Photosynthetic pathway variation in leafy members of two subfamilies of the Cactaceae

Patterns of 24-h CO2 exchange and diel fluctuations in tissue acid concentr ations were measured in leafy and leafless shoots of 10 species in the Pere skioideae and eight species in the Opuntioideae (Cactaceae). The species we re selected to represent a range of phylogenetic histories. Leafy shoots of all species in the Pereskioideae exhibited C-3 patterns of gas exchange, a nd net CO2 exchange of leafless stems in all but one species was negative d uring the day and night. Although nighttime CO2 uptake was not observed in shoots or stems of any of the pereskioid taxa, tissue acidity increased at night to a small degree in leaves of six species and stems of five species, indicative of low levels of CAM-cycling. In contrast, in leafy shoots of n early all species in the Opuntioideae, CO2 uptake occurred during the day a nd the night. Gas-exchange rates were typically greater during the day. As is typical of CAM, nighttime maximal water use efficiency often greatly exc eeded daytime values. Tissue malic acid concentrations increased overnight in leaves and stems of all eight opuntioid species. Examination of the data from a phylogenetic perspective illustrates evidence of low levels of CAM scattered among the primarily C-3 members of the more ancestral Pereskioide ae. Furthermore, such consideration of the taxa in the more derived Opuntio ideae (comparing the genera from most ancestral to most derived, that is, A ustrocylindropuntia --> Quiabentia --> Pereskiopsis --> Cylindropuntia) rev ealed that CAM became increasingly less important in the leaves of the vari ous taxa, whereas this water-conservative pathway of photosynthesis became increasingly more important in the stems. The results of this study indicat e that members of the Pereskioideae should be restricted to moister habitat s or must restrict the timing of growth to wet seasons, whereas the observe d combinations of the C-3 and CAM pathways in the opuntioid taxa should pro ve beneficial in conserving water in the sporadically arid tropical and sub tropical habitats of these plants.