Agriculture Reference
In-Depth Information
The effect is particularly striking in cultivars such as 'Mutsu' which produce
very little anthocyanin if not bagged.
High levels of nitrogen usually reduce the percentage of well-coloured fruits
at harvest time, the effect being partly due to the shading effect of the extra
foliage but primarily through a direct effect of N (Saure,
). Marsh
et al.
(
) found evidence of a detrimental effect of high N on well-exposed fruits
of 'Fuji', that they attributed to effects on fruit maturity and pigment develop-
ment. High potassium levels are often associated with the development of red
colour.
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Ethephon, an ethylene-producing compound, has been widely used to pro-
mote anthocyanin formation, mainly in early and mid-season cultivars and
their red sports. It appears to act mainly by accelerating the ripening process.
Two drawbacks to its use are that it can induce red colour with insufficient
light for fruit quality development (Saure,
), and that ethephon-treated
fruits ripen and soften too quickly. There is some evidence that endogenous
ethylene production during the ripening process increases the level of phenyl-
alanine ammonia-lyase (PAL), a rate-limiting enzyme for anthocyanin forma-
tion (Faragher and Brohier,
). Some other factors influencing anthocyanin
production do not appear to operate through effects on ethylene (Saure,
).
Fruit russeting and cracking
For a limited number of cultivars, e.g. the apple 'Egremont Russet', the skin
has a rough but finely textured, predominantly light brown surface, that is
regarded as characteristic of the cultivar. For most others russet is regarded
as a blemish acceptable either not at all or to a limited extent, and is a major
cause of downgrading.
Russet can be defined as a periderm that replaces the epidermis and forms
a continuous layer of protective tissue. Its development in a russet cultivar
is shown in Figure
. As russet develops the cuticle ruptures, anthocyanin
in the epidermis is lost and the underlying green tissues are obscured by
brown quinones in the dead, ruptured, cells (Skene,
.
). The periderm
may precede the rupture of the cuticle or be formed in a wound reaction to
physical or chemical damage to the skin, e.g. by frost or spray damage. Russet
is sometimes covered with shallow cracks up to
mm deep, when it is referred
to as rough russet. Russeting of 'Cox' starts about mid-June and the percentage
of fruit affected remains constant from mid-July to harvest (Skene,
).
Cultivars differ greatly in their propensity to russet: the characteristic is
heritable but more than one factor seems to be involved (Brown,
). Some
