Agriculture Reference
In-Depth Information
levels of within-canopy light intensity below which it is not possible to pro-
duce high-value fruits ( Jackson,
; see Figure
.
; also Chapters
and
).
Since production costs generally rise with increase of tree size ( Jackson,
;
Wagenmakers,
a), it is not desirable to have large volumes of excessively
shaded, unproductive tree canopy.
There are, consequently, major questions of optimizing orchard canopy
design so as to attain high light interception, including in the early years,
while maintaining adequate radiation levels in the fruiting zone. Unlike many
annual crops there is much scope for canopy manipulation by tree training,
pruning, control by growth regulators and vigour-controlling rootstocks as well
as planting density.
Understanding the light relationships of three-dimensional tree canopies is
essential to their effective manipulation.
Shading by stems, fruits and leaves
Some shading is caused by stems and fruits. Wagenmakers (
b) found that
the projected branch cast-shadow area for 'Comice' pear was about
%ofthe
leaf area. The relative effect of branches on shade is likely to be less than
this because of the way in which the leaves are positioned on the branches.
The projected area of the fruits can be up
% of the leaf area in some seg-
ments of heavily cropping 'Cox' trees ( Jackson,
), but much lower in
other segments. The
% found by Wagenmakers for 'Golden Delicious'
apple trees grown in high-density planting systems is probably the best
estimate.
The leaves are therefore the most important organs for shade creation as
well as for interception of light and its use in photosynthesis. The relationship
between leaf area and light interception and distribution depends on leaf
transmission and reflection of light, leaf poise in relation to the sources of
light, and the degree of leaf folding. It also depends on the extent to which
leaves are uniformly distributed within the trees, and the size and shape of the
trees and their arrangement within the orchard.
.
-
.
Leaf transmittance and reflectance
Palmer (
a) found that leaf reflectance and transmittance of PAR in the
wavelength range
nm were both highest early in the season when
the leaves were shiny and relatively thin (Table
-
.
). Absorptance ranged from
%. Both reflectance and transmittance showed a very pronounced
peak for green light at around
%to
nm. There is high transmission in the range
-
nm.
