Differential performance of two forage species, Medicago truncatula and Sulla carnosa, under water-deficit stress and recovery

The response patterns during water deficit stress and subsequent recovery of two forage species, Medicago truncatula and Sulla carnosa, were studied. After germination and pre-treatment, seedlings were individually cultivated for two months under two irrigation modes: 100% and 33% of field capacity. Measured parameters were plant growth, water relations, leaf osmotic potential, lipid peroxidation, and leaf inorganic (Na+ and K+) and organic (proline and soluble sugars) solute contents, as well as delta-1-pyrroline-5-carboxylate synthase (P5CS) and proline dehydrogenase (PDH) activities. Our results showed that under control conditions, and in contrast to roots, no significant differences were observed in shoot biomass production between the two species. However, when subjected to water-deficit stress, M. truncatula appeared to be more tolerant than S. carnosa (reduction by 50 and 70%, respectively). In the two studied species, water-deficit stress led to an increase in root/shoot ratio and leaf proline and soluble sugar contents, and a decrease in leaf osmotic potential. Enzymatic assay revealed that in the two species, P5CS activity was stimulated whereas that of PDH was inhibited under stress conditions. Despite greater accumulation of proline, sugar, and potassium in leaves of S. carnosa, M. truncatula was more tolerant to water deficit. This was essentially due to its capacity to control tissue hydration and water-use efficiency, in addition to its greater ability to protect membrane integrity. Following stress relief, M. truncatula and S. carnosa showed partial re-establishment of growth capacity

Seed priming mitigates high salinity impact on germination of bread wheat (Triticum aestivum L.) by improving carbohydrate and protein mobilization

Salinity is increasingly considered as a major environmental issue, which threatens agricultural production by decreasing yield traits of crops. Seed priming is a useful and cost-effective technique to alleviate the negative effects of salinity and to enable a fast and uniform germination. In this context, we quantified the effects of priming with gibberellic acid (GP), calcium chloride (CP), and mannitol (MP) on seed germination of three bread wheat cultivars and investigated their response when grown at high salinity conditions (200 mM NaCl). Salt exposure strongly repressed seed imbibition and germination potential and extended germination time, whereas priming enhanced uniformity and seed vigor. Seed preconditioning alleviated the germination disruption caused by salt stress to varying degrees. Priming mitigating effect was agent-dependent with regard to water status (CP and MP), ionic imbalance (CP), and seed reserve mobilization (GP). Na+ accumulation in seedling tissues significantly impaired carbohydrate and protein mobilization by inhibiting amylase and proteases activities but had lesser effects on primed seeds. CP attenuated ionic imbalance by limiting sodium accumulation. Gibberellic acid was the most effective priming treatment for promoting the germination of wheat seeds under salt stress. Moreover, genotypic differences in wheat response to salinity stress were observed between varieties used in this study. Ardito, the oldest variety, seems to tolerate better salinity in priming-free conditions; Aubusson resulted the most salt-sensitive cultivar but showed a high germination recovery under priming conditions; Bologna showed an intermediate behavior

Adjustment of photosynthetic carbon assimilation to higher growth irradiance in three-year-old seedlings of two Tunisian provenances of Cork Oak (Quercus suber L.)

Three-year-old seedlings of two Tunisian provenances of cork oak (Quercus suber L.) differing in climatic conditions at their geographical origin were subjected to increasing light intensities. Ga’four was the provenance from the driest site and Feija from the wettest site. Low-light adapted seedlings from both provenances were exposed to two light treatments: full sunlight (HL) and low light (LL, 15% sunlight) for 40 days. The CO2-response curve of leaf net photosynthesis (An-Ci curve) established under saturated photon flux density was used to compare photosynthetic parameters between leaves subjected to continuous low light (LL leaves) and leaves transferred from low to high light (HL leaves). Transfer from low to high light significantly increased net photosynthesis (An) and dark respiration (Rd) in Ga’four provenance but not in Feija. After transfer to high irradiance, specific leaf area (SLA) did not change in either provenance. This suggested that the increase in photosynthetic capacity on a leaf area basis in HL leaves of Ga’four provenance was not due to increased leaf thickness. Only the seedlings from the Ga’four provenance were able to acclimate to high light by increasing Vcmax and Jmax

Physiological and antioxidant responses of the sabkha biotope halophyte Limonium delicatulum to seasonal changes in environmental conditions

In saline biotopes, different mechanisms may contribute to the tolerance of halophytes to high soil salinity and temperature, drought and other abiotic stresses, but their relative responses and their ecological plasticity for a given species remain unknown. In this study, we examined the responses of the salt marsh halophyte Limonium delicatulum to changing environmental conditions of its natural habitats (Sabkha “El Kalbia”, Tunisia). The specific aim of the work was to check whether statistically significant correlations could be established between particular stress response mechanisms (ion uptake, activation of antioxidant systems) and soil parameters and climatic data associated with environmental stress. The results showed that the salinity of the soil increased during the months of June, July and August (dry period), concomitantly with a strong accumulation of salt ions in the aerial parts of the plant. Moreover, the highest antioxidant capacity of this halophyte (enzymes and antioxidant molecules) was reached during the same period of the year. The remaining periods (corresponding to the rainy season) did not show a significant difference in enzymes activities and level of antioxidants, with a minimum observed in the months of January and February. These results show a remarkable effect of salinity in the natural habitat on the activity of enzymes and antioxidant molecules. Other edaphic and climatic factors could also be involved to increase antioxidant capacity, such as nutrient deficiency, temperature and precipitation

