Soil salinity is one of the major abiotic stresses restricting the use of land for agriculture because it limits the growth and development of most crop plants. Improving productivity under these physiologically stressful conditions is a major scientific challenge because salinity has different effects at different developmental stages in different crops. When supplied exogenously, proline has improved salt stress tolerance in various plant species. Under high-salt conditions, proline application enhances plant growth with increases in seed germination, biomass, photosynthesis, gas exchange, and grain yield. These positive effects are mainly driven by better nutrient acquisition, water uptake, and biological nitrogen fixation. Exogenous proline also alleviates salt stress by improving antioxidant activities and reducing Na + and Cl − uptake and translocation while enhancing K + assimilation by plants. However, which of these mechanisms operate at any one time varies according to the proline concentration, how it is applied, the plant species, and the specific stress conditions as well as the developmental stage. To position salt stress tolerance studies in the context of a crop plant growing in the field, here we discuss the beneficial effects of exogenous proline on plants exposed to salt stress through wellknown and more recently described examples in more than twenty crop species in order to appreciate both the diversity and commonality of the responses. Proposed mechanisms by which exogenous proline mitigates the detrimental effects of salt stress during crop plant growth are thus highlighted and critically assessed.

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The effects of water deficit on growth, nodulation, and several physiological and biochemical processes in six symbiotic combinations involving three Moroccan alfalfa (Medicago sativa L.) populations (Tafilalet1, Adis-Tata and Demnate2), an American Moapa variety andtwo rhizobial strains (RhL9 and RhL10) were studied. The experiment was conducted under greenhouse conditions. Seedlings were separately inoculated with the suspension of two rhizobial strains and grown under two irrigation regimes: 80

The present work was a study on the adverse effects of salinity on growth, nodulation, and some physiological parameters in 4 symbiotic combinations involving 2 Moroccan alfalfa (Medicago sativa L.) populations (Demnate and Tata) and 2 rhizobial strains (rhLAr 1 and rhLAr 4). The experiment was conducted in the greenhouse at 32/22 °C day/night, 50%-80% relative humidity, and a photoperiod of 16 h. The seedlings were separately inoculated with suspensions of 2 rhizobial strains and grown under 2 NaCl treatments, 0 mM (control) and 100 mM (salt stress), in plastic pots filled with sterile sand and peat at 9/10 and 1/10 ratios, respectively. The salt stress was applied for 5 weeks and some agro-physiological and biochemical parameters related to salt tolerance were assessed. The results showed that salinity significantly reduced the height of plants, their dry biomass, and nodulation. This constraint has also negatively affected the relative water content of leaves, the membrane permeability, the stomatal conductance, the maximum quantum yield of photosystem II, and the chlorophyll contents. Comparison among the symbiotic combinations tested showed that their behavior was significantly different. Plants inoculated with rhizobial strain rhLAr 4 were more tolerant to saline conditions. Their tolerance was associated with the maintaining of adequate levels in terms of physiological and biochemical parameters studied.

Despite the agronomical and environmental advantages of the cultivation of legumes, their production is limited by various environmental constraints such as water or nutrient limitation, frost or heat stress and soil salinity, which may be the result of pedoclimatic conditions, intensive use of agricultural lands, decline in soil fertility and environmental degradation. The development of more sustainable agroecosystems that are resilient to environmental constraints will therefore require better understanding of the key mechanisms underlying plant tolerance to abiotic constraints. This review provides highlights of legume tolerance to abiotic constraints with a focus on soil nutrient deficiencies, drought, and salinity. More specifically, recent advances in the physiological and molecular levels of the adaptation of grain and forage legumes to abiotic constraints are discussed. Such adaptation involves complex multigene controlled-traits which also involve multiple sub-traits that are likely regulated under the control of a number of candidate genes. This multi-genetic control of tolerance traits might also be multifunctional, with extended action in response to a number of abiotic constraints. Thus, concrete efforts are required to breed for multifunctional candidate genes in order to boost plant stability under various abiotic constraints.