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Cowpea is widely cultivated in arid and semi‐arid regions of the world where salinity is a major environmental stress that limits crop productivity. The effects of moderate salinity on growth and photosynthesis were examined during the vegetative phase of two cowpea cultivars previously classified as salt‐tolerant (Pitiúba) and salt‐sensitive (TVu). Two salt treatments (0 and 75 mm NaCl) were applied to 10‐day‐old plants grown in nutrient solution for 24 days. Salt stress caused decreases (59 % in Pitiúba and 72 % in TVu) in biomass accumulation at the end of the experiment. Photosynthetic rates per unit leaf mass, but not per unit leaf area, were remarkably impaired, particularly in TVu. This response was unlikely to have resulted from stomatal or photochemical constraints. Differences in salt tolerance between cultivars were unrelated to (i) variant patterns of Cl− and K+ tissue concentration, (ii) contrasting leaf water relations, or (iii) changes in relative growth rate and net assimilation rate. The relative advantage of Pitiúba over TVu under salt stress was primarily associated with (i) restricted Na+ accumulation in leaves paralleling an absolute increase in Na+ concentration in roots at early stages of salt treatment and (ii) improved leaf area (resulting from a larger leaf area ratio coupled with a larger leaf mass fraction and larger specific leaf area) and photosynthetic rates per unit leaf mass. Overall, these responses would allow greater whole‐plant carbon gain, thus contributing to a better agronomic performance of salt‐tolerant cowpea cultivars in salinity‐prone regions.

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-We have previously demonstrated that salt tolerance in cowpea could be associated with lesser impairments of the photosynthetic capacity. Taking into account that photosynthesis is the main sink for reducing power consumption, our central working hypothesis is that a salt-sensitive cultivar is more prone to suffer from oxidative stress. We analyzed the long-term effects of salt stress on oxidative damage and protection against reactive oxygen species in both leaves and roots of a salt-tolerant (Pitiúba) and a salt-sensitive (TVu) cowpea cultivar. Two salt treatments (0 and 75 mM NaCl) were applied to 10-day-old plants grown in nutrient solution for 24 days. Significant salt-induced oxidative damage as demonstrated via increases in malondialdehyde concentration were noted, particularly in leaves at the end of the experiment, although such damage was found earlier in Pitiúba. In salt-stressed plants, superoxide dismutase (SOD) activity increased only in Pitiúba at 24 days from the start of salt additions (DSSA). In Pitiúba, catalase (CAT) was not significantly affected by the treatments, whereas in TVu its activity was dramatically lower in salt-stressed plants at 10 DSSA onwards. In general salt stress led to significant increases, much more pronounced in ascorbate peroxidase (APX), glutathione reductase (GR) and guaiacol peroxidase (GPX), at the end of the experiment in both cultivars. In roots, salt-induced increases in enzyme activities were particularly noted at 24 DSSA, as found for SOD and APX in Pitiúba, CAT in TVu and GR and GPX in both cultivars. Therefore, in contrast to our expectations, the present results argue, to a great extent, against a functional link between salt stress tolerance and the expression of the antioxidant system. We also demonstrated that leaves and roots should be evaluated for a full assessment of whole plant acclimation to salt stress.

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Under salinity stress, plants commonly accumulate carbohydrates for osmotic adjustment to balance the excess accumulated ions and to protect biomolecules. We selected two cowpea cultivars with contrasting response to salinity, Pitiúba (salt-tolerant) and TVu (salt-sensitive), to investigate whether the salt tolerance could be associated with changes in carbohydrate accumulation and metabolism in leaves and roots during a long-term experiment. Two salt treatments (0 and 75 mM NaCl) were applied to 10-day-old plants grown in nutrient solution for 24 days. Despite some changes in carbohydrate accumulation and carbohydrate metabolism enzymes induced by salt stress, no consistent alterations in carbohydrates could be found in leaves or roots in this study. Therefore, we suggest that tolerance to salt stress is largely unrelated to carbohydrate accumulation in cowpea.

Resumo Com o objetivo de avaliar os efeitos da salinidade da água de irrigação e doses de biofertilizante, sobre o desenvolvimento de plantas de sorgo [Sorghum bicolor (L.) Moench.] cv. BRS Ponta Negra, desenvolveu-se este trabalho em casa de vegetação. As plantas foram cultivadas em vasos contendo 23 kg de solo arenoso. Foram testados quatro níveis de salinidade: 0,2; 2,0; 4,0 e 6,0 dS m-1. Para a obtenção dos níveis de salinidade, utilizouse da água de açude adicionada com sais de NaCl, CaCl22H2O e MgCl26H2O, na proporção de 7:2:1. As doses de biofertilizante foram 75; 150; 225 e 300 L ha-1. O delineamento experimental foi o inteiramente casualizado, com quatro repetições, em esquema fatorial 4 x 4. Foram avaliadas as seguintes variáveis: massa seca total; diâmetro do colmo; altura das plantas; área foliar total; suculência foliar; massa específica foliar; teores de sódio, potássio, cloro, cálcio, magnésio e fósforo, nos colmos + bainhas e limbos foliares; teores de prolina e carboidratos nas folhas. Todas as variáveis analisadas foram afetadas negativamente pelo aumento da salinidade da água de irrigação. O aumento da concentração de biofertilizante, nos limites empregados no presente estudo, não minorou os efeitos da salinidade no crescimento de plantas de sorgo.

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