Relationship between Intracellular pH and 2, 4-D Absorption by Corn Root Protoplasts under the Influence of Metabolic Inhibitors and Antiauxins: Influence of Intracellular pH on Absorption of 2, 4-D by Corn Root Protoplasts (Part 2)

The effects of metabolic inhibitors and antiauxins on the absorption of 2, 4-D by protoplasts enzymatically isolated from the cortex of corn (Zea mays L. cv Pioneer 3377) root were investigated in relation to their effects on the intracellular pH (pHi). Five metabolic inhibitors with different types of action inhibited 2, 4-D absorption by the protoplasts. All of these chemicals reduced the pHi measured by the 5, 5-dimethyloxazolidine-2, 4-dione distribution technique, and inhibition of the 2, 4-D absorption was closely correlated with the reduction in pHi regardless of their type of action. This suggests that the pH gradient across the plasmalemma is a crucial factor for 2, 4-D absorption. In contrast, an antiauxin, p-chlorophenoxyisobutyric acid (PCIB), inhibited 2, 4-D absorption without affecting pHi. Another antiauxin, N-1-naphthylphthalamic acid (NPA), did not affect either 2, 4-D absorption or pHi. These results suggest that the mechanism of 2, 4-D absorption by the corn root protoplasts is composed of two components: 1) the passive diffusion of the undissociated 2, 4-D molecules according to the weak acid hypothesis and 2) the active transport of dissociated 2, 4-D anions mediated by a PCIB-sensitive and NPA-insensitive H+/auxin anion cotransport carrier.