Photosynthetic Pathway Variation in Leafy Members of Two Subfamilies of the Cactaceae

Abstract

Patterns of 24‐h CO₂ exchange and diel fluctuations in tissue acid concentrations were measured in leafy and leafless shoots of 10 species in the Pereskioideae and eight species in the Opuntioideae (Cactaceae). The species were selected to represent a range of phylogenetic histories. Leafy shoots of all species in the Pereskioideae exhibited C₃ patterns of gas exchange, and net CO₂ exchange of leafless stems in all but one species was negative during 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 nearly all species in the Opuntioideae, CO₂ uptake occurred during the day and the night. Gas‐exchange rates were typically greater during the day. As is typical of CAM, nighttime maximal water use efficiency often greatly exceeded 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 C₃ members of the more ancestral Pereskioideae. Furthermore, such consideration of the taxa in the more derived Opuntioideae (comparing the genera from most ancestral to most derived, that is, Austrocylindropuntia → Quiabentia → Pereskiopsis → Cylindropuntia) revealed that CAM became increasingly less important in the leaves of the various taxa, whereas this water‐conservative pathway of photosynthesis became increasingly more important in the stems. The results of this study indicate that members of the Pereskioideae should be restricted to moister habitats or must restrict the timing of growth to wet seasons, whereas the observed combinations of the C₃ and CAM pathways in the opuntioid taxa should prove beneficial in conserving water in the sporadically arid tropical and subtropical habitats of these plants.