Agriculture Reference
In-Depth Information
STONE FRUIT PHYSIOLOGY
Botanically, peach is a drupe. A drupe is a fleshy fruit with
a thin, edible outer skin (epicarp) derived from the ovary,
an edible flesh of varying thickness beneath the skin (fleshy
mesocarp), and a hard, inner ovary wall that is highly ligni-
fied (endocarp) and is commonly referred to as the 'stone'
or 'pit', which encloses a seed. Peaches have thin skins and
soft flesh. The skin, as a protective layer, is composed of
cuticle, epidermis and some hypodermal cell layers. The
cuticle is a thin coating of wax and serves to reduce water
loss and to protect the fruit against mechanical injury and
attack by pathogens. The epidermis, consisting of heavy-
walled cells, is responsible for most of the skin's mechani-
cal strength. Surface chromosomes or hairs ('fuzz') of
peach fruit are extensions of some epidermal cells. The
flesh, which is the main edible portion of the fruit, consists
mainly of storage parenchyma tissue composed of large,
relatively thin-walled cells with high water content. On the
basis of separation of stone from flesh, nectarine and peach
varieties can be divided into two groups: freestone (where
the stone does not adhere to the flesh) and clingstone
(where the stone adheres firmly to the flesh).
Upon the completion of pollination and fertilization
of the egg, the flower ovary begins to enlarge into a
developing fruit. This is 'fruit set', and it marks the
beginning of growth and development. Stone fruits have a
double sigmoidal growth curve which includes three distinct
stages of growth. Following fruit set, cell division continues
for about 4 weeks, with cell enlargement beginning and
proceeding rapidly (stage 1). Slow growth then occurs,
during which lignification of endocarp (pit hardening) and
growth of endosperm and embryo inside the seed take place
(stage 2). Cell enlargement (expansion) resumes in the flesh
(mesocarp) tissue. The fruit continues to increase in size
until it reaches full maturity, after which growth slows
markedly and finally stops (stage 3). The duration of each
stage of growth depends upon variety, climactic conditions
and some cultural practices (such as thinning or crop load
per tree, soil moisture, girdling and nutrition). Fruit density
(specific gravity) declines during Stage 1, increases during
Stage 2, then declines again during Stage 3 (final fruit
swell). During the pit hardening, the seed constitutes 25%
of the fruit weight, and this value drops to 14% during final
swell. From a postharvest standpoint, interest in Stage 3 is
greatest, since maturation, ripening and senescence occur
during this stage. Maturation is the time between final
growth and the beginning of ripening. Maturity is the end
point of maturation. An immature fruit may ripen off the
tree, but it will be of poor quality. A mature fruit will attain
good quality when ripened off the tree. Ripening involves
changes that transform the mature fruit into one ready
to eat. Changes associated with ripening include loss of
green colour and development of yellow, red and other
colours characteristic of a variety. As a fruit ripens, it
softens, its starch is converted to sugars, its acidity declines
and it produces certain volatile compounds that give it a
characteristic aroma. Increased respiration and ethylene
production rates are among the physiological changes
associated with ripening. Once a fruit ripens, it begins
senescence. Physical and chemical changes continue after
'optimum' ripeness is reached (from a flavour quality
standpoint), including further softening and loss of desirable
flavour. The final outcome of post-ripening changes is
complete breakdown and death of the tissues.
PEACH AND NECTARINE POST-HARVEST
HANDLING SYSTEMS
Fruit deterioration factors
Water loss
Fruit shrivelling occurs when fruit lose approximately
5-8% of the fruit's water content, based on weight at har-
vest. This loss is sufficient to cause visual shrivel in peaches
and nectarines (Ceponis
et al
. 1987). While there is a large
variability in susceptibility to water loss among cultivars,
all peaches and nectarines must be protected to assure the
best post-harvest life. Fruit waxes that are commonly used
as carriers for post-harvest fungicides can reduce the rate
of water loss when brushing has not been overdone.
Mineral oil waxes can potentially control water loss better
than vegetable oil and edible coatings. Because fruit shrivel
results from cumulative water loss throughout handling,
it is important to maintain low temperature and high
relative humidity throughout harvesting, packing, storage,
transport and distribution. Short cooling delays, efficient
waxing with gentle brushing, fast cooling followed by
storage under constant low temperature and high relative
humidity are the main ways of limiting water loss.
Chilling injury (CI) or internal breakdown (IB)
The major physiological cause of deterioration for
peaches and nectarines is a low-temperature or chilling
injury (CI) problem generically called 'internal break-
down' (IB) (Plate 10.1) The genetic disorder can mani-
fest itself as dry, mealy, woolly or hard-textured fruit
(not juicy), flesh or pit cavity browning and flesh trans-
lucency usually radiating through flesh from the pit. In
all of the cases, flavour is lost before visual symptoms are
evident. However, there is large variability in CI suscep-
tibility among peach and nectarine cultivars (Tables 10.1
