Ion transport and sub‐cellular compartmentation in maize root tissue as examined byin vivo133Cs nmr spectroscopy

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Abstract
133Cs NMR has been used to examine the intracellular and extracellular pools of Cs+ containing, and CsCl‐perfused, excised maize seedling roots. The 133Cs spectrum is sensitive to ionic environment and the presence of protein. In the absence of shift reagents, resonances corresponding to sub‐cellularly compartmented Cs+, in addition to extracellular Cs+, were observed. The designated assignments for each intracellular shift were established from chemical shift, spin lattice relaxation times and compartment volume ratios for different root tissue sections. Influx studies of the perfused maize tips showed a rapid uptake of Cs+ into the cytoplasm followed by a much slower movement into the vacuole. The rate constant for the cytoplasmic efflux was found to match the rate constant corresponding to vacuolar influx. Overall, Cs+ influx into the tissue was suppressed under hypoxia. Perfusion of Cs+ ‐containing tips with K+ demonstrated that cytoplasmic Cs+ exchanged directly for K+ while vacuolar Cs+ levels remained constant