31P NMR Measurements of the Cytoplasmic and Vacuolar Pi Content of Mature Maize Roots: Relationships with Phosphorus Status and Phosphate Fluxes

Abstract

The vacuolar and cytoplasmic inorganic phosphate (Pi) content of the mature regions of maize roots was measured by a 31P NMR technique which used an external standard to avoid the need for tissue extraction and which exploited the relatively rapid relaxation of cytoplasmic Pi in order to improve the detection of this pool in fully-vacuolated cells. In mature roots of maize growing with abundant external phosphate, the concentration of Pi in the cytoplasm was approximately 6.5 mol m−3. When these plants were deprived of external phosphate, the vacuolar Pi content of the roots decreased rapidly, but the cytoplasmic Pi concentration initially remained constant and did not begin to decline until P-stress became severe. Calculations show that withdrawal of Pi from the vacuoles into the cytoplasm under these conditions would be against an electrochemical gradient. During P-starvation, an increased capacity for Pi influx developed, preceding any detectable change in the cytoplasmic Pi content of the roots. This response is considered in terms of parallel effects on transport sites for phosphate at the plasmalemma and at the tonoplast. Comparisons of simultaneous rates of influx and net uptake implied that phosphate efflux accounted for < 10% of influx in plants of a steady or declining P-status. However, direct measurements of efflux suggested that this process may be temporarily accelerated when plants are recovering from P-stress.