Agriculture Reference
In-Depth Information
Severe winter pruning, which stimulates vigorous shoot growth, results in
lowerfruitCaconcentrationsandmorebitterpitthansummerpruningsystems
that result in numerous weak, fruiting twigs that cease growth early in the year
(Schumacher
et al.
,
). This is
at least partly a consequence of the low cropping and large fruits that can be
induced by heavy pruning but may also involve effects of competition.
Calcium can be withdrawn from fruits in the later stages of their growth,
apparently in response to water stress. Fruits commonly shrink during the
day as a result of water loss due to equilibration with transpiration-induced
tensions in the xylem (Tukey,
; Deckers and Missotten,
; Link,
). Bitter pit, indicative
of Ca deficiency, is frequently associated with vascular tips within the fruits
and hot dry conditions leading to water stress (Smock,
,
; Tromp,
).
However, this water is replaced during the night and spur-wood xylem sap
may have ten times as high a concentration of Ca as fruit xylem sap. Lang
and Volz (
; Perring,
), working with 'Royal Gala' which shows relatively little
decline in xylem function late in the season, found evidence that this cycling of
xylem sap out of and back into the fruit contributes to fruit Ca accumulation.
This may also explain why leaf transpiration, leading to accumulation of Ca
in conducting tissues, increases the Ca content of nearby fruits.
,
Movement within the fruits
Differences in Ca content in different parts of apple fruits and movement
between these are well documented. The core, even after removing the seeds,
has a much higher Ca concentration than the inner cortexwhich, in turn,
has a higher concentration than the outer cortex(Perring and Pearson,
;
Ferguson and Watkins,
). The peel concentration is intermediate between
those in the core and the cortex(Perring and Pearson,
). It falls steeply,
with increasing cell size, from the fruit surface to a depth of
mm. During the
post-harvest period Ca may move from the middle and outer cortical tissues
to the core zone. This is followed by development of bitter pit lesions.
Surface application to the fruits
Movement within the fruit is basic to the common use of orchard sprays and
post-harvest dips with Ca salts to increase fruit Ca content and reduce the
incidence of post-harvest disorders (Table
).
Although orchard sprays increase the Ca content of both leaves and fruits
(Van der Boon,
.
; Ferguson and Watkins,
), bagging experiments have shown that the fruit must be
directly exposed to the Ca sprays for its Ca content to rise (Ford,
). Ca
intercepted by the leaves is not translocated to the fruit. Natural openings in
the fruit surface such as stomata, lenticels (which are derived from stomata)
