Mokhtar Lachaâl

Long‐term effects of mild salt stress on growth, ion accumulation and superoxide dismutase expression of Arabidopsis rosette leaves

Arabidopsis thaliana plants (wild-type accessions Col and N1438) were submitted to a prolonged, mild salt stress using two types of protocols. These protocols allowed salt-treated plants to absorb nutrients either through a part of their root system maintained in control medium (split-rooted plants) or during episodes on control medium alternating with salt application (salt alternation experiment). Full-salt treatments (salt applied continuously to whole root system) resulted in severe (but non-lethal) growth inhibition. This effect was partly alleviated in split-rooted plants on mixed salt-control medium and in plants submitted to salt-control medium alternation. The activity of the various isoforms of superoxide dismutases (SODs) did not appreciably change with the treatments. The abundance of the mRNAs of the seven SOD genes present in Arabidopsis genome was determined using real-time polymerase chain reaction. The two protocols gave qualitatively identical results. The expression level was increased by full-salt treatments for some genes and diminished for other genes. However, the nature of these genes differed according to the accessions: the responses to salt of FSD1 and MSD were opposite in Col and N1438. In Col, salt treatments inhibited the expression of FSD1 and strongly stimulated that of CSD1 and MSD. In N1438, the stimulation by salt concerned FSD1 and CSD1 and MSD expression being inhibited. In both accessions, the expression of CSD2 and CSD3 was lowered by salt. For all genes, the treatments that mitigated stress partially restored SOD expression to control level. Thus, the changes in SOD transcript abundance accurately reflected the severity of the salt stress.

Salt effects on the growth, mineral nutrition, essential oil yield and composition of marjoram (Origanum majorana)

The aim of this work was to investigate the growth, mineral nutrition and essential oil composition of marjoram aerial part. Seedlings were cultivated for 20xa0days on nutrient solution, and then transferred to hydroponic solution with different NaCl concentrations (0, 50, 100, 150xa0mM). Plants were harvested after 17xa0days of treatment. Mineral nutrition and essential oil composition of shoots were determined. Results showed that growth, water content and development of the different organs of marjoram plant were affected just at the highest NaCl concentration (150xa0mM). Furthermore, salt did not seem to affect leaf area and root length but reduced the number of leaves. An increase in the total leaf surface and its thickness was observed at different NaCl concentrations. At 50xa0mM NaCl, sodium was primarily accumulated in roots but at 150xa0mM, it was strongly accumulated in leaves. However, Cl− accumulation was lower at higher NaCl concentrations. Essential oil yield of marjoram shoots was 0.12% in the control and 0.10% at 50xa0mM but an important decrease was observed at 100xa0mM (0.05%). Thirty-three components were identified belonging to different chemical classes. In the control, the essential oil was found to be rich in trans-sabinene hydrate (47.67%), terpinen-4-ol (20.82%) and cis-sabinene hydrate (7.23%). The proportions of these main compounds were differently affected by salt.

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor QA− to QB by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, QB, and the S2 state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of QA− reoxidation.

The impact of genotype and salinity on physiological function, secondary metabolite accumulation, and antioxidative responses in lettuce.

Salinity inhibits plant growth due to osmotic and ionic effects. However, little is known about the impact of genotype and salinity on biochemical and molecular processes in the leafy vegetable lettuce. We report here evaluations of two lettuce types, Verte (NaCl tolerant) and Romaine (NaCl sensitive), under iso-osmotic 100 mM NaCl and 77 mM Na(2)SO(4) treatments. As compared to Romaine, NaCl-treated Verte displayed better growth, contained lower levels of inorganic cations in leaves, and possessed superior antioxidative capacity due to enhanced carotenoid and phenolics biosynthesis and more active antioxidative enzymes resulting in reduced membrane damage. Both genotypes had relatively similar growth patterns under Na(2)SO(4) treatment, but Romaine showed enhanced root lignification, greater malondialdehyde formation, and suppressed Fe-superoxide dismutase expression in roots as compared with Verte.

