Growth and physiological responses of Phaseolus spp. under salinity

To study the behavior of physiological parameters and plant growth analysis under salt stress, Phaseolus species were grown under greenhouse conditions, in nutrient solution at four different salinity levels (0, 30, 60, and 90 mM NaCl). Relative growth rate (RGR; g g(-1) d(-1)), net assimilation rate (NAR; g m(-2) d(-1)), leaf area ratio (LAR; m(2) g(-1)), specific leaf area (SLA; m(2) g(-1)) and leaf weight ratio (LWR; g g(-1)) were calculated for 20 d. The increase in salinity significantly reduced RGR, NAR, LAR and SLA, without affecting LWR. In all species, except in P. filiformis, NAR, but not LAR, was significantly correlated with RGR, thus indicating that NAR is an important factor underlying the salinity-induced differences among species. In P. filiformis neither NAR nor LAR were affected by increased salinity, and consequently RGR was maintained. Increasing salinity progressively decreased the stomatal conductance. The CO2 assimilation rate decreased gradually with salinity, but significant reductions were showed only at the highest salt levels (90 mM NaCl). In P. filiformis, however, neither stomatal conductance nor CO2 assimilation were affected by salt stress. Leaf water and osmotic potentials declined significantly as stress conditions intensified, but osmotic adjustment permitted the maintenance of positive turgor throughout the growth period. Salinity increased the concentration of total carbohydrates in leaves and stems; this carbohydrate accumulation was associated to the osmotic adjustment at the initiation or the saline period. Salt tolerance in P. filiformis is associated to a better stomatal control and photosynthetic efficiency through osmotic adjustment.