Agriculture Reference
In-Depth Information
and cracks in the cuticle provide the main avenues of entry. Some Ca transport
can also occur across the cuticle itself (Ferguson and Watkins,
; Harker
and Ferguson,
). Surfactants and 'sticking agents' may increase the uptake
of applied Ca (Sharples
et al.
,
). Penetration into young fruits can be to
a depth of
cm but only to a few millimetres under the skin of older fruits
except via cracks. However, surface-applied Ca seems to move by diffusion in
the free space and higher concentrations can be found in mid-cortical tissues
after spraying. Movement within the fruit may, though, be relatively slow.
Chittenden
et al.
(
) painted one half of 'Cox' apples seven times in the
month prior to harvest with
% calcium nitrate and found a
% incidence of
bitter pit on the untreated side compared with only
% on the treated side after
months' storage. Calcium chloride injected into the core cavity at harvest is
as effective as that applied to the surface in reducing bitter pit in the outer and
mid-cortex(Perring and Pearson,
).
The effectiveness of post-harvest application of Ca to the skin or to the core,
in reducing Ca deficiency disorders in the fruit flesh and in reversing softening,
suggests that the individual cells and their middle lamellae remain active as
sinks for Ca well into their post-harvest life. This supports the concept that the
decline in Ca importation into the fruit in the later stages of their growth in
the orchard is a function of an impaired transport system through the stalk,
not simply to a decline in demand with the cessation of cell division.
Effects of other nutrients on calcium fluxinto fruits
As well as influencing uptake from the soil, as noted earlier, other nutrients
can influence the movement of Ca into fruits. Orchard sprays with zinc sul-
phate, especially early in the season, can result in increases in fruit Ca content,
possibly by releasing bound Ca from various chelating and complexing agents
such as lignin, organic acids and proteins for transport to the shoot (Shear,
). Copper can have similar effects. Addition of zinc chloride (
.
%) to
post-harvest dips in
% CaCl
can double the uptake of Ca from the latter,
increasing fruit Ca in different cultivars by
g compared
with that following dipping in CaCl
alone (Testoni and Pizzocaro,
.
to
.
mg/
).
Cultivar effects on fruit Ca level
Clonal or seedling rootstocks provide the root systems of almost all commer-
cially grown apple and pear trees and might be expected to affect Ca uptake
and transport through the graft union. There is indeed some evidence for
rootstock effects on the content of Ca in scion leaves (Kennedy
et. al.
,
).
The largest effects of rootstocks on fruit Ca are, however, mediated by their
effects on crop load and fruit size, as discussed earlier. An exception to this is
