Osmotic stress affects water relations, growth, and nitrogen fixation in Phaseolus vulgaris plants

The effects of mannitol-mediated osmotic stress on water relations, plant growth and symbiotic N-2-fixation in four common bean (Phaseolus vulgaris) lines (Coco Blanc, BAT 477, BRB 77 and Flamingo) were studied. After germination, seedlings were inoculated with a reference strain (Rhizobium tropici CIAT 899) and aerohydroponically grown in a glasshouse. Osmotic stress was applied by 50 mM mannitol. Plants were harvested 4 weeks after osmotic stress application. Measured parameters were plant water relations, growth, nodule development, and symbiotic N-2-fixation (SNF) as well as leghemoglobin contents. Osmotic stress induced significant changes in water relations, growth and symbiotic N-2-fixation in stressed plants compared to control ones in all lines studied. A noticeable different behaviour was observed in the end of the treatment: Flamingo was the most tolerant line, whereas Coco blanc was the most sensitive, the two other lines exhibited an intermediate behaviour. The four bean lines displayed significant differences in their responses to osmotic stress. This study indicated that the relative tolerance of Flamingo line seems to be due to its ability to maintain higher leaf water potential, adequate leaf area and abundant and efficient nodular system, which in turn determines an important rate of SNF.