Agriculture Reference
In-Depth Information
During the growing season the main factors influencing individual fruit growth
are light intensity and leaf area development, controlling potential supply of
photosynthate, and competition for this between shoots and fruits and between
fruits.
The changes in light intensity with position in the canopy are the most
important source of within-tree variation in fruit growth in most types of apple
and pear trees. Jackson (
% of within-tree variation in
weight of 'Cox' apples was accounted for by fruit location effects that were
clearlyrelatedtoshade.Wherecanopystructurewassuchastomakevertically-
summed leaf area indexrele vant to shade, apples in the upper third of the size
distribution were not found beneath an LAI greater than
) found that
-
b).
Part of this effect is a consequence of poor spur development under shaded
conditions, such spurs having lower photosynthetic potential in terms of leaf
area and specific leaf weight (Tustin
et al.
,
.
( Jackson,
). Major responses to shade are,
however, demonstrated by artificial shading of previously well-exposed trees
or branches immediately post-anthesis.
Fruit size is reduced by imposed shade even though the number of fruits is
also reduced. The reduction in fruit size is primarily due to reduction in cell
division and cell number but also involves reduction in cell size. With light
intensities of
%,
% and
% of full daylight, respective final fruit weights
×
per fruit and mean
were
,
and
g, cell numbers
,
and
.
×
−
mm
( Jackson
et al.
,
cell volumes
). Part of the
effect of shade may be associated with reduced fruit temperatures (Thorpe,
.
,
.
and
)
as well as with effects on leaf and fruit photosynthesis. Shading fruits without
shading the adjacent leaves can considerably reduce their growth and size at
harvest (Schrader and Marth,
), hence reduced fruit metabolism and sink-strength (Minchin
et al.
,
).
Sources of photosynthate for fruit growth change over the season. The
early development of flower clusters depends on stored carbohydrate and
bud and flower photosynthesis (Chapter
). Spur leaf emergence starts before
blossoming and at two weeks after bloom spur leaf assimilate is almost equally
distributed between the spur leaves themselves and the bourse shoots and
fruitlets which develop more or less simultaneously. Bourse shoots initially
compete with the fruitlets for spur leaf assimilates and also retain and utilize
their own assimilates (Tustin
et al.
,
). Subsequently more and more of
the spur leaf assimilate feeds the developing fruitlets and the bourse shoot
leaves become exporters to the fruitlets. The fruits then become dominant
importers of assimilates from extension shoot, bourse shoot and non-fruiting
as well as fruiting spur leaves. Proximity between sources and sinks, modified
by vascular connection patterns, determines which demands are satisfied first
