Agriculture Reference
In-Depth Information
During summer the xylem sap contains appreciable amounts of P (Hansen,
) and it seems that both xylem and phloem transport are involved in the
supply of P to the fruits. Accumulation of P in leaves reaches a maximum in
July. That in fruit continues up to harvest (Wilkinson and Perring,
), closely
following the weight increase of the fruit. Low fruit content of P predisposes
apple fruits to low temperature breakdown in storage. Spraying the fruits with
P compounds between mid-June and mid-July increases their P content at
harvest and reduces the incidence of low temperature breakdown, senescent
breakdown and, in some years, superficial scald and coreflush ( Johnson and
Yogaratnam,
). The beneficial effects
are, however, fairly limited and sprays with KH
PO
may increase fruit K
content and have adverse effects on bitter pit.
As noted in Chapter
; Yogaratnam and Sharples,
(root systems), apple tree roots are mycorrhizal.
Factors that reduce mycorrhizal infection, e.g. the use of herbicides overall in
the orchard, can reduce P uptake (Atkinson,
) and the concentration of P
in fruits ( Johnson
et al.
,
).
Potassium nutrition
Apple trees have high potassium requirements in terms of both gross and
net uptake (Table
). In England young trees frequently show K deficiency
symptoms which must be remedied if growth is not to be stunted. Subsequent
fertilizer requirementdepends largely oncropping levels. Excesssoilpotassium
leads to inhibition of calcium and magnesium uptake. It also leads to high fruit
K and increased problems if Ca concentrations are low. In many fruit-growing
regions little or no K fertilization is needed. Possibly as a result of previous
fertilizer practice, up to around
.
% of orchards in Santa Catarina, Brazil
showed above-normal leaf K (Basso and Wilms,
). A survey in Germany
found
% of orchard soils to contain levels of K which were excessive with
regard to susceptibility to physiological disorders of apple fruit (Quast,
).
Potassium movement through the soil to the root surface is primarily by
diffusion. The soil water content is a dominating factor for this and K uptake
canbeseverelylimitedbydrought.Althoughpassiveaswellas'active'processes
participate in the movement of potassium within plants, the overall rate of K
uptake by apple trees appears to be under metabolic control. Tromp (
)
subjected apple trees to a range of environmental conditions and found that
K uptake was linearly related to growth, i.e. to metabolic demand. The main
transport vehicle for potassium appears to be the transpiration stream in the
xylem, but it also moves freely in the phloem. Fruits are very strong sinks for
potassium (Hansen,
) and there is a strong positive correlation between
fruit K and soluble dry matter or acid content (Wilkinson,
; Perring and
