In summary, this study demonstrates that foliar applications
of picomole quantities of L(+)-adenosine to apical leaves elicit within seconds a transitory change in Ca2+, Mg2+, and
K+ concentration within the solution phase of the stem apoplast.
The fact that the affected tissue is distant (>10 cm)
from the treated leaves suggests the involvement of a biophysical
signaling system within the plant. It is not clear if
the observed changes in ionic concentration are an integral
part of the signaling system or a secondary response.
The rapid transient change in extracellular ionic concentration
of the magnitude observed in this study is of physiological
importance. Modulations in the ionic environment of
either leaf or root cells in response to a stimulus would alter
the electrochemical potentials across the plasma membrane
of those cells. An increased influx of Ca2+ into the cytoplasm
in response to transient increases in apoplastic Ca2+ concen-tration, as reported here, would modulate the activity of
important regulatory enzymes within cells by binding to such
receptor proteins as calmodulin (Marmé, 1986; Evans et al.,1991)