Ahmedou M. Vadel

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

Summary 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.

NaCl stress affects growth and essential oil composition in rosemary (Rosmarinus officinalis L.)

Summary The effect of irrigation with increasing concentrations of salt [4.5 (control), 25, 50, 75, 100, 150, and 200 mM] on shoot growth and the essential oil (EO) composition of Rosmarinus officinalis L. was investigated in 2-year-old plants growing on dune-sand in pots. Shoot extension was not affected by low NaCl concentrations in the soil solution (≤ 25 mM); but it decreased with higher concentrations. NaCl at 150 mM and 200 mM was lethal and caused the plants to wilt after 1 month of culture. The Na+ content of both leaves and roots increased, whereas the K+ content decreased with the progressive increase in NaCl concentration in the growth medium. Rosemary plants maintained a higher K+:Na+ ratio in the shoots compared to roots. It appeared that rosemary is moderately salt tolerant. This tolerance is apparently due to the ability of these plants to accumulate Na+ in their old leaves and to maintain a higher K+: Na+ ratio in the leaves compared to the roots. During the period of study, the level of salinity in the soil solution affected the EO composition. There was a 50% decrease in the content of 1,8-cineole with increasing NaCl concentrations, concomitant with a slight increase in borneol content.