Aspects of potassium accumulation by stomata of Vicia faba

Abstract

Potassium accumulation by guard cells during the light opening of V. Jaba stomata in epidermal strips was determined quantitatively with 42K and 86Rb tracers. The sodium cobaltinitrite stain for potassium was also used. Particular attention was paid to errors arising from the presence of intact epidermal cells. When epidermal strips obtained from darkened leaves were floated on potassium chloride solution (usually 10 mM) in light plus CO2-free air, guard cell potassium content and stomatal aperture increased in parallel, both approaching a maximum after about 300 min. Flux measurement suggested that the steady maximum potassium content (and aperture) arises because of a decline in the initially high potassium influx. With opening of stomata in the light, external calcium reduced in a parallel manner both opening and potassium uptake at potassium concentrations ranging from nearly zero to 50 mM. Also, maximum stomatal opening and potassium accumulation in the presence or absence of calcium bore linear relationships to log external potassium concentration. These results are discussed in relation to potassium accumulation in other plant systems. In experiments with epidermal strips, guard cell potassium content showed a consistent linear relationship to stomatal aperture. For 16 experiments spanning two years and many treatments affecting aperture, the mean slope of this relationship was 2·6 nmoles cm−2 μ−1. This is equivalent to a change in potassium concentration of 40 mM μ-1. The increase in guard cell osmotic pressure attributable to potassium plus anion accumulation is estimated to be 1·5 bars μ−1, which is a major portion of the observed increase in osmotic pressure with opening (2·0 bars μ−1). © 1972 CSIRO.