Abstract:
Toxic and osmotic effects of NaCl on growth and nodulation of alfalfa plants (Medicago sativa L. cv. Gilboa) were studied in a controlled environment in a greenhouse. PEG (polyethylene glycol), NaCl, KCl, Na2SO4, K2SO4 or CaCl2, all at 130 mOsm, had reduced plant dry matter production by 50% after 55 days of growth. Nodule number was mainly affected by the toxicity of the salts, rather than by the osmotic stress induced by the PEG treatments. Delaying NaCl application for 33 days resulted in increased nodule number and nodule weight as compared with early salt application. Nitrogenase specific activity and nodule development (as defined by the average weight of one nodule), were not significantly affected by any of the above iso-osmotic treatments applied either early or late, but nodule weight was affected by PEG. The reduction in nodule number, induced by the above salts at 250 mOsm in the root zone, was 70, 65, 55, 22 and 2% for Na2SO4, NaCl, KCl, K2SO4 and CaCl2, respectively. Light microscopy study of roots of 4-day-old seedlings grown on Fahraeus slides showed that, at 200 mOsm NaCl, there was a significant reduction in root-hair length and in the proportion of root hairs containing an infection thread. This effect was not observed with PEG. The results indicate that under salt levels up to 200 mOsm (90 mM NaCl), initiation of alfalfa nodules is more sensitive to the toxic effects of sodium and chloride ions than to the osmotic stress induced by the salt.
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