John Stanley wrote: “What puzzles me is that most orchid leaves seem to
have a pretty impermeable, if not waxy, outer layer (at lest on their
dorsal surface) and I would have thought the stomatal pores to be too
small for the ingress of water (unless assisted by a wetting agent that
would create all sorts of other problems).”
John while it certainly looks and feels like the top of orchid leaves is
covered in an impermeable layer of wax, the layer of epicuticular wax is
not solid. The waxy layer purpose is to regulate water loss and leaching
of mineral during rain. Another minor barrier in foliar feeding is
surface tension, and many people on this forum have already commented on
the use detergent for breaking the surface tension. A little breeze will
also do the trick. To give you a cited source and more detailed
explanation of cuticle and epicuticular wax, Mineral Nutrition of Higher
Plants, Horst Marschner:
“This outer wall is covered by the cuticle (cuticle proper) and a layer
of epicuticular waxes which are often well and typically structured
(Bartloth, 1990). These waxes are excreted by the epidermal cells and
consist of long-chain alcohols, ketones, and esters long chain fatty
acids. Waxes also occur ‘intracuticularly’ within the cuticle and in the
cutinized layer (Fig. 4.2). The cuticle consists mainly of cutin, a
mixture of long-chain fatty acids. The chemical and physical properties
of the cuticle differ between outer and inner surfaces, a distinct
gradient occurring from the hydrophobic (lipophilic) outer surface to a
more hydrophilic inner surface of the cutinized layer. The cutinized
layer is normally the thickest part of the epidermal wall (Fig. 4.2) and
consists of a cellulose skeleton incrusted with cutin, wax and pectin.
The cuticle and the cutinized layer (Fig. 4.2) have diverse functions. A
major function is to protect the leaf from excessive water loss by
transpiration. The control of water economy in terrestrial higher plants
by the stomata is dependent on the remaining surface of the plant being
very low in hydraulic conductivity. The other main function of these
structures is to protect the leaf against excessive leaching of
inorganic and organic solutes by rain (Section 4,4). It has to be kept
in mind that mineral nutrients and other solutes entering the leaves via
the xylem are in the apoplasm of the leaf tissue, and a waterproof’
barrier is required to act as an apoplasmic boundary thereby playing a
similar role to that of the Casparian band in the endodermis of the
roots (Section 27). The relative importance of these two main functions
of the cuticle depends on climatic conditions (arid zones versus humid
tropics), In addition the cuticle is involved in temperature control,
optical properties of leaves and plays a role in defense against pests
and diseases (Chapter 31).”
John Stanley asks: “I wonder if there is anyone out there who can
explain exactly how foliar feeding works, irrespective of what is
assimilable by the plant? Without digging into ancient research, which
must go back to, at least, the sixties, what is the principle of
adsorption/absorption by the leaf tissues?”
Also from Mineral Nutrition of Higher Plants, Horst Marschner:
“Permeation of low-molecular-weight solutes (e.g., sugars, mineral
elements) and evaporation of water through the cuticle (peristomatal or
cuticular transpiration) takes place in hydrophilic pores within the
cuticle. The majority of these pores in the cuticle have a diameter of
less than 1 nm, and a density of about 10 to the 10th power pores cm -2
has been calculated (Schonherr, 1976). These pores are readily permeable
to solutes such as urea (radii 0.44 nm) but not to larger molecules such
as synthetic chelates (e.g. FeEDTA). These small pores are lined with
fixed negative charges (presumably mainly from polygalacturonic acids)
increasing in density from the outside of the cuticle the inside (i.e.
the cutinized layer and the cell wall interface, Fig. 4.2). Accordingly,
permeation of cations along this gradient is enhanced whereas anions are
repulsed from this region (Tyree et al. 1990). Uptake of cations by
leaves is thus faster than that of anions (e.g. NH4+ compared with NO3-)
and is particularly fast for small, uncharged molecules such as urea.
However, when applied at high concentrations as foliar sprays,
differences in uptake rates of nitrogen from urea, ammonium and nitrate
become negligible (Bowman and Paul, 1992).
Cuticular pore density is higher in cell walls between guard cells and
subsidiary cells (Maier-Maercker, 1979). This explains the commonly
observed positive correlation between number or distribution of stomata,
for example, between the upper (adaxial) and the lower (abaxial) leaf
surface, and the intensity of mineral nutrient uptake from foliar sprays
(Levy and Horesh, 1984). Not only is the number of the cuticular pores
larger around guard cells (or
