Mechanisms for drought tolerance in two Mediterranean seasonal dimorphic shrubs

Pressure-volume curves were constructed on a seasonal basis in two malacophyllous, drought semi-deciduous Mediterranean species (Phlomis fruticosa L. and Cistus creticus L.). Summer reduction of water potential at the turgor loss point in both species was less pronounced than corresponding changes in relative water content (RWC) at the turgor loss point, implying the existence of turgor maintenance mechanisms. However, actual summer leaf water potentials and RWCs in the field indicate that plants may experience zero or negative turgor pressure during the dry period. In both species, gradual decreases in osmotic potential and apoplastic relative water content, as well as increases in tissue elasticity, were observed during the summer. In particular, P. fruticosa exhibited a remarkable elasticity throughout the year. Indications for osmotic adjustment processes were also recorded. Critical (sublethal) RWCs measured during the winter and summer were lower not only from the corresponding RWCs at turgor loss point but also from the actual midday RWCs usually observed in the field. Both species seem to use a combination of elastic and osmotic adjustments in order to maintain their turgidity during the dry period. However, even though turgor may be lost during part of the summer, water deficits do not usually surpass critical levels under field conditions. This is probably achieved through the occurrence of two leaf populations (summer and winter leaves) with differential physiological drought tolerance.