Turgor-mediated Leaf Movements in Analogy With Stomatal Function and Under the General Aspect of Water Flux Through the Plant

Summary Microautoradiographic methods applied to the pulvinar region of Phaseolus vulgaris L. revealed specific patterns of ion distribution according to the phases of upward or downward movements of the leaves: When feeding was taking place during the dark phase of the diurnal cycle, the main source of radioactivity was the dorsal groove of the vascular core on the flexor side. During the phase of leaf opening, 86 Rb and 43 K were mainly found within the parenchymatous bundle sheath. As an additional similarity to the guard cell physiology, toxic metal ions were accumulated in the cells of the bundle sheath when introduced into the plant during the light phase. Additional experiments on the influence of osmotic gradients supported the view that the characteristic distribution of ions during the two phases of the diurnal cycle is the result of regulatory processes controlling water and solute flux from the sites of storage to the evaporating surfaces. The conclusion was that in light the cells of the bundle sheath removed the ions from the flexor sites and that this process is substantially controlled by transpiration rates. Findings of other authors were discussed in view of the hypothesis that K + movements in the pulvinar region depend on regulatory systems other than active ion pumps. Among the processes probably involved in the initiation of primary water flux changes are the import of carbohydrates into the cells of the bundle sheath and starch hydrolysis.