Enhancement of NaCl tolerance in Arabidopsis thaliana by exogenous L‐asparagine and D‐asparagine

The tolerances of Columbia Arabidopsis thaliana (L.) Heynh. to NaCl, L‐asparagine (L‐Asn) and D‐asparagine (D‐Asn) during seedling establishment on sterile agar medium were determined. Germination and the establishment of upright seedlings with expanded green cotyledons were increasingly inhibited by NaCl concentrations from 20 to 180 mM and radicle growth was prevented at 225 mM NaCl. Tolerance of established seedlings to NaCl was similar at these concentrations. Seedling establishment was prevented at 20 mM L‐Asn and 60 mM D‐Asn, but L‐Asn was not toxic to established seedlings. At lower concentrations, exogenous L‐ and D‐Asn enhanced NaCl tolerance during germination and seedling establishment. Inhibition of seedling establishment by NaCl concentrations below 225 mM was reduced by the addition of L‐ and D‐Asn to the medium. Maximal reduction of NaCl inhibition occurred between 2 and 4 mM for both L‐ and D‐Asn. Higher concentrations of NaCl prevented establishment whether exogenous Asn was present or not. Reduction of NaCl inhibition occurred to the same extent whether L‐Asn was presented simultaneously with the NaCl or preloaded for up to 24 h. The total seedling content of Na+ increased about 4‐fold to 55 μg (mg dry weight)−1 as the medium concentration of NaCl was increased from 9 μM to 150 mM NaCl. Total K+ content declined about 80% from about 34 μg (mg dry weight)−1 over the same range of NaCl concentrations. The Na+ uptake and K+ efflux by whole seedlings were similar whether or not NaCl tolerance was increased by exogenous Asn.