| Salinity tolerance during germination and early seedling growth was evaluated for 24 accessions representing four wild Phaseolus species (P. angustissimus, P. filiformis, P. leptostachyus, and P. microcarpus) and four accessions of cultivated common bean ( P. vulgaris) at 0 to 180 mM NaCl. Wild Phaseolus filiformis accessions were the most tolerant to salinity stress, as indicated by rapid germination, relative stability, and greater seedling growth. In contrast, P. leptostachyus, P. microcarpus and P. angustissimus accessions were salt sensitive in terms of late germination, intermediate sensitivity index, and reduced seedling growth. Cultivated accessions of P. vulgaris had rapid germination, low sensitivity index, and intermediate seedling growth.; We further investigated salinity tolerance by studying the effect of NaCl during vegetative growth in 142 Phaseolus accessions. The greatest differences in relative tolerance to NaCl occurred in wild species of P. filiformis, P. macvaughii, P. micranthus, P. lunatus, and P. vulgaris and among accessions of cultivated species of P. acutifolius, P. coccineus, and P. lunatus. These species were able to grow and function normally at 180 mM NaCl. In contrast, P. leptostachyus, P. microcarpus, P. glabellus and P. oligospermus were grouped by cluster analysis, as salt-sensitive species unable to sustain growth under NaCl stress.; Salt-sensitive Phaseolus species were characterized by low relative growth rate (RGR). Salt stress significantly reduced RGR, unit leaf rate (ULR), and leaf area ratio (LAR). Unit leaf rate (ULR), but not LAR, was significantly correlated with RGR, indicating that ULR was an important factor underlying the salinity-induced differences in RGR among species. Partial stomatal closure accounted for nearly all of the reduction in the CO2 assimilation rate per unit leaf area. Leaf and osmotic potentials declined significantly as salinity stress conditions intensified. However, a positive turgor was maintained through the entire growth period. Therefore, in addition to osmotic effects and the inhibitory effects of high concentrations of Cl and Na, an imbalance of essential nutrients contributed to the reduction in plant growth under saline conditions. Salinity tolerance of P. filiformis was associated with Na exclusion, maintenance of higher leaf K concentrations, and osmotic adjustment. The results confirm that wild Phaseolus species, and in particular P. filiformis, represent a genetic resource for the improvement of salinity tolerance in common bean. |
