Agriculture Reference
In-Depth Information
cells, or both (Pratt
et al.
,
). Some red sports when used as parents pass
on the red colour to their progeny. This is shown by comparing the progeny
of crosses between the colour sport and a yellow-skinned cultivar with those
of the original cultivar and the same yellow-skinned cultivar. Other colour
mutants do not pass on the colour to their progeny. In the former case the
mutation must have taken place in layer
of the meristem which produces
the sub-epidermal cells and the gametes. The non-heritable colour mutation
must have been in the superficial layer of the meristem which develops into the
epidermis. 'Richared', a red sport of 'Delicious', is a good example of one in
which increased anthocyanin production is heritable. It has the same number
of pigmented cells in the epidermis but twice as many in the sub-epidermis as
'Delicious' (cf. review by Brown,
). The frequency of occurrence of colour
sports and the ease with which they can be identified in the orchard, together
withtheheritabilityofthecharacteristic,hasbeenparticularlyimportantinthe
'Delicious' group of apples but is now also important in many other cultivars.
Red-skinned mutants of several important pear cultivars, with anthocyanin
production in either epidermal ('Starkrimson' mutation of 'Clapp's Favorite')
or sub-epidermal ('Red Bartlett') layers, are also available.
The ratio of leaves to fruits has a marked effect on red colour development.
Magness (
) found that, for 'Delicious' and 'Winesap', fruits with only ten
leaves per fruit had
% of their surface red-coloured even with almost perfect
light exposure, whereas fruits with
leaves had
% red colour. The sugar
contents of these fruits were
% and
%, respectively. Other fruit-thinning
trials(reviewedbyWalter,
a)showedsimilareffectsonredcolour.Faragher
(
) found that ripening, as measured by ethylene production, as well as skin
anthocyanin production, was delayed in heavily cropping relative to lightly
cropping trees. This may implicate internal carbohydrate conversion within
the fruits, as well as gross carbohydrate supply, in the effect of leaf-to-fruit ratio.
If a small portion of the skin of a potentially red fruit is protected from light
by an overlapping leaf or opaque adhesive tape, that protected area will not
show anthocyanin development and will remain green. A number of enzymes
involved in anthocyanin synthesis, including dihydroflavonal reductase (DFR)
and phenylalanine ammonia-lyase (PAL), are light-dependent. However, there
is no clear evidence that light effects on these control anthocyanin levels (Lister
et al.
,
;Ju
et al.
,
) although some correlations have been noted.
Artificial shading of small trees has a very marked effect both on the propor-
tion of the skin developing a red colour and on the intensity of this colour
