Effect of sodium chloride stress and nitrogen source on respiration, growth and photosynthesis in lucerne (Medicago sativa L.)

The effects of salinity at different light intensities on fresh weight growth and on carbon dioxide influx and efflux were examined in young plants of lucerne (Medicago sativa L.) that had been grown in solution culture with nitrogen supplied either as nitrate or by a symbiotic rhizobium. Although the inoculated plants grew more slowly than those supplied with nitrate, NaCl at a level equivalent to an osmotic stress of -0.3 MPa did not reduce the growth rate of either type of plant under a 12 h day-length in a growth chamber. With a day-length of 5 h salt stress (0.0 to -0.6 MPa) did not greatly affect gross photosynthesis of plants grown on nitrate but respiration rate reached a maximum at -0.3 MPa and declined at larger salt concentrations. Salt diminished both gross and net photosynthesis in the inoculated plants at a 5 h day-length without stimulating respiration.The relationship between photosynthesis and respiration as the photon flux density was successively decreased was used to infer the effect of salt on maintenance respiration of the plants supplied with nitrate. Growth and maintenance components of respiration could not clearly be separated in the inoculated plants suggesting that these were unable to maintain themselves under the combined stresses of salt and low light intensity. This view was supported by chemical analysis of the plant material. We conclude that the failure of the inoculated plants to adapt to these conditions could be attributed to the greater demand for assimilates by the rhizobium.