Sclerophylly and Plant Water Relations in Three Mediterranean Quercus Species

The possible role in drought resistance played by sclerophylly was studied in the Mediterranean oaks Quercus ilex, Q. suber and Q. pubescens. Studies were conducted on leaves at 30, 50 and 80% of their final surface area, as well as on mature leaves of the current year's growth in June and September and on 1-year-old leaves. Leaves of different ages of the three species showed quite different degrees of sclerophylly (DS). Q. ilex leaves reached the definitive DS of 1.75 g dm⁻² during leaf expansion; Q. pubescens leaves hardened at the end of their expansion, with a final DS of 0.93 g dm⁻²; Q. suber showed the lowest DS of 0.76 g dm⁻². Leaf conductance to water vapour (g₁) of 1-year-old leaves of Q. ilex, measured in the field, showed a duration of the g₁ peak values about twice that of the other two species. The minimum leaf relative water content (RWC), however, was near the same in the three species, indicating that water loss was recovered partly by Q. ilex leaves. This was apparently due to the higher bulk modulus of elasticity ($ɛ¯$_max) as resulting from leaf water potential isotherms. High correlation coefficients were noted between DS and $ɛ¯$_max but large changes in DS were needed to display minor changes in $ɛ¯$_max. When both DS and $ɛ¯$_max were correlated to the minimum RWC, an increase in DS of 133 % (corresponding to an increase in $ɛ¯$_max of only 12 %) was needed to cause an increase in RWC of 2%. Therefore, sclerophylly cannot be considered as significantly related to a drought-avoiding strategy.