Abstract
Morphological and physiological traits known to be involved in plant resistance to water deficit were studied in leaves of two olive (Olea europaea L.) cultivars, 'Meski' and 'Chemlali', reputed to be drought-sensitive and drought-tolerant, respectively. The objective of this study was to compare defense mechanisms against water stress in the leaves of these two cultivars. Withholding irrigation decreased the relative water content (RWC), net photosynthesis rate and stomatal conductance in leaves of both cultivars. This decrease was more acute in 'Meski' than in 'Chemlali'. The former cultivar was, therefore, more sensitive to water deficit. 'Chemlali' had higher leaf trichome and stomatal densities. However, when leaves of both cultivars were kept in the dark, the desiccation curves were comparable, implying that the two varieties had similar leaf cuticle transpiration. Under acute water deficit, leaves of both cultivars accumulated soluble sugars and proline, possibly for osmotic adjustment. The levels of RWC at which soluble sugar concentrations started to increase and decline were the same for both cultivars; nevertheless, 'Meski' leaves accumulated more soluble sugars. Detached leaves of 'Chemlali' had lower soluble sugar and higher proline concentrations than attached leaves. It is possible that soluble sugars were diverted to proline synthesis in response to injury. The RWC levels at which proline accumulation started and ended differed between the two cultivars, being higher in 'Meski' than in 'Chemlali'. In our study, high leaf trichome and stomatal densities and effective osmotic adjustment with proline may explain the greater drought-tolerance of cultivar 'Chemlali' compared with cultivar 'Meski'.