When Mesembryanthemum crystallinum is grown at 23 C under continuous photoperiods (24L)for 5 weeks, the fourth juvenile leaf (J4), but not J5, shows expression of P-enolpyruvate carboxylase (PEPc) activity and accumulation of titratable acidity with no diurnal fluctuation. CO2 uptake is primarily during the daytime. Transferring these plants from 24L to photoperiods of 12L:12D results in a diurnal fluctuation of acidity but still daytime CO2 uptake, a metabolic condition call CAM-cycling. Stressing the latter (24L moved to 12L:12D)plants by irrigation with 0.5 M NaCl causes a reduction in daytime CO2 uptake and an increase in nighttime CO2 fixation consistent with full CAM activity. Plants grown concomitantly on 12L:12D photoperiods at the same temperature do not show PEPc activity, organic acid synthesis, or nocturnal CO2 uptake. Salt stress had no effect on these plants. The data indicate that the enzyme induction phase of CAM (here measured by PEPc activity) can be separated from diurnal organic acid fluctuation by photoperiodic manipulation. Furthermore, although salt stress results in a shift to full CAM, salt only affects the shift after PEPc is expressed. We conclude from these data that the expression of CAM enzymes is most likely developmentally regulated, and the shift to full CAM can be induced by environmental stress once the CAM enzymes are expressed. Thus there is both a developmental and and environmental aspect of CAM induction in Mesembryanthemum crystallinum.

Key words: CAM, Mesembryanthemum crystallinum, photosynthesis, stress