It has been known since a long time that certain gases stimulate fruit ripening eventhough its scientific study dates back to the beginning of the 20th century. Ancient Chinese knew that fruits stored in a room where incense is burnt ripen quickly (Salisbury and Ross, 1986). Denny (1921) reported that ethylene might help in fruit ripening.
A conclusive evidence however was ultimately provided by GAne (1934) who reported the role of ethylene synthesized by plants themselves contributing to the ripening of the fruits. Ethylene may be regarded as a volatile hormone as it is a gas under ordinary temperature. Maximum rate of ethylene production has been reported in many fruits just before senescence. Ethylene production has also been seen in etiolated pea seedlings. In light however ethylene production drops down.
Biosynthesis of ethylene:
Available evidences seem to point out that methionine; one of the amino acids may act as the precursor of ethylene (Morris and Mapson, 1964). Methionine is first converted to S – adenosylmethionine (SAM), the reaction requiring the participation of ATP. This is next converted to ACC (1-aminocyclopropane -1-carboxylic acid) under the influence of the enzyme ACC synthase (Adams and Yang, 1979).
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Philips (1971) has suggested two alternate pathways of ethylene synthesis with methionine as the precursor.
Physiological Roles of Ethylene:
1. Abscission:
Ethylene promotes the changes that occur before the abscission of leaves, flowers and fruits. It promotes cellulase activity.
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2. Ripening and climacteric rise:
Climacteric rise is a term employed to denote the increased rate of respiration in mature and senescing tissues such as ripe fruits. Originally it was believed that ethylene is a product and not cause of senescence. Recent studies however have shown that ethylene production begins even before the climacteric rise.
3. Activation of enzymes:
Ethylene activates the action of certain enzymes like malic enzyme and pyruvate decarboxylase. Chlorophyllase, an enzyme that degrades chlorophyll is also activated by ethylene.
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4. Elongation of roots and stems:
Ethylene inhibits root and stem elongation in dicots.
5. Effects on flowering:
Although, ethylene generally inhibits flowering in most plants, in some of the bromeliads like-pineapple, ethylene is known to initiate the flowering. An ethylene releasing substance has been commercially produced (ethrel) to promote flowering in pineapple. Ethylene also produces the degreening effect in citrus and banana fruits.