Salinity reduces membrane-associated calcium in corn root protoplasts.

Calcium is an important factor in the ability of plants to resist salt stress, possibly because of its role in maintaining membrane integrity. We studied the effects of NaCl stress on membrane-associated Ca in corn root protoplasts (Zea mays L. cv Pioneer 3377) using the fluorescent Ca probe chlorotetracycline (CTC). Protoplasts were isolated from the cortex of primary roots of corn seedlings (Gronwald and Leonard, Plant Physiol 1982 70: 1391-1395). After a 30 minute incubation in 50 micromolar CTC, the protoplasts were exposed to isosmotic treatment solutions containing various concentrations of NaCl just before fluorimetric analysis. Increasing NaCl concentrations caused a progressive reduction in net CTC fluorescence, to 50% of control values at 150 mm NaCl. NaCl did not displace CTC from the cells, nor did it directly interfere with Ca-CTC binding. Tests with CsCl, RbCl, KCl, LiCl, Na(2)SO(4), NaNO(3), and NaBr indicated that the reduction in CTC fluorescence was not specific to either Na or Cl, but may have been due to increased ionic strength of the treatment solutions. Like CTC fluorescence, root growth of intact corn seedlings was not specifically sensitive to Na, but was inhibited by several monovalent cations in the order Li > Cs >> Rb > Na > K. CTC fluorescence at 100 mm NaCl was restored to unstressed levels by increasing Ca concentrations. Since our salt treatments were isosmotic, we conclude that the ionic component of salt stress displaces Ca from membranes of corn root cells.