEAF WATER-CONTENT ON THE C-3-CAM TRANSITION IN MESEMBRYANTHEMUM-CRYSTALLINUM

Changes in leaf water content, night-time accumulation of malic (Delta -malate) and citric acid (Delta-citrate) and phosphoenolpyruvate carbo xylase (PEPC, EC 4.1.1.31) activity were followed for 60 d after germi nation in well watered and salt-stressed plants of the facultatively h alophytic ephemeral Mesembryanthemum crystallium L. To separate the ef fects of development, salt stress and water deficit on crassulacean ac id metabolism (CAM) induction plants were stressed initially 10 d afte r germination and then successively at l-wk intervals (five sets). Rel ated to dry mass or organic matter (i.e. dry mass corrected for the ma ss of inorganic ions) water content started to decrease during the lat e embryonal phase of the life cycle. Water content on a dry mass basis was always lower in salt-stressed than in well watered individuals. H owever, on an organic matter basis no difference was detectable. This indicated that salt treatment did not reduce leaf water content but fa lsified the basis (dry mass). Increases in leaf succulence and in pres sure potential prevented long-term water deficit in well watered and i n salt-stressed plants. Instead, these changes displayed enhanced vacu olisation, which is an essential prerequisite for the development of C AM. The end of that differentiation process might allow the initiation of nocturnal malic acid accumulation in a threshold response. At the onset of each salt treatment, short-term water deficits occurred due t o an incomplete osmotic adaptation independent of plant age. As Delta- malate only appeared when plants were c. 35 d old this water deficit w as unlikely to be a decisive CAM-inducing factor. About 2 wk after ger mination water content began to decline during the light periods in pl ants of all treatments. This pattern disappeared again when CAM had be en fully established. Daytime transpirational water loss is therefore unlikely to be the decisive factor because it failed to induce the met abolic shift in young plants. Environmental stress (e.g. salt or draug ht) can therefore only induce Delta-malate when leaf and plant differe ntiation has reached a certain stage.