[Effect of water stress on phosphoenolpyruvate carboxylase activity in Mesembryanthemum crystallinum (L.)]

Four-and-a-half-week-old plants of Mesembryanthemum crystallinum (L.) were transferred to cooled nutrient solution or to nutrient solution of low oxygen content. These stress treatments are known to reduce the absorption of water by plants. Examination of the second foliar leaves showed that the stress treatments result in a malate accumulation in the leaf mesophyll during the night which is paralleled by a drastically increased activity of phosphoenolpyruvate carboxylase. The results are discussed in relation to alterations of leaf growth, leaf water content and leaf water saturation deficit of the plants treated as described.

[Studies on NaCl-induced crassulacean acid metabolism in Mesembryanthemum crystallinum]

Mesembryanthemum crystallinum plants were grown for 10 weeks in soil culture and, during the final 3 weeks, watered with NaCl solutions varying in concentration between 50 and 500 mM. Plants show an increasingly less negative net CO2 balance and increased malate accumulation in the leaves during the dark with increasing NaCl concentration. There is a positive net CO2 balance above 100 mM with a maximum at 350 mM NaCl. Plants watered with 350 mM NaCl do not display this characteristic feature of crassulacean acid metabolism (CAM) initially, i.e. during the first week after the commencement of NaCl treatment. However, after this adaption period net CO2 balance during the dark begins to approach a positive value. Malate accumulation in the leaves during the dark increases rapidly after the first week of NaCl treatment, reaching a constant level after about 3 weeks. Leaves which develop after commencement of NaCl treatment produce less malate during the dark than leaves which were already present at the onset of NaCl watering. Na(+) content in the leaves is higher than Cl(-) content when plants are watered with NaCl solutions. Four-and-a-half-month-old plants grown under soil culture without addition of NaCl also exhibit CAM, probably because of ion uptake from the soil which causes increase in Na(+) and Cl(-) contents in the leaves. Na2SO4, KCl and K2SO4 have effects similar to those of NaCl on gas exchange reactions of Mesembryanthemum crystallinum.

[Discovery of NaCl induced crassulacean acid metabolism in a second member of the Aizoaceae family: Carpobrotus edulis]

Carpobrotus edulis grown for 24 days in nutrient solution plus 400 mM of NaCl shows the typical CO2 gas exchange reactions observed in CAM plants. Control plants grown in nutrient solution alone exhibit CO2 gas exchange reactions typical for C3 plants.

[CO2-fixation metabolism in the halophytic species Mesembryanthemum crystallinum grown under different environmental conditions]

The correlation of CO2-fixation metabolism to various environmental conditions such as NaCl content of culture medium, air humidity and light intensity was investigated in the halophytic species Mesembryanthemum crystallinum. The data obtained demonstrate that a change in photosynthesis from C3-pathway to crassulacean acid metabolism (CAM) is observed not only in NaCl treated plants as reported earlier but also in control plants grown in non-saline medium when environmental conditions (high light intensity, low air humidity) cause a water deficit in the leaves. It is suggested that water stress plays an important role in regulation between C3- and CAM-pathway of photosynthesis in Mesembryanthemum crystallinum.