Effects of 2,4-Dichlorophenoxyacetic Acid, Antiauxins, and Metabolic Perturbations on Cytoplasmic and Vacuolar pH of Corn Root Tips Measured by in Vivo31P-NMR

Abstract

Cytoplasmic pH (pH cyt) and vacuolar pH (pH vac) of corn ( Zea mays L. cv Pioneer 3377) root tips measured by in vivo 31P-NMR were typically 7.6 and 5.5, respectively, and both were stable under the external pH variation (6.5 to 4.5). The treatment with 200 μ M 2,4-Dichlorophenoxyacetic acid (2,4-D) did not affect pH vac and ATP peaks on 31P-NMR spectra, but slightly reduced pH cyt. This suggests that 2,4-D accumulation occurs in cytoplasm rather than in vacuoles. Antiauxins 100 μ M p-chlorophenoxyisobutyric acid (PCIB) and 100 μ M N-1-naphthylphthalamic acid did not affect pH vac, pH cyt, nor ATP peaks. In contrast, anaerobic conditions and the treatment with an uncoupler, 50 μ M carbonylcyanide- m-chlorophenylhydrazone, caused a remarkable reduction in pH cyt with decreasing ATP peaks. The pH vac was relatively stable even under these conditions. These results support the hypothesis that metabolic inhibitors inhibit 2,4-D accumulation in plant cells by reducing pH cyt and an antiauxin, PCIB, inhibits 2,4-D accumulation without affecting pH cyt.