Habib Khemira

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.

How reliable is the double-ended pressure sleeve technique for assessing xylem vulnerability to cavitation in woody angiosperms?

The reliability of a double-ended pressure sleeve technique was evaluated on three woody angiosperm species with contrasting maximum vessel lengths. Vulnerability curves (VCs) were constructed by varying sample length and the size of the pressure sleeves. VCs were compared against curves obtained with reference techniques. For the two diffuse-porous species, Betula pendula and Prunus persica, VCs built with shoot segments shorter than maximum vessel length strongly overestimated species vulnerability. Furthermore, increasing the size of the pressure sleeve also tended to lead to overestimated VCs. For the ring-porous species Quercus robur, the technique strongly overestimated vulnerability to embolism, whatever the sample length or chamber tested. In conclusion, the double-ended pressure sleeve technique only gives reliable VCs on diffuse-porous angiosperms with short pressure sleeves, only when segments are longer than maximum vessel length.

Improvement to the air-injection technique to estimate xylem vulnerability to cavitation

Several techniques have been developed to quantify the degree of embolism of the xylem using hydraulic conductance. Although there have been several improvements to these techniques, their reliability is still questionable and many technical pitfalls persist. We are proposing here a manometric approach to improve the accuracy of xylem cavitation measurement by the original air-injection technique which uses twigs exposed to pressurized air to cause cavitation. The measured parameter is air bubble production (Pb) caused by xylem cavitation in birch (Betula pendula Roth) twigs from which the percent increase in bubble production is calculated to quantify xylem cavitation. Data produced by three different methods (bench-drying, air-injection, and manometric approach) are compared. Xylem vulnerability curves (VCs) constructed by the reference and reliable bench-drying technique and the manometric approach show similar sigmoid “S” shape, but a small anomaly appeared in the VC constructed by the original air-injection technique. The xylem pressure inducing 50% of embolism (P50) was the same with the three techniques. Furthermore, there was a strong positive correlation between the estimators of xylem cavitation measured by the three different methods. For its reliability, precision and ease we recommend the manometric technique as an improved version of the original hydraulic air-injection method.

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.

NaCl stress-induced changes in the essential oil quality and abietane diterpene yield and composition in common sage -

Aim: The purpose of this study was to evaluate how increasing NaCl salinity in the medium can affects the essential oils (EOs) composition and phenolic diterpene content and yield in leaves of Salvia officinalis L. The protective role of such compounds against NaCl stress was also argued with regard to some physiological characteristics of the plant (water and ionic relations as well as the leaf gas exchanges). Materials and Methods: Potted plants were exposed to increasing NaCl concentrations (0, 50, 75, and 100 mM) for 4 weeks during July 2012. Replicates from each treatment were harvested after 0, 2, 3, and 4 weeks of adding salt to perform physiological measurements and biochemical analysis. Results: Sage EOs were rich in manool, viridiflorol, camphor, and borneol. Irrigation with a solution containing 100 mM NaCl for 4 weeks increased considerably 1.8-cineole, camphor and β-thujone concentrations, whereas lower concentrations (50 and 75 mM) had no effects. On the contrary, borneol and viridiflorol concentrations decreased significantly under the former treatment while manool and total fatty acid concentrations were not affected. Leaf extracts also contained several diterpenes such as carnosic acid (CA), carnosol, and 12-O-methoxy carnosic acid (MCA). The concentrations and total contents of CA and MCA increased after 3 weeks of irrigation with 75 or 100 mM NaCl. The 50 mM NaCl had no effect on these diterpenes. Our results suggest a protective role for CA against salinity stress. Conclusion: This study may provide ways to manipulate the concentration and yield of some phenolic diterpenes and EOs in sage. In fact, soil salinity may favor a directional production of particular components of interest.

Differential response of two almond rootstocks to chloride salt mixtures in the growing medium

