Exogenous proline mitigates the inhibition of growth of Nicotiana tabacum cultured cells under saline conditions

The addition of exogenous proline (10 mM) to Na100-saline culture medium, m odified LS medium (Linsmaier and Skoog 1965: Physiol. Plant., 18, 100-127) with 100 mM NaCl promoted the growth of tobacco (Nicotiana tabacum L., cv. Bright Yellow-2) suspension cells unadapted to salt stress without maintain ing a high ratio of K+ to Na+ ions under salinity conditions. The addition of exogenous glutamic acid or alanine were not comparable to that of exogen ous proline. The proline contents of the NaCl-unadapted cells became much h igher when the cells were grown in Na100-saline culture medium with 10 mm p roline than when the cells were cultured without proline. The accumulation of K+, Na+, and counter ions was sufficient to compensate for the increase of the water potential of the cells caused by salinity. These results sugge st that exogenous proline does not act as a nitrogen store and that proline may act as a protectant for enzymes and membranes against salt inactivatio n rather than as a compatible solute in tobacco suspension cells.