How succulent leaves of Aizoaceae avoid mesophyll conductance limitations of photosynthesis and survive drought

Abstract

In several taxa, increasing leaf succulence has been associated with decreasing mesophyll conductance (g M) and an increasing dependence on Crassulacean acid metabolism (CAM). However, in succulent Aizoaceae, the photosynthetic tissues are adjacent to the leaf surfaces with an internal achlorophyllous hydrenchyma. It was hypothesized that this arrangement increases g M, obviating a strong dependence on CAM, while the hydrenchyma stores water and nutrients, both of which would only be sporadically available in highly episodic environments. These predictions were tested with species from the Aizoaceae with a 5-fold variation in leaf succulence. It was shown that g M values, derived from the response of photosynthesis to intercellular CO2 concentration (A:C i), were independent of succulence, and that foliar photosynthate δ13C values were typical of C3, but not CAM photosynthesis. Under water stress, the degree of leaf succulence was positively correlated with an increasing ability to buffer photosynthetic capacity over several hours and to maintain light reaction integrity over several days. This was associated with decreased rates of water loss, rather than tolerance of lower leaf water contents. Additionally, the hydrenchyma contained ~26% of the leaf nitrogen content, possibly providing a nutrient reservoir. Thus the intermittent use of C3 photosynthesis interspersed with periods of no positive carbon assimilation is an alternative strategy to CAM for succulent taxa (Crassulaceae and Aizoaceae) which occur sympatrically in the Cape Floristic Region of South Africa.

Recent developments in mesophyll conductance in C3, C4, and crassulacean acid metabolism plants

“…These manuscripts report a range of g m from 0.44 to 0.85 μmol m −2 sec −1 Pa −1 , which are on the lower end of values reported for C 3 species g m , such as those found in gymnosperms (Flexas et al , ). Low g m in CAM species could be a result of compromised between mesophyll requirement for C 4 acid storage, minimize the loss of CO 2 from the leaf during decarboxylation of phase III and the availability of CO 2 for both PEPC in cytosol and RuBisCO in the chloroplast (Maxwell et al , , ; Gillon et al , ; Ripley et al , ). However, there has been very little work on the variation of g m across different CAM plant groups and a significant amount of more research is needed in this area.…”

From facilitative to competitive interactions between woody plants and plants with crassulacean acid metabolism (CAM): The role of hydraulic descent

“…In this study, CAM plants transpire at night but their transpiration rate is relatively low because of the low gradients in water vapor concentration existing during nighttime (Lüttge, ; Ogburn & Edwards, ). Moreover, CAM plants typically have a relatively low mesophyll conductance (e.g., Flexas et al, ; Nelson & Sage, ; Ripley, Abraham, Klak, & Cramer, ) and thus a low transpiration rate (Equation ) because water storage restricts the gas space available for the diffusion of CO 2 into the photosynthetic tissue. These conditions favor the maintenance of relatively high soil moisture levels in the shallow soil and thus provide the conditions favorable for hydraulic descent (Yu & Foster, ).…”

The competitive advantage of a constitutive CAM species over a C4 grass species under drought and CO2 enrichment

“…1; Drennan and Nobel 2000). This likely occurs because CO 2 uptake at night in ambient CO 2 conditions is still restricted by a low mesophyll conductance limiting the supply of CO 2 to the photosynthetic tissue (Drennan and Nobel 2000, Nelson and Sage 2008, Ripley et al 2013). C. imbricata performed a large fraction (>70-80%) of CAM (C 4 ) photosynthesis during the night (Fig.…”