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Global climate change tends to intensify water unavailability, especially in semi-arid regions, directly impacting agricultural production. Cowpea is one of the crops with great socio-economic importance in the Brazilian semi-arid region, cultivated mainly under rainfed farming and considered moderately tolerant to water restriction. This species has physiological and biochemical mechanisms of adaptation to these stress factors, but there is still no clear vision of how these responses can not only allow survival, but also ensure yield advances in the field. Besides acclimation mechanisms, the exogenous application of abiotic (salicylic acid, silicon, proline, methionine, and potassium nitrate) and biotic (rhizobacteria) elicitors is promising in mitigating the effects of water restriction. The present literature review discusses the acclimation mechanisms of cowpea and some cultivation techniques, especially the application of elicitors, which can contribute to maintaining crop yield under different water scenarios. The application of elicitors is an alternative way to increase the sustainability of production in rainfed farming in semi-arid regions. However, the use of eliciting substances in cowpea still needs to be carefully explored, given the difficulties caused by genotypic and edaphoclimatic variability under field conditions.

Silicon and proline play important physiological, metabolic and functional roles in plants, especially under water deficit conditions. Their application can mitigate the adverse effects of stress in crops by increasing water use efficiency and antioxidant activity. The objective of this study was to evaluate silicon (Si) as attenuator of the effects of water deficit on cowpea cultivars, through physiological, biochemical and growth indicators. The experimental design was randomized blocks with four cultivars (BRS Guariba, BRS Itaim, BRS Aracê and BRS Rouxinol) and four irrigation treatments associated or not with Si application (W100 - 100% ETo; W50 - 50% ETo; W50+100Si and W50+200Si, with 100 and 200 mg L-1 silicon, respectively), in a 4 x 4 factorial scheme, with five replicates. Leaf water potential, proline concentration, antioxidant enzymes and growth indicators were evaluated in cowpea plants. Under water deficit conditions, all cultivars showed reductions in leaf water potentials, which compromised plant growth. However, Si applications of 200 mg L-1 in the cultivar BRS Guariba and 100 and 200 mg L-1 in the cultivar BRS Itaim minimized the effects of stress, by increasing leaf water potential and the activity of the enzyme ascorbate peroxidase, in both cultivars, besides increasing proline concentration in the former and reducing proline concentration in the latter, which ensured the maintenance of growth. Despite the no contribution to the increase in water potential, Si applications of 100 and 200 mg L-1 in BRS Rouxinol and BRS Aracê, respectively, reduced the deleterious effects of the stress on their growth by regulating the enzymatic metabolism and proline.

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Salinity in soil can affect Jatropha seedling metabolism, interfering with plant establishment. In this study, the effect of salinity on the mobilization of reserves during the development of Jatropha seedlings was tested. Two genotypes of Jatropha were used and three concentrations of NaCl were applied between the 4th and 8th days after germination. The effects of salinity on seedling growth, in terms of fresh and dry phytomass, ionic partition, and sugar quantification, starch, proteins, amino acids, and lipids were evaluated in cotyledon leaves, hypocotyls, and roots. There was an increase in the content of all classes of macromolecules analyzed in at least one of the organs. It is hypothesized that the hypocotyls acted as an accumulating organ of Na+. The accumulations of amino acids and protein in roots suggest that metabolic responses occurred in response to the ionic and osmotic effects of NaCl, although this accumulation did not appear to prevent biomass losses in seedlings. Furthermore, the findings of this study demonstrate that salinity inhibits the mobilization of lipids and carbon stocks from cotyledon leaves to the rest of the plant, and together with the synthesis of proteins and amino acids that occurred primarily in roots, contributed to response of these plants to salinity.

ABSTRACT Dry land systems spread all over the world and comprise 41.3% of the terrestrial area, which host 34.7% of the global population, so it is convenient to propose crops able to grow there. Jatropha curcas is a plant adapted to arid and semiarid regions as well as sub-humid conditions, being a potential source of biodiesel. The challenge is to understand the physiology of J. curcas, which enables it to live under saline and drought conditions. The seeds of J. curcas used came from Ciego de Ávila Province, Cuba. Seven-day-old seedlings were cultivated in 1.5 L pots with half strength Hoagland solution for 42 days under semi-controlled conditions. NaCl added to solutions in pots provided 75 or 150 mM treatments for 240 h before measurements. Leaf growth, net photosynthesis and stomatal pore area were affected by 150 mM NaCl. Non-photochemical quenching of leaves was only changed by 150 mM NaCl after 24 h; the electron transport rate had a tendency to decrease in leaves under saline conditions. The gene expression pattern changed for SOS1 and HKT1 according to the NaCl used in the medium, indicating active mechanism to deal with Na+ in the cell. In general, Cuban J. curcas plants were able to grow and perform photosynthesis under 75 mM NaCl, which represents 7 dS m-1, a condition that restricts growth for many plant species.

Exogenous applications of chemical compounds stimulate changes in plant metabolism and promote tolerance to different environmental stresses, including water deficit. This study aimed to evaluate the ability of salicylic acid (SA) and proline (PRO) to reduce water stress in a traditional variety of cowpea in a typical Brazilian semiarid climate. A completely randomized design was used in a 2 × 4 factorial scheme, with five replicates. Two irrigation regimes were evaluated corresponding to 100% (W100) and 50% of daily evapotranspiration (W50), respectively, with the addition of the following four attenuators: control (distilled water), SA (550 mg L-1), PRO (690 mg L-1), and 690 mg L-1 PRO + 550 mg L-1 SA. The treatments promoted changes in osmotic and antioxidant metabolism, which may contribute to the tolerance mechanisms of cowpea plants to water stress. The application of SA increased osmoregulator synthesis and protein concentrations, and modulated antioxidant enzyme activity in the cowpea plants under water stress. PRO concentrations increased synergistically in plants treated with PRO and SA, particularly in 50% of water replacement.

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