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

The uptake of K⁺ by plant roots is matched to the demand forK⁺ for growth. The growing shoot must communicate its K⁺ requirementto the root. It has been suggested that this might be effectedby varying the amount of K⁺ retranslocated from the shoot tothe root via the phloem. It is predicted that less K⁺ is returnedto the roots in K⁺-deficient plants and that this promotes compensatoryK⁺ uptake from the external medium. These experiments addressthis hypothesis. Rye (Secale cereale L.) was grown hydroponically in completenutrient solutions containing either 100 aM or 400 µMK⁺. Plant development, shoot fresh weight (FW) and plant drymatter accumulation did not differ between seedlings grown atthese K⁺ concentrations. However, root FW was lower in seedlingsgrown in solutions containing 100 µM K⁺, which resultedin a greater shoot/root FW ratio. Seedlings from both treatmentshad similar shoot K⁺ concentrations, but the root K⁺ concentrationof seedlings grown In solutions containing 100 µM K⁺ wasless 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 µMK⁺ in the nutrient solution was greater than that into seedlingsgrown with 400 µM K⁺ in the nutrient solution, indicatingan increased K⁺ influx capacity in the former. Furthermore,K⁺ (⁸⁶Rb⁺) influx into seedlings grown and assayed at 100 µMK⁺ was greater than that into seedlings grown and assayed at400 µM K⁺. Since net K⁺ uptake was lower in the seedlingsgrown at 100 µM K⁺, this indicates a greater unidirectionalK⁺ efflux from roots in solutions containing 100 µM K⁺. An empirical model, based on the immobility of calcium in thephloem, was used to describe quantitatively K⁺ fluxes in seedlings14 d after sowing. As primary data, the composition of xylemsap and the accumulation of elements in root and shoot tissueswere determined. Xylem sap was collected either as root-pressureexudate or from excised roots immersed in nutrient solutionand subjected to a pneumatic pressure of 0.4 MPa. The K:Ca ratioin these saps differed, and led to contrasting conclusions concerningthe effect of K⁺ nutrition on the recirculation of K⁺. Basedon the K:Ca ratio in the sap obtained following the applicationof pneumatic pressure, which is thought to resemble that ofintact transpiring plants, it was calculated that the K⁺ fluxfrom the shoot to the root was higher in seedlings maintainedin solutions containing higher K⁺ concentrations. This suggeststhat a negative feedback mechanism based on K⁺ recirculationfrom the shoot to the root via the phloem could be a primarysignal decreasing K⁺ influx. Key words: K⁺ influx, K⁺ recirculation, regulation, root, rye, Secale cereale L