Influence of Nitrogen Form on Extracellular pH and Bentazon Uptake by Cultured Soybean (Glycine max) Cells

Abstract

The influence of nitrogen form on bentazon uptake and toxicity to soybean cell cultures and medium pH was examined. Soybean cell growth was completely inhibited at 40 μM bentazon when cells were cultured in a Murashige and Skoog medium containing 20 mM ammonium and 38 mM nitrate. However, bentazon, up to 100 μM, did not affect the growth of cells cultured in B5 medium (Gamborg B5 medium containing 2 mM ammonium and 25 mM nitrate). Twenty-minute14C-bentazon uptake experiments on 48- and 96-h-old cells demonstrated that14C uptake was nitrogen-type dependent, with cells cultured in AMM medium (B5 medium containing 38 mM nitrate and 12 mM ammonium) accumulating 5 to 10 times more14C than those in B5 or NIT media (B5 containing 50 mM nitrate). Differential bentazon uptake between ammonium and nitrate-containing media appears to be attributable to the effect of nitrogen form on pH. From an initial pH of 5.5, the pH of AMM medium decreased to 4 by 48 h. The pH of NIT or B5 medium increased to approximately 7 from 5.5. Changes in pH are likely attributable to cellular maintenance of ionic balance; NH4+and NO3−uptake results in H+ and OH- efflux, respectively. Methylamine at 12 mM did not result in a decrease in medium pH. Bentazon, a weak acid (pKa 3.3), would be expected to penetrate the plasma-membrane faster in the molecular form at low pH. Support for a nitrogen pH theory is provided by the finding that14C-bentazon uptake was similar for ammonium and nitrate-containing media when media were buffered.