Evidence for stomatal uptake of solutes by leaves without the application o
f surfactants or pressure has recently been provided (Eichert et al,, 1998)
. In the present study, experimental conditions were refined in that the wa
ter potential was held at <0 on the physiologically inner side of the epide
rmal strips (ES) by use of a ceramic plate or the proper mesophyll of the p
lant. The penetrated substances were immobilized on the inner side by ion e
xchange membranes. The influence of humidity, light, stomatal density, and
re-wetting on the uptake of anions (fluorescein) and cations (Fe3+) was inv
estigated, using leaves of Allium porrum, Commelina communis and Sedum tele
phium. Uptake increased with humidity, stomatal aperture and stomatal densi
ty. It was restricted to stomatal areas, and was especially high below the
rims of drying droplets, Again, penetration of stomatal pores was observed.
Uptake was strongly correlated with the number of penetrated stomata, alth
ough usually less than 10% of the stomata contributed to uptake. The number
of stomata that had been penetrated was highly variable, increasing extrem
ely significantly with the number of repeated drying/wetting cycles. These
results indicate that stomatal uptake can be a major pathway for the foliar
uptake of ionic solutes. It is a dynamic process, depending on environment
al conditions and history of the residues on the leaf, aspects that had bee
n neglected in previous concepts.