Comparative impacts of water stress on the leaf anatomy of a drought-resistant and a drought-sensitive olive cultivar

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 more in '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.