Agriculture Reference
In-Depth Information
Foliar application of nitrogen
Above-ground plant parts have a basic capability to absorb mineral nutrients
and water although this may be reduced by barriers to water loss. The use-
fulness of absorption of nutrients by leaves may, however, be limited by lack
of ability to direct them subsequently. For example, Ca absorbed by leaves
from foliar sprays is not translocated to the fruits where it is needed. The value
of foliar sprays of nitrogenous compounds was greatly enhanced when it was
realized that N supplied to leaves just prior to leaf fall might move into storage
tissues as part of the natural cycling of N within the trees (Oland,
,
;
Shim
et al.
,
). Nitrogen applied to leaves post-harvest prolongs their period
of photosynthetic activity but does not give unwanted further shoot growth or
fruit enlargement and softening. It can also be applied at much higher concen-
trations, giving more uptake, than if applied to buds, flowers and developing
fruits which might show phytotoxicity. Foliar applications of urea after harvest
of apples and pears has been found to add to tree reserves of N (Shim
et al.
,
) and to have generally appreciable effects on apple
fruit set and shoot growth in the following spring. Effects on blossom fertility
have been discussed in Chapter
; Sanchez
et al.
,
.
Apple leaf cuticles are traversed by polysaccharide microfibrils which may
facilitate penetration, and the cutin itself is not totally impermeable (Swietlik
and Faust,
). Nutrients may enter relatively easily through stomatal pores,
in which the cuticle is hydrated and wax-free, and trichomes. Cuticles are
-
b).
Urea is absorbed more rapidly by intact leaves than any mineral nutrient, and
facilitates the penetration of other nutrients through isolated cuticles and into
intact leaves (Yamada
et al.
,
times more permeable to urea than to inorganic ions (Yamada
et al.
,
a).
After passing through the cuticle there is some evidence for passage of
urea through the cellulose cell walls by way of thread-like structures called
ectodesmata (Swietlik and Faust,
).
Absorption by the lower surface of apple leaves is very rapid within the first
hours and then levels off, whereas the upper surface absorbs urea more
slowly and steadily (Boynton
et al.
,
). Shim
et al.
(
) found
%ofa
%
solution of urea applied to senescing apple leaves was absorbed in
hours.
Leaf-absorbedNmustbemetabolizedbeforeitcanbeutilized.Thisinvolves
the hydrolysis of urea, reduction of nitrate and incorporation of ammonium
into amino acids. Absorbed urea can be broken down in the leaves by urease
(Oland,
). The N is then incorporated into amino acids and proteins (Shim
et al.
,
a, b). The proteins in turn are converted to amino acids, transported
to the storage tissue and re-assembled into proteins. The urea may also be
exported as such. This export can take place very quickly. Boynton
et al.
(
)
reported that about
% of the urea N absorbed by apple leaves on shoots
was translocated out of the leaves within
hours.
