Changes in malate concentration and activity of NADP-dependent malic enzyme were observed as the effect of Botrytis cinerea infection of C3 or CAM-performing Mesembryanthemum crystallinum plants. Biotic stress applied on C3 plants led to increase in malate concentration during the night and in consequence it led to increase in Δ-malate (day/night fluctuations) in infected leaves on the 2nd day post infection (dpi). It corresponded with induction of additional isoform of NADP-malic enzyme (NADP-ME3). On the contrary, CAM-performing M. crystallinum plants exhibited decrease in malate concentration and decay in its diurnal fluctuations as a reaction to B. cinerea infection. This correlated with significant decrease in activities of NADP-malic enzyme isoforms on the 2nd dpi as well as no fluctuations in their activities on the 9th dpi. Presented results point out to differences between C3 and CAM plants in the direction of changes in primary metabolism providing energy, reducing equivalents and carbon skeletons for defense responses to halt the pathogen growth.