Influence of nitrate—ammonium ratio on growth and nutrition of Arabidopsis thaliana

Arabidopsis thaliana seedlings were grown in hydroponic culture under controlled conditions in mediums with different NO3−/NH4+ ratios. The mediums were not buffered, against pH change, but their pH was reset at pHxa05.5 each 2xa0days. When ammoniacal N source was predominant, the whole plant biomass deposition was diminished, parallely to several parameters which revealed degraded leaf physiological status, such as leaf chlorosis associated with diminished chlorophyll content, lowered K+, Ca2+, and water content, along with restricted leaf expansion, and apparition of cellular stress symptoms. However, leaf functioning for biomass production did not seem affected, since biomass production rate per unit leaf surface area remained unchanged. Indeed, the main factor of decrease biomass production was leaf elimination and/or repression of leaf initiation. The results are discussed in the context of literature data on the effects of NH4+ on ion fluxes at the root level and properties of K+ transport systems in Arabidopsis. Disturbance of mineral (cationic) nutrition by NH4+ seemed to be a major cause of the detrimental effect of ammoniacal nutrition.

Different antioxidant responses to salt stress in two different provenances of Carthamus tinctorius L.

Seedlings of two Tunisian Carthamus tinctorius L. provenances (Kairouan and Tazarka) differing in salt sensitivity were hydroponically grown at 0 and 50xa0mMxa0NaCl over 21xa0days. Leaves of Kairouan (salt-sensitive) showed a 48% restriction in their growth at 50xa0mM NaCl although they accumulated less sodium than those of Tazarka (less salt-sensitive) that maintained an unchanged growth. Salt treatment induced oxidative stress in C. tinctorius and the effect was more pronounced in the leaves of the more salt sensitive provenance, Kairouan. Both provenances exhibited a stimulation of antioxidant enzyme activities with higher catalase (CAT) and superoxide dismutase (SOD) activities in Tazarka and higher peroxidase (POD) activity in Kairouan. But, it seems that antioxidant activities were more correlated with polyphenol content. Actually, leaves of Tazarka experienced higher polyphenol and antioxidant activity than Kairouan at 50xa0mM NaCl. Hence, moderate salinity (3xa0g NaClxa0L−1) enhanced bioactive molecule yield in the less salt sensitive provenance, Tazarka. In addition, C. tinctorius was found rich in ascorbic acid, but the moderate salt stress enhanced its production only in the sensitive provenance.

Comparative effect of potassium on K and Na uptake and transport in two accessions of Arabidopsis thaliana during salinity stress

Potassium-sodium interaction was compared in two natural accessions of Arabidopsis thaliana, Columbia-0 and NOK2. Seedlings were grown in the presence of 0 or 50 mM NaCl and 0.1; 0.625 or 2.5 mM K(+). At the lowest K(+) concentration, salt treatment inhibited both K(+) uptake and growth. Increasing the K(+) availability did not modified salt response in Columbia-0, but restored nearly normal net K(+) uptake in NaCl condition and alleviated NaCl growth reduction in NOK2. The effect of K(+) and NaCl on transcript level of several K(+) and Na(+) transporters in both shoots and roots was assessed using semi-quantitative RT-PCR. The mRNA abundance of the NHX1 and SOS1 Na(+)/H(+) antiporters was significantly increased by 50 mM NaCl in the two accessions. NHX1, which is responsible for Na(+) sequestration into vacuoles, was more up-regulated in NOK2 leaves than in Columbia-0s in NaCl stress condition. AKT1, which is the major channel involved in K(+) absorption, was down-regulated in salt stress condition, but was not responding to K(+) treatments. Only in NOK2, SKOR and AKT2, which respectively control xylem and phloem K(+) transport, were markedly up-regulated by 2.5 mM K(+) in both roots and shoots, independently of NaCl. Phenotypic and gene expression analyses suggest that the relative salt tolerance of NOK2 is mainly due to a high ability to sequester Na(+) in the vacuole and to take up and transport K(+). Up-regulation of SKOR and AKT2 by K(+), and of NHX1 by NaCl could participate in determining this phenotype.

