Brinjal and Apples become dark when they are kept open after cutting. Why?

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 (C2H2).

Apart from fruit ripening, some other notable impacts of Ethylene are as follows:

In pollination, when the pollen reaches the stigma, the precursor of the ethylene, ACC (1-Aminocyclopropane-1-carboxylic acid) , is secreted to the petal, the ACC releases ethylene with ACC oxidase.

• Ethylene stimulates leaf and flower senescence and senescence of mature xylem cells in preparation for plant use.
• Induces leaf abscission and seed germination
• Induces root hair growth — increasing the efficiency of water and mineral absorption
• Induces the growth of adventitious roots during flooding
• Affects gravitropism
• Inhibits stem growth and stimulates stem and cell broadening and lateral branch growth outside of seedling stage.
• Induces flowering in pineapples

Process of Fruit Ripening

The process of fruit ripening is chiefly regulated by a gaseous plant hormone called ethylene. Most fruits have elevated ethylene levels during ripening and sometimes just a peak in ethylene levels, just before the process of ripening begins. Ethylene regulates the expression of several genes involved in fruit ripening so as to modulate the activity of various enzymes involved in the process of ripening. These enzymes act to soften the `skin’ of the fruit and also convert complex polysaccharides into simple sugars. The chemical commonly used to ripen fruits commercially is ethephon (2-chloroethylphosphonic acid), which penetrates into the fruit and decomposes to ethylene. Incidentally, chemicals (e.g. calcium carbide) that produce acetylene, an analogue of ethylene, are also used in some places posing dangers of explosion and carryover of toxic materials to consumers. Ethylene is induced by several cues such as higher temperature, wounding, disease etc. Higher levels of ethylene and enhanced respiration might contribute to ripening when stored at higher temperatures.