Influence of drought on mitochondrial activity, photosynthesis, nocturnal acid accumulation and water relations in the CAM plants Prenia sladeniana (ME-type) and Crassula lycopodioides (PEPCK-type)

The activity of photosynthesis and mitochondrial respiration, nocturnal org anic acid accumulation and water relations were investigated in Prenia slad eniana L. Bol. [malic enzyme (ME)-type] and Crassula lycopodioides Lam. [ph osphoenolpyruvate carboxykinase (PEPCK)-type] to compare the physiological responses to water deficit in crassulacean acid metabolism (CAM) plants dif fering in their decarboxylating enzyme systems. Withholding water inhibited daytime gas exchange within 2 d while night time CO2 gain and malic acid a ccumulation remained relatively unchanged in both species. In P. sladeniana , maximum photochemical efficiency (F-v/F-m) and photosynthetic electron tr ansport declined to nearly the same degree as CO2 supply was restricted dur ing drought. Despite limited CO2 availability, photosynthetic activity was largely unaffected in C. lycopodioides, as were mitochondrial properties. T here is no indication of a drought-induced increase in the capability to to tally oxidize malate, yielding 4 CO2, in either species. Nevertheless, the enhanced ratio of malate to glycine oxidation may have increased the in viv o capability for malate oxidation in P. sladeniana. Although pressure poten tial was maintained throughout the experiment in both species, active osmot ic adaptation occurred only in P. sladeniana. The observed decrease in phot osynthetic and mitochondrial activity may have resulted from the large incr ease in osmotic concentration in this species. (C) 2000 Annals of Botany Co mpany.