The effect of irradiance on carboxylating/decarboxylating enzymes and fumarase activities in Mesembryonthemum crystallinum L. exposed to salinity stress

In Mesembryanthemum crystallinum plants, treated for 9 days with 0.4 M NaCl at low light intensities (80-90 or 95-100 muE m(-2) s(-1); lambda = 400-700 nm), no day/night malate level differences (Delta malate) were detected. At high light (385-400 muE m(-2) s(-1)) strong stimulation of PEPC activity, accompanied by a Delta malate of 11.3 mM, demonstrated the presence of CAM metabolism. This indicates that, to evolve day/night differences in malate concentration, high light is required. Salt treatment at low light induces and increases the activity of NAD- and NADP-malic enzymes by as much as 3.7- and 3.9-fold, while at high light these values reach 6.4- and 17.7-fold, respectively. The induction of activity of both malic enzymes and PEPC (phospoenolpyruvate carboxylase) take place before Delta malate is detectable. An increase in SOD (superoxide dismutase) was observed in plants cultivated at high light in both control and salt-treated plants. However, in salt-treated plants this effect was more pronounced. Carboxylating and decarboxylating enzymes seem to be induced by a combination of different signals, i.e., salt and light intensity. Plants performing CAM, after the decrease of activity of both the decarboxylating enzymes at the beginning of the light period, showed an increase in these enzymes in darkness when the malate pool reaches higher levels. In CAM plants the activity of fumarase (Krebs cycle) is much lower than that in C(3) plants. The role of mitochondria in CAM plants is discussed.