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 freshweight growth and on carbon dioxide influx and efflux were examinedin young plants of lucerne (Medicago sativa L.) that had beengrown in solution culture with nitrogen supplied either as nitrateor by a symbiotic rhizobium. Although the inoculated plantsgrew more slowly than those supplied with nitrate, NaCl at alevel equivalent to an osmotic stress of –0.3 MPa didnot reduce the growth rate of either type of plant under a 12h day-length in a growth chamber. With a day-length of 5 h saltstress (0.0 to –0.6 MPa) did not greatly affect grossphotosynthesis of plants grown on nitrate but respiration ratereached a maximum at –0.3 MPa and declined at larger saltconcentrations. Salt diminished both gross and net photosynthesisin the inoculated plants at a 5 h day-length without stimulatingrespiration. The relationship between photosynthesis and respiration as thephoton flux density was successively decreased was used to inferthe effect of salt on maintenance respiration of the plantssupplied with nitrate. Growth and maintenance components ofrespiration could not clearly be separated in the inoculatedplants suggesting that these were unable to maintain themselvesunder 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 adaptto these conditions could be attributed to the greater demandfor assimilates by the rhizobium. Key words: Medicago sativa, Nitrogen source, Salt stress