Agriculture Reference
In-Depth Information
Rootstocks affect fruit size indirectly by influencing the proportion of fruits
growing in high-light environments on branches and spurs well exposed to
sunlight in both the current and the previous season. They also have intrinsic
effects on fruit size which are not consequent on effects on tree vigour and
size. Apple trees grown on 'MM.
' rootstock (semi-dwarfing) give smaller
fruits than those on 'M.
'even after taking into account effects
of crop load per unit of tree size or of light interception (Blasco,
' and 'M.
). Some
recently selected rootstocks, e.g. 'AR
-
' (Webster
et al.
,
) and 'Jork
'
(Callesen,
) induce fruits as large as or larger than those from trees on
'M.
a).
The pollenizer cultivar can influence fruit size independently of effects on
seed number. Use of 'Fuji' as a pollenizer has particularly beneficial effects
(Keulemans
et al.
,
'. Others, e.g. 'Mark' and 'PI' have adverse effects (Barritt
et al.
,
).
Fruit skin colour, russet and cracking
Apple and pear cultivars are, for purposes of marketing, largely defined by the
appearanceoftheirfruits,andcolouranddegreeofrussetingoftheskinsurface
play a large part in their description. The major distinctions are between
cultivars with red, partially red, brown or green skin and between those which
are smooth-skinned, can have a degree of russeting or are mainly russeted.
Within cultivars the degree of red colour and of russet may also be used to
definequalitygrade.Thisisbasedonvisualattractivenessbutalso,withinsome
cultivars, the fact that only those fruits most exposed to sunlight develop deep
redcolourhasledtoanassociationbetweencolour,sweetnessandotheraspects
of flavour and texture influenced by light climate. Russeted fruits may also be
prone to water loss, both in storage and subsequently, with some consequent
loss of crispness.
The apple skin
The term skin is usually used to refer to the outer protective tissues of the fruit
consisting of cuticle, epidermis and hypodermis. At blossom time the cuticle
is about one micrometre (
µ
m) thick: at harvest its thickness varies between
cultivars, being about
µ
m in 'Golden Delicious' (Meyer,
) and
µ
m
in 'McIntosh Red' (Tukey and Young,
).
The epidermal cells have much larger radial than tangential dimensions
at blossom time but the cells subsequently become rounder and after June
become tangentially elongated (Figure
). In some cultivars, e.g. 'Golden
Delicious', the identity of the epidermis is lost as the fruit grows.
At blossom time the hypodermal cells are very similar to those of the cortex
althoughslightlysmaller(TukeyandYoung,
.
).Latertheircellwallsthicken
