Abstract
The absorption of 2, 4-D by protoplasts enzymatically isolated from the cortex of corn (Zea ways L. cv Pioneer 3377) root was investigated in relation to their extracellular and intracellular pH. The 2, 4-D absorption increased as the extracellular pH (pHo) decreased. The ratio of the 2, 4-D concentration inside to outside the protoplasts (ci/co) reached ca. 1.2, 4.0, 10-12 and 30-40 at pHo 7.5, 6.5, 5.5 and 4.5 respectively, within 75min incubation. At pHo 3.5 and 2.8, ci/co ratio reached ca. 90 and 150 respectively within 15min, but the absorbed 2, 4-D was rapidly released after 15min incubation due to the death of the protoplasts. The determination of the overall intracellular pH (pHi) by the 5, 5-dimethyloxa-zolidine-2, 4-dione distribution technique indicated that the change of the pHi as influenced by the pHo variation (7.5 to 4.5) was relatively small (6.9 to 6.6). Using the measured pHi, the theoretical ci/co ratio at each pHo was estimated based on the weak acid hypothesis. Although the pHo dependence of the theoretical ci/co roughly resembled the observed tendency, the measured ci/co ratios at pHo 6.5 and 7.5 were significantly greater than their respective theoretical maxima. These results suggest that the absorption of 2, 4-D by protoplasts cannot be accounted for simply by the weak acid hypothesis and that an active transport is involved at least at pHo 6.5 and 7.5.
