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Measurement of cytosolic nitrate is one of the factors required for the resolution of factors controlling nitrate uptake and assimilation in plants and for identifying likely nitrate transport mechanisms at both the plasma membrane and tonoplast. This paper reviews methods and reported measurements of cytosolic nitrate in higher plants and concludes that nitrate-selective microelectrodes are the best approach. These microelectrodes have been used to measure intracellular nitrate activities in barley and maize root cells. Triplebarrelled electrodes, incorporating a pH-sensing barrel have been used to identify the compartmental location of the nitrate-selective tip giving unequivocal estimates of vacuolar and cytosolic nitrate activities. The microelectrode measurements are used to discuss the possible mechanisms of nitrate transport at both the tonoplast and plasma membrane. The energetics of possible proton-coupled transport systems are described and the feasibility of the mechanism is discussed.
Miller et al. (Mon,) studied this question.