Ethylene (Gaseous
Phytohormone)
Almost
all plant tissues produce ethylene gas in minute quantities.
In 1924, Denny found that ethylene stimulates
the ripening of lemons. In 1934, R. Gane
found that ripe bananas contain abundant ethylene. In 1935, Cocken et al., identified ethylene as a natural plant hormone.
Maximum
synthesis occurs during climacteric ripening of fruits (see Box info) and tissues undergoing senescence. It is formed in
almost all plant parts like roots, leaves, flowers, fruits and seeds.
Ethylene
can easily diffuse inside the plant through intercellular spaces.
It is a
derivative of amino acid methionine, linolenic acid and fumaric acid.
Ethylene
can be measured by gas chromatography. This technique helps in the detection of
exact amount of ethylene from different plant tissues like lemon and orange.
·
Ethylene stimulates respiration and ripening in
fruits.
·
It stimulates radial growth in stem and root and
inhibits linear growth.
·
It breaks the dormancy of buds, seeds and storage
organs.
·
It stimulates formation of abscission zone in
leaves, flowers and fruits. This makes the leaves to shed prematurely.
·
Inhibition of stem elongation (shortening the
internode).
·
In low concentration, ethylene helps in root
initiation.
·
Growth of lateral roots and root hairs. This
increases the absorption surface of the plant roots.
·
The growth of fruits is stimulated by ethylene in
some plants. It is more marked in climacteric fruits.
·
Ethylene causes epinasty.
·
Ethylene normally reduces flowering in plants
except in Pine apple and Mango.
·
It increases the number of female flowers and
decreases the number of male flowers.
·
Ethylene spray in cucumber crop produces female
flowers and increases the yield.
Climacteric fruits: In most of the plants,
there is sharp rise in respiration rate near the end of the development of
fruit, called climacteric rise. Such fruits are called climacteric fruits. The
ripening on demand can be induced in these fruits by exposing them to normal
air containing about 1 ppm of ethylene. A liquid called ethephon is being used
in fruit ripening as it continuously releases ethylene.
Example: Tomato, Apples, Banana, Mango.
Non climacteric fruits: All fruits cannot be
ripened by exposure to ethylene. Such fruits are called non-climacteric fruits
and are insensitive to ethylene.
Example: Grapes, Watermelon, Orange.
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