Effects of NaCl or Na2SO4 salinity on plant growth, ion content and photosynthetic activity in Ocimum basilicum L.

Basil (Ocimum basilicum L., cultivar Genovese) plants were grown in Hoagland solution with or without 50xa0mM NaCl or 25xa0mM Na2SO4. After 15xa0days of treatment, Na2SO4 slowed growth of plants as indicated by root, stem and leaf dry weight, root length, shoot height and leaf area, and the effects were major of those induced by NaCl. Photosynthetic response was decreased more by chloride salinity than by sulphate. No effects in both treatments on leaf chlorophyll content, maximal efficiency of PSII photochemistry (Fv/Fm) and electron transport rate (ETR) were recorded. Therefore, an excess of energy following the limitation to CO2 photoassimilation and a down regulation of PSII photochemistry was monitored under NaCl, which displays mechanisms that play a role in avoiding PSII photodamage able to dissipate this excess energy. Ionic composition (Na+, K+, Ca2+, and Mg2+) was affected to the same extent under both types of salinity, thus together with an increase in leaves Cl−, and roots SO42− in NaCl and Na2SO4-treated plants, respectively, may have resulted in the observed growth retardation (for Na2SO4 treatment) and photosynthesis activity inhibition (for NaCl treatment), suggesting that those effects seem to have been due to the anionic component of the salts.

Varied tolerance to NaCl salinity is related to biochemical changes in two contrasting lettuce genotypes

Salt stress perturbs a multitude of physiological processes such as photosynthesis and growth. To understand the biochemical changes associated with physiological and cellular adaptations to salinity, two lettuce varieties (Verte and Romaine) were grown in a hydroponics culture system supplemented with 0, 100 or 200xa0mM NaCl. Verte displayed better growth under 100xa0mM NaCl compared to Romaine, but both genotypes registered relatively similar reductions in growth under 200xa0mM NaCl treatment. Both varieties showed differences in net photosynthetic activity in the absence of salt and 8 days after salt treatment. These differences diminished subsequently under prolonged salt stress (14 days). Verte showed enhanced leaf proline and restricted total cations especially Na+, lesser malondialdehyde (MDA) formation and lignification in the roots under 100xa0mM NaCl salinity. Membrane damage estimated by electrolyte leakage increased with elevated salt concentrations in roots of both varieties, but Verte had significantly lower electrolyte leakage relative to Romaine under 100xa0mM NaCl. Moreover, Verte also accumulated greater levels of carotenoids under increasing NaCl concentrations compared to Romaine. Taken together, these findings suggest that the greater tolerance of Verte to 100xa0mM NaCl is related to the more restricted accumulation of total cations and toxic Na+ in the roots and enhanced levels of antioxidative metabolites in root and leaf tissue.

Effect of salt stress on growth, fatty acids and essential oils in safflower (Carthamus tinctorius L.)

This study examined the influence of salt treatment on the growth parameters (fresh and dry weights), the mineral content (K+ and Na+), total lipid contents, fatty acid composition, yields and chemical composition of the essential oil of safflower (Carthamus tinctorius L.) grown in hydroponics for 2xa0weeks. Results showed that the application of 50xa0mM NaCl reduced the fresh weight of aerial parts (shoots and leaves) while it enhanced those of the roots. The reduction of dry weight was found to be more pronounced in the aerial parts. Salt treatment increased markedly the concentrations of Na+ in both plant parts while it reduced those of K+ which resulted in a sharp reduction of K+/Na+ ratio. In response to salt treatment, total lipids contents decreased in both plant parts and great qualitative changes in the fatty acids profiles were observed. Whatever the plant parts analysed, a redirection of the lipidic metabolism towards synthesis of unsaturated fatty acids as revealed by the increase of double bond index and linoleic desaturation ratio was pointed out. The increased unsaturation index was found to be more important in roots than in aerial parts. Such treatment also reduced the essential oil yields and induced marked quantitative changes in the chemical composition of the essential oils from both plant parts. Of all the identified components, oxygenated components display a prominent salt-induced synthesis and/or accumulation in both roots and aerial parts.