The foreword to the special issue on diversity of CAM plant photosynthesis (crassulacean acid metabolism) highlights some of the key contributions of the Australian plant biologist Professor Charles Barry Osmond to our understanding of the CAM pathway of photosynthesis and provides a brief introduction to the research papers of this issue.

Portulaca oleracea has emerged as a model system to answer the intriguing question of how two carbon concentration mechanisms (C₄ and CAM) can co-exist within a single leaf. Recent progress has been made with the study of C₄ and CAM functional genomics, but similar molecular approaches have not been possible in C₄-CAM facultative species. Essential tools for functional gene analysis are now available for P. oleracea, which may accelerate C₄-CAM photosynthesis research and the future application of these valuable photosynthetic adaptations within crop biotechnology.

In two Cistanthe species from the Atacama desert, CO₂ uptake and leaf acidification patterns were observed that are typical of water-use efficient crassulacean acid metabolism (CAM) photosynthesis. CAM expression in the perennial C. sp. aff. crassifolia was facultative whereas CAM in the annual C. sp. aff. longiscapa was constitutive. Cistanthe becomes the sixth genus known to exhibit CAM within the family Montiaceae.

By identifying physiological traits more common in drier environments, it is possible to understand the ways in which tropical trees have adapted to deal with drought. By analysing a genus from Central and South America, we were able to test if it is more beneficial to prevent or tolerate water loss. Our results show that preventing water loss has a greater benefit to living in drier niches, which has implications for the ways in which future climates will affect tropical flora.

To better understand CAM-related metabolites and water-deficit stress responses of Opuntia ficus-indica, comparative metabolic profiling was performed on mesophyll and epidermal tissues collected from well-watered and water-deficit stressed cladodes. A total of 382 metabolites, including 210 (55%) named and 172 (45%) unnamed compounds, were characterised across both tissues. This study revealed a total of 34 unnamed metabolites that accumulated in response to water-deficit stress indicating that such compounds might play important roles in water-deficit tolerance.