The halotolerant rizhobacterium Glutamicibacter sp. alleviates salt impact on Phragmites australis by producing exopolysaccharides and limiting plant sodium uptake

Salinity is a widespread abiotic stress, which has strong adverse effects on plant growth and crop productivity. Exopolysaccharides (EPS) play a crucial role in plant growth-promoting rhizobacteria (PGPR)-mediated improvement of plant stress tolerance. This study aimed to assess whether Glutamicibacter sp. strain producing large amounts of EPS may promote tolerance of common reed, Phragmites australis (Cav.) Trin. ex Steud., towards salt stress. This halotolerant rizhobacterium showed tolerance to salinity (up to 1 M NaCl) when cultivated on Luria-Bertani (LB) medium. Exposure to high salinity (300 mM NaCl) significantly impacted the plant growth parameters, but this adverse effect was mitigated following inoculation with Glutamicibacter sp., which triggered higher number of leaves and tillers, shoot fresh weight/dry weight, and root fresh weight as compared to non-inoculated plants. Salt stress increased the accumulation of malondialdehyde (MDA), polyphenols, total soluble sugars (TSSs), and free proline in shoots. In comparison, the inoculation with Glutamicibacter sp. further increased shoot polyphenol content, while decreasing MDA and free proline contents. Besides, this bacterial strain increased tissue Ca and K content concomitant to lower shoot Na and root Cl accumulation, thus further highlighting the beneficial effect of Glutamicibacter sp. strain on the plant behavior under salinity. As a whole, our study provides strong arguments for a potential utilization of EPS-producing bacteria as a useful microbial inoculant to alleviate the deleterious effects of salinity on plants.Tunisian Ministry of Higher Education and Scientific Research, Grant/Award Number: LR10CBBC0

Plant growth-promoting rhizobacteria alleviate high salinity impact on the halophyte suaeda fruticosa by modulating antioxidant defense and soil biological activity

Plant growth-promoting rhizobacteria (PGPR) are considered as bio-ameliorators that confer better salt resistance to host plants while improving soil biological activity. Despite their importance, data about the likely synergisms between PGPR and halophytes in their native environments are scarce. The objective of this study was to assess the effect of PGPR (Glutamicibacter sp. and Pseudomonas sp.) inoculation on biomass, nutrient uptake, and antioxidant enzymes of Suaeda fruticosa, an obligate halophyte native in salt marshes and arid areas in Tunisia. Besides, the activity of rhizospheric soil enzyme activities upon plant inoculation was determined. Plants were grown in pots filled with soil and irrigated with 600 mM NaCl for 1 month. Inoculation (either with Pseudomonas sp. or Glutamicibacter sp.) resulted in significantly higher shoot dry weight and less accumulation of Na and Cl in shoots of salt-treated plants. Glutamicibacter sp. inoculation significantly reduced malondialdehyde (MDA) concentration, while increasing the activity of antioxidant enzymes (superoxide dismutase; catalase; ascorbate peroxidase; and glutathione reductase) by up to 100%. This provides strong arguments in favor of a boosting effect of this strain on S. fruticosa challenged with high salinity. Pseudomonas sp. inoculation increased shoot K and Ca content and lowered shoot MDA concentration. Regarding the soil biological activity, Pseudomonas sp. significantly enhanced the activities of three rhizospheric soil enzymes (urease, ß-glucosidase, and dehydrogenase) as compared to their respective non-inoculated saline treatment. Hence, Pseudomonas sp. could have a great potential to be used as bio-inoculants in order to improve plant growth and soil nutrient uptake under salt stress. Indole-3-acetic acid concentration in the soil increased in both bacterial treatments under saline conditions, especially with Glutamicibacter sp. (up to +214%). As a whole, Glutamicibacter sp. and Pseudomonas sp. strains are promising candidates as part of biological solutions aiming at the phytoremediation and reclamation of saline-degraded areas.This study was carried out in the framework of project AGL2009-12530-C02-02 from Ministerio de Economía y Competitividad (Spain). The financial support of the Tunisian Ministry of Higher Education and Scientific Research and the International Centre for Biosaline Agricultura (ICBA, Dubai, United Arab Emirates) is gratefully acknowledged. The financial support of the Arab German Young Academy of Sciences and Humanities (AGYA) is gratefully acknowledged

Exogenous proline mediates alleviation of cadmium stress by promoting photosynthetic activity, water status and antioxidative enzymes activities of young date palm (Phoenix dactyliferaL.)

International audienceThe ability of exogenous compatible solutes, such as proline, to counteract cadmium (Cd) inhibitory effects in young date palm plants (Phoenix dactylifera L. cv Deglet Nour) was investigated. Two-year-old date palm plants were subjected for five months at different Cd stress levels (0, 10 and 30mg CdCl2kg(-1) soil) whether supplied or not with exogenous proline (20mM) added through the irrigation water. Different levels of Cd stress altered plant growth, gas exchanges and chlorophyll content as well as water status, but at different extent among them. In contrast, an increase of antioxidant enzymes activities of Cd-treated plants in association with high amounts of proline content, hydrogen peroxide (H2O2), thiobarbituric acid reactive substances (TBARS) and electrolyte leakage (EL) were observed. Interestingly, exogenous proline mitigated the adverse effects of Cd on young date palm. Indeed, it alleviated the oxidative damage induced by Cd accumulation and established better levels of plant growth, water status and photosynthetic activity. Moreover, proline-treated plants showed high antioxidant enzymes activities (superoxide dismutase, catalase and glutathione peroxydase) in roots and leaves as compared to Cd-treated plant