The evaluation of biochemical markers is important for the understanding of the mechanisms of tolerance to salinity of Phaseolus beans. We have evaluated several growth parameters in young plants of three Phaseolus vulgaris cultivars subjected to four salinity levels (0, 50, 100, and 150 mM NaCl); one cultivar of P. coccineus, a closely related species reported as more salt tolerant than common bean, was included as external reference. Biochemical parameters evaluated in leaves of young plants included the concentrations of ions (Na+, K+, and Cl−), osmolytes (proline, glycine betaine, and total soluble sugars), and individual soluble carbohydrates. Considerable differences were found among cultivars, salinity levels, and in their interaction for most traits. In general, the linear component of the salinity factor for the growth parameters and biochemical markers was the most important. Large differences in the salinity response were found, with P. vulgaris cultivars “The Prince” and “Maxidor” being, respectively, the most susceptible and tolerant ones. Our results support that salt stress tolerance in beans is mostly based on restriction of Na+ (and, to a lesser extent, also of Cl−) transport to shoots, and on the accumulation of myo-inositol for osmotic adjustment. These responses to stress during vegetative growth appear to be more efficient in the tolerant P. vulgaris cultivar “Maxidor”. Proline accumulation is a reliable marker of the level of salt stress affecting Phaseolus plants, but does not seem to be directly related to stress tolerance mechanisms. These results provide useful information on the responses to salinity of Phaseolus.

Genetic improvement of crop drought tolerance has become an urgent need for increasing agricultural yields and food production, to feed a growing human population in the context of global climate change. To get insights into the most relevant mechanisms underlying drought resistance in beans, we have analysed the responses to water deficit of three Phaseolus vulgaris (common bean) and one P. coccineus (runner bean) cultivars, focusing on the accumulation of specific osmolytes, a conserved response to abiotic stress in plants. Changes in osmolyte levels were correlated with the relative tolerance to water stress of the studied cultivars. Drought tolerance in Phaseolus largely depends on the accumulation of myo-inositol; glycine betaine may also contribute to tolerance in P. coccineus (but not in P. vulgaris). Proline, another common osmolyte, is a reliable marker of the level of stress affecting bean plants, but is not directly involved in tolerance mechanisms, as its drought-dependent accumulation is lowest in the most tolerant cultivar. We suggest that, by measuring the levels of proline and myo-inositol in water-stressed plants, a large number of cultivars could be easily and rapidly screened to select promising candidates to be used in breeding programmes for improving drought tolerance in beans.