Agriculture Reference
In-Depth Information
Table
.
Effects of shading, from
, on the structure of leaves from the
middle regions of extension shoots on
June
Sept
% full daylight
SED
Leaf dry wt (mg cm
−
)
.
∗∗∗
.
∗∗∗
.
∗∗∗
.
. df
∗∗∗
∗∗∗
∗∗∗
Leaf thickness (
µ
m)
.
Depth of palisade
layer (
µ
m)
∗∗∗
∗∗∗
∗∗∗
.
Number of cells in palisade
Layer
. .
∗
.
∗
.
.
df
.
∗
.
∗
Layer
.
.
.
..
∗∗∗
.
∗∗∗
.
∗∗∗
Layer
.
.
∗∗∗
.
∗∗∗
.
∗∗∗
Layer
.
.
∗
,
∗∗
,
∗∗∗
denote differences from the unshaded control at the
%,
% and
.
% levels
of significance.
From Jackson and Palmer (
). Reproduced with permission.
but the rootstock influence on scion leaf structure is inconsistent. Hard-pruned
young trees of dwarfing rootstocks have much thicker leaves than unpruned
older trees.
Spur leaves are thinner than extension shoot leaves, with fewer layers of
palisade parenchyma, shorter palisade parenchyma cells, less chlorophyll and
lower specific leaf weight (Ghosh,
). Basal leaves on shoots tend to have
thinner layers of palisade cells than apical leaves (Cowart,
). These two
effects of leaf position may be associated with differences in exposure to light,
the effects of which are discussed below.
Cultivars also differ in the number of stomata per unit leaf area, ranging
from around
mm
−
(Cowart,
). The leaves
of vigorous apple rootstock cultivars have a much greater stomatal density than
those of dwarfing ones (Beakbane and Majumder,
to around
; Slack,
).
Basal leaves have many fewer stomata per unit area than apical leaves,
the increasing frequency being associated with increasing structural density
(Cowart,
).
Perhapsthemostimportantvariationsinleafstructurearethoseattributable
to variations in light intensity within the tree canopy. Leaf thickness and
specific leaf weight (mg cm
−
) are positively correlated with incident radi-
ation (Barden,
,
; Barritt
et al.
,
; Jackson and Beakbane,
;
Tustin
et al.
,
). The extreme cases are obvious 'sun' and 'shade' leaves.
Jackson and Beakbane (
) showed that the percentage of air space in
the spongy mesophyll decreased and the thickness of the palisade layer in-
creased with light intensity. Artificial shading experiments confirmed these
