The reason for the darkening of the fruits, when kept open after cutting can be related to the mechanism of ripening. Hence, it is essential to know about the ripening mechanism at first and then extend it to explain the reason, for the question asked.
In many fruits, especially the fleshy types, there are marked changes in their `respiration rate' after they mature. In certain fruits, there is a considerable increase in the respiration rate at the time of `ripening', which is known as climacteric rise.
Even after the fruits are harvested, they continue to respire, which leads to consumption of food, water and other cellular fluids. This results in the breakdown of tissues eventually and causes the perishability of fruits.
The onset of fruit ripening happens due to the release of ethylene (H{-2}C=CH{-2}). Earlier it was mentioned that there is climacteric respiratory rise during ripening. In other words, the climacteric respiration rise and release of C{-2}H{-4} complement each other in facilitating the ripening.
As a consequence of this, metabolic and physical transformations happen such as:
-Hydrolysis of starch to sugar.
-`Softening' (i.e.) increase in the permeability of the cell walls.
- Changes in the `pigmentaiton' and `flavor' and
eventually resulting in overall breakdown of tissues.
Hence, if the climacteric rise is appropriately controlled, then one can regulate the C{-2}H{-4} release, which might help in controlling the rate of ripening.
When a fruit is cut and kept open, then there is greater availability of Oxygen (O{-2}). So, there is an increase in the respiration rate (and concomitant increase in the release of C{-2}H{-4}), resulting in the above-mentioned metabolic transformations more readily (i.e.) faster breakdown of tissues.
Thus, the darkening is nothing but ``over-ripening''. In a sense, the fruit ripening may be regarded as a special case of senescence.
The respiration rate is calculated by experimentally measuring CO{-2} liberated{rcub}
Note: Apple, banana exhibit climacteric behaviour, while citrus fruits like lemon, orange do no exhibit this behaviour.
Published in The Hindu on Dec 13, 2001.
