The regulation of K+influx into roots of rye (Secale cerealeL.) 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 aM 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 14 d 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.