It was examined how essential cations, Ca2+ and K+, can mitigate the toxic effects of NaCl on two different almond species (Prunus amygdalus Batsch) rootstocks, Garnem (GN15) and Bitter Almond. The tree growth parameters (water potential (Ψw), gas exchange, nutrient uptake) and leaf chlorophyll (Chl) content were measured in control and NaCl-treated plants with or without KCl or CaCl2 supplements. The addition of CaCl2 and KCl to Bitter Almond trees reduced their dry weight, shoot growth and leaf number although net photosynthetic assimilation rate (A) was not affected. These results indicated that changing of photo-assimilates flux to proline and/or soluble sugars synthesis may help to increase leaf Ψw. The Garnem trees also did not respond to the CaCl2 and KCl addition indicating that the plants are already getting enough of these two cations (Ca2+ and K+). In both rootstocks, NaCl in the medium reduced growth attributes, Ψw, A, stomatal conductance (gs), and leaf Chl content. When CaCl2 and KCl fertilizers were added together with NaCl to Bitter Almond trees, leaf K+ and Ca2+ contents increased while Na+ and Cl– decreased leading to higher Ca/Na and K/Na ratios, but shoot growth was not improved and even declined compared to NaCl-treated trees. It appears that the addition of salts further aggravated osmotic stress as indicated by the accumulation of proline and soluble sugars in leaf tissues. The addition of KCl or CaCl2 to NaCl-treated GN15 trees did not increase A, leaf Ψw, and shoot growth but improved ionic balances as indicated by higher Ca/Na and K/Na ratios. The reduction in A was mainly due to non-stomatal limitations in GN15, possibly due to the degradation of Chl a, unlike Bitter Almond, for which the reduction of A was due to stomata closure. The improvement in ionic balances and water status of Bitter Almond trees in response to addition of KCl or CaCl2 was apparently offset by a high sensitivity to Cl–; therefore, no-chloride salts should be the preferred forms of fertilizers for this rootstock. Both rootstocks were sensitive to soil salinity and cation supplements were of limited value in mitigating the effect of excessive salt concentrations.

Non-thermal dielectric barrier discharge (DBD) plasma affects germination of coffee and grape seeds

Seeds of many plant species often fail to germinate even when moisture and temperature are adequate because they are dormant. Commonly, these seeds need to be exposed to cold temperature or some other forms of stress to release their dormancy. In the nursery, they use strong acids or hot water for the same purpose. Low-temperature plasma has been shown to be effective in improving germination of seeds. In the present study, we have used an atmospheric pressure dielectric barrier discharge (DBD) operated in Helium gas to treat dormant grape and coffee seeds. The seeds were treated at fixed power of 50 W for different durations (control, 30s, 60s, 120s and 240s). Exposure to plasma has affected germination rate and germination vigor of the seeds differently depending on the species. The germination rate of coffee seeds decreased after short exposure to plasma as compared to non-treated seeds. When the seeds were exposed to plasma for 120s germination was advanced but germination rate was the same as that of the control. However, for grape seeds, all plasma treatments increased germination rate as compared to the control treatment. Treating the seeds for 240s advanced germination by 10 days and more than doubled the rate of germination. The effect of plasma seems to be partly due to accelerated water can enhance the vigor of coffee and grape seeds; this should improve seedling establishment and growth.Seeds of many plant species often fail to germinate even when moisture and temperature are adequate because they are dormant. Commonly, these seeds need to be exposed to cold temperature or some other forms of stress to release their dormancy. In the nursery, they use strong acids or hot water for the same purpose. Low-temperature plasma has been shown to be effective in improving germination of seeds. In the present study, we have used an atmospheric pressure dielectric barrier discharge (DBD) operated in Helium gas to treat dormant grape and coffee seeds. The seeds were treated at fixed power of 50 W for different durations (control, 30s, 60s, 120s and 240s). Exposure to plasma has affected germination rate and germination vigor of the seeds differently depending on the species. The germination rate of coffee seeds decreased after short exposure to plasma as compared to non-treated seeds. When the seeds were exposed to plasma for 120s germination was advanced but germination rate was the same as that of...

Composition of the root mycorrhizal community associated with Coffea arabica in Fifa Mountains (Jazan region, Saudi Arabia)

Arbuscular mycorrhizal fungi (AMF) constitute a key functional group of soil biota that can greatly contribute to crop productivity and ecosystem sustainability. They improve nutrient uptake and enhance the ability of plants to cope with abiotic stresses. The presence of AMF in coffee (Coffea arabica L.) plant roots have been reported in several locations but not in Saudi Arabia despite the fact that coffee has been in cultivation here since ancient times. The objective of the present study was to investigate the diversity of AMF communities colonizing the roots of coffee trees growing in two sites of Fifa Mountains (south‐west Saudi Arabia): site 1 at 700 m altitude and site 2 at 1400 m. The AMF large subunit rDNA regions (LSU) were subjected to nested PCR, cloning, sequencing, and phylogenetic analysis. Microscopic observations indicated higher mycorrhizal intensity (24.3%) and spore density (256 spores/100 g of soil) in site 2 (higher altitude). Phylogenetic analysis revealed 10 phylotypes, six belonging to the family Glomeraceae, two to Claroideoglomercea, one to Acaulosporaceae and one to Gigasporaceae family. Glomus was the dominant genus at both sites and the genus Gigaspora was detected only at site 2. This is the first study reporting the presence of AMF in coffee roots and the composition of this particular mycorrhizal community in Saudi Arabia.