Effect of ammonium on the regulation of sodium transport in Helianthus annuus

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Abstract
SUMMARY: The electrical potential difference between exuding sap of detopped sunflower plants and rooting media containing different NH4 ion concentrations was measured, together with the Na and K concentrations in root tissue and their fluxes in the xylem exudate. It was found that adding NH4 ions to the medium made the electrical potential difference less negative with respect to the external solution and decreased the water conductivity of roots. Moreover, fewer K and Na ions were transported to the xylem sap and the K concentration in root tissue after 3 h had decreased whereas that of Na had increased. These results suggest that the sunflower root acts as an efficient accumulator for Na rather than simply as a barrier to Na transport. It is suggested that NH4 ions caused a decrease in charge separation across one of the barriers where Na is actively transported so that adding NH4 ions to the medium increased Na accumulation in root cells. This barrier may have been at the symplast.