The effects of drought on several major morphological and anatomical features of leaves were investigated in an attempt to explain the origin of the difference in drought resistance between two olive (Olea europaea L.) cultivars, (‘Chemlali’ and ‘Meski’) previously demonstrated to be drought-resistant and drought-sensitive, respectively. Under water deficit conditions, ‘Chemlali’ maintained higher rates of photosynthetic assimilation and lower rates of transpiration compared to ‘Meski’. In the present study, we found cultivar-dependent differences in leaf morpho-anatomical adaptations to drought stress. When subjected to water stress, the leaves of‘Chemlali’ increased the thickness of their upper palisade and spongy parenchyma by 17% and 22%, respectively, compared with only 9% and 13% in the case of ‘Meski’. A thicker palisade parenchyma could contain larger numbers of CO₂-fixation sites, while a thicker spongy parenchyma could result in easier diffusion of CO₂ to these sites. Furthermore, stomatal density (SD) in ‘Chemlali’ leaves increased by 25%(vs. 7% for ‘Meski’ leaves) during drought treatment, which could also enhance the external supply of CO₂. Other morpho-structural traits implicated in the control of water loss were enhanced morein ‘Chemlali’ than in ‘Meski’ leaves. Under conditions of lower water availability, leaf size decreased by 24% in ‘Chemlali’ (vs. 15% in ‘Meski’), trichome density (TD) increased by 25% (while remaining unchanged in ‘Meski’), and the thickness of the upper and lower epidermis increased by 32% and 25%, respectively (while remaining unchanged in ‘Meski’). The above morpho-anatomical adaptations should improve the water-use efficiency of the tree. These differential changes in leaf morphology and anatomy can explain, at least in part, the difference in drought resistance between the two cultivars. In particular, the upper palisade parenchyma, the spongy parenchyma, SD, and TD could be considered key structural features of leaves that govern the ability of a tree to withstand water stress. They could therefore be used as criteria to select olive cultivars that are more resistant to drought.

We examined the effects of drought on the water relations, osmotic adjustment and xylem vulnerability to embolism of olive (Olea europaea L.) varieties, ‘Meski’ and ‘Chemlali’. Two-year-old self-rooted cuttings growing in sand-filled pots in a greenhouse were subjected to water stress by withholding water for 60 days. Water relations and gas exchange measurements showed that ‘Chemlali’ was more drought resistant than ‘Meski’ and had a greater capacity for osmotic adjustment through solute accumulation. However, when water stress was acute, the effect of osmoregulation on leaf cell turgor was largely counteracted by xylem cavitation. Cavitation vulnerability curves showed that both varieties were highly resistant to embolism formation. The xylem water potential inducing 50% loss of stem conductivity approached −7 MPa in ‘Meski’ and only slightly less in ‘Chemlali’. Although the difference between varieties in susceptibility to xylem embolism was small, it appears to account in large part for the difference between them in the ability to tolerate severe drought.

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’.