Initial attempts to measure K+ activity (aK) in vacuoles of barley leaf epidermal cells using triple-barrelled K+- selective microelectrodes gave values that were only about one-third of those expected. This was due to high (c. 200 mM) NO-3 concentrations in the vacuoles interfering with the K+-sensor. The effect of NO-3 was on 1,2,-dimethyl-3-nitrobenzene (DNB) used as a plasticizer in the K+-sensor. Replacing DNB with dibutyl sebacate, but not with 2-nitrophenyl octyl ether, overcame this problem and the modified sensor gave acceptable calibration curves with no interference elecfrom physiological concentrations of other ions. These far electrodes were used successfully to measure a mean aK of 235 mM in vacuoles of epidermal cells of K+- replete barley leaves.
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Cuin et al. (1999) studied this question.