Martin C. E., Wallace R. S., Photosynthetic pathway variation in leafy members of two subfamilies of the Cactaceae, International Journal of Plant Sciences, 2000, 161(4):639-650, doi: 10.1086/314285
The study combines measurements of daily gas exchange with measurements of leaf and stem acidity in eighteen leafy cacti. It distinguishes uptake of atmospheric CO₂ from nocturnal recycling of respiratory carbon. It shows varying contributions of C₃ and CAM photosynthesis in individual organs and species.