Succulence and Crassulacean Acid Metabolism in the Genus Clusia

Abstract

Succulent plants can be found across the world in semi-arid and seasonally dry ecosystems. Succulent anatomy exists in different forms; either via large chlorenchyma cells, specialised water-storage tissue called hydrenchyma, or some combination of the two. In addition, succulent chlorenchyma tissue is often accompanied by Crassulacean acid metabolism (CAM); an altered form of photosynthesis which minimises transpirational water loss. By comparing the hydrenchyma to the large photosynthetic chlorenchyma cells, it was shown that only succulence in the hydrenchyma provides capacitance to the leaf. In addition, the degree to which succulence and CAM contribute to leaf turgor loss point (TLP) was explored. The presence of hydrenchyma drives the TLP up, whereas the presence of CAM does not. To develop this, the ecological significance of hydrenchyma and CAM was tested, by investigating how these adaptations affect species’ climatic niches. CAM, and not hydrenchyma, is an important adaptation for species ability to inhabit arid environments. As hydrenchyma and CAM were deemed to be playing discretely different, non-overlapping roles in both the physiology and ecology of CAM, it was expected that they would have different impacts on the vascular architecture of Clusia. Vein density was significantly lower in CAM leaves, whilst hydrenchyma has no impact, suggesting that the lower diel stomatal conductance of CAM plants requires less hydraulic conductance to replace lost water, but that the capacitance generated by hydrenchyma does not have this effect. Finally, as CAM plays an ecologically important role in species distributions, but does not affect the TLP, it was hypothesised that the relevance of this adaptation lies outside of maintaining cell turgor during drought. Accordingly, the CAM drought response was characterised by higher photosynthetic assimilation and mitochondrial respiration rates, suggesting that the purpose of CAM is to sustain metabolic rates during drought, rather than prevent drops in water potentials