Agriculture Reference
In-Depth Information
becomes a centre for Ca accumulation (Faust,
). Movement into leaf cells
is thought to be by active uptake at the plasmalemma depending on metabolic
energy supplies (Bowling,
).
Reducing fruit transpiration of 'Gala' apples by bagging them reduces their
Ca concentration (Tomala,
), but the effects of bagging 'Golden Delicious'
apples are relatively small ( Jones and Samuelson,
). Calculation of poten-
tial Ca fluxinto fruits of 'Golden Delicious' and, especially, 'Bramley' assuming
mass flow of xylem sap to meet net water requirements for fruit growth and
evaporation, underestimates Ca uptake into the apples early in the season
and overestimates it in the month prior to harvest ( Jones
et al.
,
). In fact
net Ca uptake by the fruits is often restricted to the early part of the season,
and there is little movement into fruits of
Ca supplied to the roots in early
August (Ford and Quinlan,
) although it is detectable in the fruit stalk. In
contrast, accumulation of dry matter and fresh weight usually continues up
to harvest so the percentage of Ca in the fruits falls with time. Circumstances
or treatments that enhance late-season fruit growth therefore tend to reduce
fruit Ca concentration at harvest (Figure
).
Late harvesting, and its associated larger fruits with lower Ca concentra-
tions, results in more senescent breakdown,
Gloeosporium
rotting and watercore
(Perring and Pearson,
.
), but effects on bitter pit incidence are inconsistent
(Perring and Pearson,
).
With fruits harvested at optimum maturity, in general the larger the fruit the
lower the Ca concentration and the greater the incidence of all Ca deficiency
disorders (Perring and Jackson,
; Ferguson and Watkins,
). Under South
African conditions the maximum fruit diameter to ensure complete freedom
from bitter pit is
; Terblanche
et al.
,
). Market demand for large
fruits rules out control of fruit size as a technique for controlling Ca content.
Instead, it is realized that procedures to increase fruit size, e.g. fruit thinning,
will increase the risk of Ca deficiency disorders (Sharples,
mm (Terblanche
et al.
,
; Johnson,
) and, with cultivars prone to Ca deficiency, must be accompanied by
other practices to maintain fruit Ca at desirable concentrations. Exposure to
high light intensities, which has beneficial effects on many aspects of quality,
has negative effects on bitter pit. These are very largely accounted for by effects
of exposure on fruit size (Figure
) and leaf Ca concentrations, which result
in lower fruit Ca concentrations ( Jackson
et al.
,
.
).
Thethinning-inducedeffectoffruitsizeonfruitcalciumdeficiencydisorders
is not specifically dependent on cell number or cell size but is an integrated
effect (Sharples,
). Ca concentrations tend to be higher the greater the
number of seeds in the fruit (Bramlage
et al.
,
).
Consequent to this, supplementary pollination can increase seed number and
the concentration of Ca even in fruits of the same size. Netting to give poor
pollination reduces seed number and Ca concentration (Volz
et al.
,
; Tomala and Dilley,
b).
