How does leaf succulence relate with plant drought resistance in woody shrubs?

Abstract

Succulence describes the amount of water stored in cells or organs, regardless of plant life-form, including woody and herbaceous plants. In dry environments, plants with greater survival often have greater leaf succulence. However, it is unclear how leaf succulence relates with plant drought resistance strategies, including isohydry (closing stomata to maintain leaf water status) and anisohydry (adjusting cell turgor to tolerate low leaf water status), which exist on a continuum that can be quantified by hydroscape area (larger hydroscape area indicates more anisohydric). We evaluated 12 woody species with differing leaf succulence in a glasshouse dry-down experiment to determine relationships among leaf succulence (degree of leaf succulence, leaf succulent quotient, leaf thickness) and plant drought response (hydroscape area, plant water use, turgor loss point, pre-dawn leaf water potential when transpiration ceased). Hydroscape areas ranged from 0.72 MPa2 (Carpobrotus modestus; CAM plants) to 7.01 MPa2 (Rhagodia spinescens; C3 plants), suggesting that C. modestus was more isohydric and R. spinescens was more anisohydric. More isohydric species C. modestus, C. rossii and Disphyma crassifolium (CAM plants) had greater leaf succulence, lower root allocation, used stored water and ceased transpiration at higher pre-dawn leaf water potential, shortly after reaching their turgor loss point. The remaining nine species that are not CAM plants had larger hydroscape areas and ceased transpiration at lower pre-dawn leaf water potential. Greater leaf succulence was not related with cumulative water loss until transpiration ceased in drying soils. All 12 species had high turgor loss points (-1.32 MPa to -0.59 MPa), but turgor loss point was not related to hydroscape area or leaf succulence. Our data suggests that overall greater leaf succulence was related with isohydry but this may have been influenced by the fact that these species were also CAM plants.