Dynamics of fruit
growth, biochemistry and molecular biology of fruit maturation
Fruit growth
The angiosperm fruit is formed from mature carpel or
several carpels generally with seeds and with or without accessory structures.
Usually, seed is formed from ovule and fruit from ovary. Ovary wall or pericarp
is generally differentiated into three layers. The outer layer is exocarp and
forms skin, the middle layer mesocarp is fleshy and inner layer is called
endocarp.
Dynamics
The growth in fruit is occurred by cell division or by
cell elongation. In majority of fruits, such as apples, most of the cell
division occurs prior to anthesis. There are a few division cycles in anthesis
but the growth in fruit size is due to cell enlargement. Fruit growth is
measured by increase in length, diameter, dry weight or fresh weight. There are
two patterns for all types of development – sigmoid or double sigmoid. Sigmoid
growth pattern is shown by the fruits like apple, pear, date palm and orange.
Double sigmoid curve is the characteristic of stone fruit such as mango,
almonds and fig, olive etc. Different stages of growth in fruits show distinct
morphological changes.
In the orange, the first period of slow growth increases
the thickness of peel by cell division. The development of the juice vesicles
occurs in the second stage of rapid growth and in the third stage, the growth
is slow and maturation of fruit takes place. Those fruits which exhibit a
double sigmoid growth curve have two periods o rapid growth which can be
separated by a period of slow development. In the stone fruit this slow period
is similar to cellularization of endosperm and hardening of the endocarp to
form the pit. During the development period, fruits change their shape. In
general, there is a reduction in the ratio of fruit length to diameter due to
periclinal cell division and radial cell enlargement.
Biochemistry
A large quantity of water and sugar get collected in most
of the fruits during their development and after the maturation, fruits contain
80- 90% water and 10- 20% sugar. While in some fruits, lipid contents are high.
Besides water and sugar, organic acids are also stored in fruits which give the
typical flavor to the fruits. In addition to it, volatile aromatic compounds are also found.
Auxins, gibberlins and cytokinins are produced actively by developing seeds
which are found in high level in the endosperm, pericarp. The control of fruit
growth by seeds can taken place by mobilization of such nutrients which are
necessary for their own growth and also for the changes required for the tissue
surrounding the fruit.
Molecular biology of Maturation
Maturation is defined as ‘The changes which occur from
the later stage of growth and development prior to senescence.’ The ripening of
a fruit is associated with a series of changes involving the color, the aroma,
the consistency and the flavor. There are a few physiological changes which
takes place during matuaration-
Respiratory
Climatric-
There is a rapid increase in rate of respiration in some
fruits as apple, mango, peach, pear etc., they are called climatric species
whereas in the non- climatric species such as cherry, fig and lemon the
respiratory pattern is downwards.
RNA metabolism-
There is an increase in RNA synthesis and at the same
time in RNA – splitting. The protein content also increases.
Enzymes-
Generally, there is an increase in the activity of many
hydrolytic enzymes as polygalacturonase, pectin methylesterase, lipase and
cellulose.
Pigment formation-
The maturation of fruit is clear by change in color. This
is due to the breakdown of chlorophyll molecules and formation of carotenoids
(red pepper fruit) or anthocyanins (apples, strawberries).
Changes in permeability-
The ripening fruits show a raised permeability.
Hormonal Regulation
Auxins, cytokinins, gibberlins, abssisic acid and
ethylene are involved in maturation of fruit. Auxins, cytokinins and gibberlins
slow down the process, while abssisic acid and ethylene are necessary for the
maturation of fruit.
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