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 trichomes), but the pores also seem to
have different permeability characteristics (Schonherr and Bukovac,
1978) and are most likely the sites where larger solute molecules (e.g.
FeEDTA) penetrate the cuticle and are taken up by the leaf cells.
The differences in resistance to solute penetration at various parts of
the cuticle are shown schematically in Fig. 4.4. It is unlikely that
direct penetration of solutes from the leaf surface through open stomata
into the leaf tissue plays an important role, because a cuticular layer
(the internal cuticle) also covers the surface of the guard cells in
stomatal cavities. Furthermore, ion uptake rates from foliar sprays are
usually higher at night, when the stomata are closed, than during the
day, when the stomata are open.”
This information dispels the notion that large solute molecules-
synthetic chelates- (e.g. FeEDTA) cannot be absorbed through the leaf.
It also dispels the notion that the stomata play a large role in the
penetration of solutes. While apply high concentrations of nitrate make
the difference in uptake rates of nitrogen from urea, ammonium and
nitrate negligible, why bother using a high nitrate fertilizer when
there are fertilizers on the market that are cheap and whose nitrogen
comes in the form of ammonical nitrogen and urea.
There are many studies on foliar feeding on food plants, but I don’t
know of any on orchids. I don’t think that matters much. In Fundamentals
of Orchid Biology, Joseph Arditti only mentions nutrient leaf uptake,
but does not back it up. “A fourth feature of orchids is their ability
to take up minerals both through roots and leaves.” You don’t know if he
is talking about minerals in an aqueous form or just the uptake of
nitrogen and carbon though a gaseous form as Jerry explained in his
posting. As many orchid are epiphytic, it would seem that the uptake of
minerals through both roots and leaves whether in aqueous or gaseous
form would be advantageous.
I think possibly the biggest misconception about foliar feeding is that
it somehow accelerates growth and flowering of an orchid that is
healthy. In what I have read, there is a great possibility of leaf
damage as the result of a local nutrient imbalance in the leaf tissue.
Foliar feeding of crop plants is not used to increase growth, but as a
rapid response to nutrient deficiency.
Do I foliar feed? Not on any regular basis. When I have foliar sprayed
an orchid it is usually to add nitrogen to the leaves to green up a
premature yellowing of a leaf or plant. Example: I received a very
yellow Laelia lundii through mail order. I imagine the grower I bought
it from had tried to expose the orchid to some highlight in order to get
it to bloom. I foliar fed the Laelia regularly for months until the
leaves and the plant as a whole greened up. It now blooms regularly. The
plant and leaves were once a dull yellow, are now a bright light to
medium green.
Mark Sullivan
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