Dissecting succulence: Crassulacean acid metabolism and hydraulic capacitance are independent adaptations in Clusia leaves.

Succulence is found across the world as an adaptation to water-limited niches. The fleshy organs of succulent plants develop via enlarged photosynthetic chlorenchyma and/or achlorophyllous water storage hydrenchyma cells. The precise mechanism by which anatomical traits contribute to drought tolerance is unclear, as the effect of succulence is multifaceted. Large cells are believed to provide space for nocturnal storage of malic acid fixed by crassulacean acid metabolism (CAM), whilst also buffering water potentials by elevating hydraulic capacitance (C ). The effect of CAM and elevated C on growth and water conservation have not been compared, despite the assumption that these adaptations often occur together. We assessed the relationship between succulent anatomical adaptations, CAM, and C , across the genus Clusia. We also simulated the effects of CAM and C on growth and water conservation during drought using the Photo3 model. Within Clusia leaves, CAM and C are independent traits: CAM requires large palisade chlorenchyma cells, whereas hydrenchyma tissue governs interspecific differences in C . In addition, our model suggests that CAM supersedes C as a means to maximise CO assimilation and minimise transpiration during drought. Our study challenges the assumption that CAM and C are mutually dependent traits within succulent leaves.