The regulation of K+ influx into roots of rye (Secale cereale L.) seedlings by negative feedback via the K+ flux from shoot to root in the phloem

Abstract

The uptake of K+ by plant roots is matched to the demand for K+ for growth. The growing shoot must communicate its K+ requirement to the root. It has been suggested that this might be effected by varying the amount of K+ retranslocated from the shoot to the root via the phloem. It is predicted that less K+ is returned to the roots in K+-deficient plants and that this promotes compensatory K+ uptake from the external medium. These experiments address this hypothesis. Rye (Secale cereale L.) was grown hydroponically in complete nutrient solutions containing either 100 μM or 400 μM K+. Plant development, shoot fresh weight (FW) and plant dry matter accumulation did not differ between seedlings grown at these K+ concentrations. However, root FW was lower in seedlings grown in solutions containing 100 μM K+, which resulted in a greater shoot/root FW ratio. Seedlings from both treatments had similar shoot K+ concentrations, but the root K+ concentration of seedlings grown in solutions containing 100 μM K+ was less than their counterparts grown at 400 μM K+. When assayed at the same K+ concentration, unidirectional K+ (86Rb+) influx into 14-d-old seedlings grown with 100 μM K+ in the nutrient solution was greater than that into seedlings grown with 400 μM K+ in the nutrient solution, indicating an increased K+ influx capacity in the former. Furthermore, K+ (86Rb+) influx into seedlings grown and assayed at 100 μM K+ was greater than that into seedlings grown and assayed at 400 μM K+. Since net K+ uptake was lower in the seedlings grown at 100 μM K+, this indicates a greater unidirectional K+ efflux from roots in solutions containing 100 μM K+. An empirical model, based on the immobility of calcium in the phloem, was used to describe quantitatively K+ fluxes in seedlings 14d after sowing. As primary data, the composition of xylem sap and the accumulation of elements in root and shoot tissues were determined. Xylem sap was collected either as root-pressure exudate or from excised roots immersed in nutrient solution and subjected to a pneumatic pressure of 0.4 MPa. The K : Ca ratio in these saps differed, and led to contrasting conclusions concerning the effect of K+ nutrition on the recirculation of K+. Based on the K : Ca ratio in the sap obtained following the application of pneumatic pressure, which is thought to resemble that of intact transpiring plants, it was calculated that the K+ flux from the shoot to the root was higher in seedlings maintained in solutions containing higher K+ concentrations. This suggests that a negative feedback mechanism based on K+ recirculation from the shoot to the root via the phloem could be a primary signal decreasing K+ influx.