K + Gradients in the Pulvinus of Phaseolus coccineus during Leaf Movement*

Abstract

We used quantitative histochemistry to investigate the tissue-specific compartmentation of potassium in the laminar pulvinus of Phaseolus coccineus L. at day and night positions of diurnal leaf movement. The assay was based on the potassium-dependent activation of pyruvate kinase. Total potassium levels of pulvini were higher in the light than in the dark [0.88 and 0.57mol (kg dry weight)−1, respectively]. Transverse compartmentation of potassium was studied on three tissue slices, representing the middle part, petiolar and laminar sides of individual pulvini. These were dissected further into 10 distinct subsamples (bundle; motor tissues: extensor, flexor; flanks). In the day position the amount of potassium in the extensor was higher than in the night position [1.92 and 1.50 mol (kg dry weight)−1, respectively]. Flexor changes were opposite [1.13 and 1.65 mol (kg dry weight)−1, respectively]. In the day position there was a steep and consistent increase in potassium content from the innermost to the outermost zones of the extensor. In the night position this was much more variable. Comparable gradients were not detected in flexor samples. Here highest amounts of potassium were recovered from the middle of the motor tissue. The data specify distinct tissue regions involved in osmotic adjustment during leaf movement in Phaseolus coccineus.