Agriculture Reference
In-Depth Information
channels exchange will be much more important (Ferguson and Watkins,
).
One consequence of the stele acting partly as an ion exchange column is
that it can act as a store for Ca. This is particularly important with respect to
the supply of Ca during the initial stages of fruit growth. Fuhr and Wienecke
(
) found that
Ca supplied during one growing season is found in all new
plant parts in the following spring. They estimated that about
% of the total
Ca in apple fruits in one season originated from Ca reserves deposited in the
previous year, the precise sites of storage being uncertain.
Bell and Biddulph (
) put forward the concept that calcium ascent is
based not on the loss of water from the individual plant parts but on the
metabolic removal of the ions from the exchange columns leading to the
individual parts. This results in the various tissues acquiring nutrient ions in
proportion to their metabolic utilization but also subject to the influence of
massflowinthemajorconduitsandthesupplyofCa,othercationsandorganic
acids.
Some effects on fruit calcium content seem explicable in terms of factors
controlling the general upward flux. The leaves and stems, as well as the fruits,
in the upper and outer parts of the tree canopy tend to have less Ca per unit
of dry matter than corresponding organs nearer to the roots, which is com-
patible with abstraction of Ca from the transport system
en route
(Preuschoff,
). Many
nutritional treatments increase both leaf and fruit Ca concentrations (Huguet,
; Jackson
et al.
,
; Haynes and Goh,
; Barritt
et al.
,
). Moreover, the accumulation of Ca by fruits is
positively dependent on the area of primary and bourse shoot leaves subtend-
ing the fruit (Ferree and Palmer,
; Terblanche
et al.
,
; Jones and Samuelson,
; Proctor
and Palmer,
a). Primary leaves are the more ef-
fective. Enclosing the leaves in polythene bags has a similar effect to their
removal, confirming the effect of leaf transpiration on the supply of Ca to
the fruits ( Jones and Samuelson,
; Volz
et al.
,
,
). It is obviously important to maximize
early spur-leaf area.
Fluxinto the fruits
The final stages of movement into organs and tissues are at least partly under
metabolic control.
Within the shoot system Ca is translocated preferentially towards the shoot
apexeven through the transpiration rate of young apple leaves is much lower
than that of older leaves (Shear and Faust,
). Transport to the
growing point is believed to be induced by IAA, synthesized in the shoot apex,
stimulating a proton effluxpump in the elongation zones of the shoot apex.
This increases the formation of cation exchange sites so that the growing tip
; Faust,
