Influence of light and darkness and the experimental design on kinetics of K+ (86Rb) influx in roots of intact spring wheat

Seedlings of spring wheat ( Triticum aestivum L. cv. Svenno) were cultivated at 20°C in continuous light or darkness with the roots in nutrient solutions for six days. The plants were starved for K⁺ during different periods of time to produce plants with various K⁺ status. In one cultivation light-grown plants were pretreated in darkness, and vice versa, before the uptake experiment. In all experiments, roots were put in a complete nutrient medium containing 2.0 m M K⁺ radiolabelled with ⁸⁶Rb. The uptake time was varied (5, 60 or 120 min).
The K⁺ concentration in the roots, K⁺]_root, increased during the course of the uptake experiments, especially in light and at initially low K⁺]_root, At the same time K⁺ (⁸⁶Rb) influx in the roots decreased. The simoidal relationship obtained between K⁺ (⁸⁶Rb) influx and K⁺]_root was affected by these changes, and Hill plots gave various Hill coefficients, n_H, depending on the duration of the uptake experiments. n_H from three apparently straight line segments of the same plot, in different K⁺]_root - intervals, indicated a falling degree of interaction between the binding sites as K⁺]_root increased. For the dark-grown plants negative cooperativity could not be demonstrated.