# Stronger expression of crassulacean acid metabolism (CAM) requires effective cuticular transpiration barriers but not necessarily strong succulence

## TL;DR

This study shows that strong CAM function in plants relies more on effective cuticle barriers than on high succulence, using Aeonium species as a model.

## Contribution

The study reveals a tight evolutionary link between cuticular transpiration barriers and CAM performance in Aeonium.

## Key findings

- A negative correlation was found between g_min and ΔH+, highlighting the cuticle's role in CAM function.
- More succulent species maintain CAM mode when stress is reduced, despite no overall correlation between ΔH+ and succulence.
- Strong CAM expression is linked to reduced g_min, improving water-use efficiency.

## Abstract

Discovering functional and evolutionary interdependencies of hydraulic traits and crassulacean acid metabolism (CAM) is crucial to understand CAM phenotype diversity and convergence. In complex traits such as CAM, the co‐option of associated traits strongly impacts the evolutionary outcome. Here we study Aeonium (Crassulaceae), a diverse Macaronesian genus that exhibits a broad array of CAM expression, focusing on two CAM‐associated traits, minimum conductance (g
min) and succulence.At the heart of the study, there are two experiments: a comparative cultivation experiment to monitor nocturnal acidification (ΔH+) under drought and heat treatments and a leaf‐drying curve experiment to quantify g
min. Our study group was comprehensively sampled to cover its phylogenetic and ecological diversity.We found a consistently negative correlation of g
min and ΔH+, indicating a critical role for the cuticle in the function of CAM. Although ΔH+ and succulence were overall not positively correlated, we found evidence that more succulent species remain in the CAM mode when stress is relaxed.We conclude that there is a tight evolutionary link between cuticular transpiration barrier properties and CAM performance. Thereby, the stronger CAM plants express diurnal stomatal closure typical of CAM, the more they may optimise water‐use efficiency through reduced g
min.

Discovering functional and evolutionary interdependencies of hydraulic traits and crassulacean acid metabolism (CAM) is crucial to understand CAM phenotype diversity and convergence. In complex traits such as CAM, the co‐option of associated traits strongly impacts the evolutionary outcome. Here we study Aeonium (Crassulaceae), a diverse Macaronesian genus that exhibits a broad array of CAM expression, focusing on two CAM‐associated traits, minimum conductance (g
min) and succulence.

At the heart of the study, there are two experiments: a comparative cultivation experiment to monitor nocturnal acidification (ΔH+) under drought and heat treatments and a leaf‐drying curve experiment to quantify g
min. Our study group was comprehensively sampled to cover its phylogenetic and ecological diversity.

We found a consistently negative correlation of g
min and ΔH+, indicating a critical role for the cuticle in the function of CAM. Although ΔH+ and succulence were overall not positively correlated, we found evidence that more succulent species remain in the CAM mode when stress is relaxed.

We conclude that there is a tight evolutionary link between cuticular transpiration barrier properties and CAM performance. Thereby, the stronger CAM plants express diurnal stomatal closure typical of CAM, the more they may optimise water‐use efficiency through reduced g
min.