Drought avoidance strategy in Ceratonia siliqua L., a mesomorphic-leaved tree in the xeric Mediterranean area

We report measurements of diurnal changes in leaf relative water content (RWC), water potential (ψ1) and conductance to water vapour (Cwv) during leaf ontogeny in Ceratonia siliqua L.. This evergreen broad-leaved tree, although having quite mesomorphic leaves (with large intercellular spaces and no fibres in their mesophyll) is a typical species of the most thermophilic plant community occurring in Sicily (Olea-Ceratonion community) where sclerophylls (and microphylls) dominate. C. siliqua mature leaves appeared to withstand water stress by a water spending strategy. They appeared to lose water substantially during the day but maintained their RWC at a constant level. This was achieved by dropping ψ1 for minimal water losses which in turn was due to a substantial and transient drop in turgor pressure. The bulk modulus of elasticity of mature leaves was about 29 MPa (i.e. mesophyll cells have relatively rigid cell walls). The concept of sclerophylly is discussed in terms of the different functional significance of leaf ‘rigidity’ as due to a large number of mechanical cells in the mesophyll or to more rigid cell walls of parenchymatous cells. Young leaves of C. siliqua appeared to be potentially very vulnerable to water stress but escape drought because leaf growth is completed before the dry period begins.