Microelectrode and 133Cs nuclear magnetic resonance evidence for variable cytosolic and cytoplasmic nitrate pools in maize root tips

The effect of nitrate uptake on the subcellular distribution of tissue nitrate in 2-5 mm maize root tips was investigated by two complementary methods. First a novel in vivo analysis using Cs-133 nuclear magnetic resonance (NMR) was used to demonstrate changes in the cytoplasmic and vacuolar pools during caesium nitrate uptake. This method depended on interpreting the nitrate-induced changes in the positions of the cytoplasmic and vacuolar caesium signals. The assignment of the signals was confirmed by using in vivo(39)K NMR to observe the displacement of cytoplasmic potassium into the vacuole during caesium uptake, and in vivo(133)Cs NMR to observe the displacement of cytoplasmic caesium into the vacuole during potassium uptake. Secondly nitrate-selective microelectrodes were used to quantify the change in the cytosolic nitrate activity that occurred in the outermost cells of root tips under the same conditions. Both methods showed that the detected nitrate pool increased over a period of 8-10 h in the presence of 10 mM nitrate and it is concluded that the data provide support for the view that homeostasis in the cytosolic and cytoplasmic nitrate pools is not necessarily an invariant characteristic of root tips.