Short-term studies on the uptake and transport of Cl− by soybean cultivars differing in salt tolerance

Short-term tracer experiments (³⁶Cl) were conducted with the differentially salt susceptible soybean cultivars “Lee” (moderately tolerant) and “Jackson” (sensitive) to elucidate the pattern of Cl^? uptake and translocation in relation to the physiology of salt tolerance. Rates of Cl^? uptake by excised roots of “Jackson” were much greater in the lower (0.1–0.5 mM NaCl) and particularly in the higher concentration range than by the more tolerant cultivar. The transfer rate to the shoot was significantly higher in “Jackson” than in “Lee” and increased with time of treatment. The cultivar “Lee” translocated a relatively high amount of Cl^? during the onset of salt treatment, but in contrast to “Jackson” was then able to slow down Cl^? translocation into the shoot to a degree about proportional to the increment of dry matter. In experiments on secondary translocation both cultivars extruded substantial amounts of ³⁶Cl^? to the nutrient solution during the period in inactive solution with constant salinity following labeling. Possibly, some Cl^? that had moved into the leaves during labeling was retranslocated and extruded via the roots. The absolute efflux rate was presumably greater for “Jackson” than for “Lee” although it appeared not efficient enough to compensate for the high rate of influx into the root. After 5–6 days of secondary translocation a lesser amount of Cl^? was shifted from the root to the shoot in “Lee” as compared with “Jackson”. Chloride accumulation in the upper root and lower stem, similar to that reported for Na⁺ in several Na⁺ excluding species, was not observed. From the results it may be concluded that the cultivar “Jackson” cannot sufficiently control the uptake of Cl^? and its translocation, particularly into the mature leaves; this contributes causally to the development of severe injury under continuous salt stress.