Agriculture Reference
In-Depth Information
(Bramlage
et al.
,
b). They also show high respiration rates. However, if fruit
nitrogen levels in 'Cox' fall below
g susceptibility to breakdown is
increased, especially if calcium levels are also low (Sharples,
mg/
). Johnson
et al.
(
) found that a beneficial effect of late sprays of calcium nitrate on
texture was associated with an increase in the ratio of N to C, rather than of
Ca to C, in the primary cell wall and middle lamella. Many of the adverse
effects of high N are associated with its effect in increasing fruit size and vigour
of shoot growth, which adversely affect Ca concentrations, and also with the
direct effects of the ammonium ion on Ca uptake.
High potassium concentrations have a strong positive effect on fruit acidity
(Wilkinson,
), which is an important aspect of the taste of
some cultivars. Fruits high in K are, however, more susceptible to breakdown,
bitter pit and other disorders associated with low calcium status. These effects
relate to the interaction between K and Ca in cells and high K can induce Ca
deficiency.
Magnesium concentrations in apple fruits tend to be proportional to those
of K and have similar associations with fruit acidity, bitter pit, etc. Magnesium
salt sprays or dips can greatly increase bitter pit incidence. Sharples
et al.
(
; Johnson,
)
attributed this to direct competition with Ca at the cellular level. Hopfinger
and Poovaiah (
) found very high Mg concentrations in pitted tissue and
concluded that bitter pit is due to a localized magnesium toxicity which can be
prevented by calcium treatment, but Ferguson and Watkins (
) considered
this Mg-induced pitting to differ from true bitter pit.
Low phosphorus levels in fruits lead to increased senescent breakdown and
low temperature breakdown. Phosphorus sprays can reduce the incidence of
these disorders (Yogaratnam and Sharples,
).
Carbohydrate supply is important with respect to fruit carbohydrate con-
centration and associated quality factors. Shade reduces total fruit dry matter,
alcohol-insoluble and alcohol-soluble matter and starch per unit fresh weight
( Jackson
et al.
,
). Fruit thinning results in higher sugar (brix) concentration
in 'Fuji' apples (Fukuda and Takishita,
). It also increases fruit firmness
despite the fruits on thinned trees being larger and lower in Ca concentration
( Johnson,
).
Recommended reading
Atkinson, D., Jackson, J.E., Sharples, R.O. and Waller, W.M. (eds.) ().
Mineral Nutrition of Fruit Trees.
London: Butterworths.
Combrink, J.C. (ed.) ( ).
Integrated Management of Post-harvest Quality
.
Stellenbosch: Infruitec.
Ferguson, I.B. and Watkins, C.B. (). Bitter pit in apple fruit.
Horticultural
Reviews
, -.
