How does an unripe fruit turn sweet a few days after it is plucked ?
SNEHA ANTONY,Thiruvananthapuram
Ripening is a physiological process involving the induction of a variety of metabolic process, making the fruit sweeter and more palatable. Plants send unique ripening signals using hormones and most of the processes are enzymatically regulated and catalyzed. The process of fruit ripening is primarily regulated by a gaseous plant hormone called ethylene (CH). This ethylene is produced and released by rapidly-growing plant tissues.
It is released by the growing tips of roots, flowers, damaged tissue, and ripening fruit. Hence, the act of picking matured green fruit can cause a wound which activates ethylene production and induce the ripening process.
This phytohormone (ethylene) is said to regulate the expression of several genes involved in fruit ripening so as to modulate the activity of various enzymes involved in the process of ripening. New enzymes are made because of this ethylene signal and they catalyze reactions to alter the characteristics of the fruit. These include hydrolases to help break down chemicals inside the fruits, amylases to accelerate hydrolysis of starch into sugar, pectinases to catalyze degradation of pectin.
In simpler words, the action of these enzymes causes the ripening responses. Chlorophyll is broken down and sometimes new pigments are made so that the fruit skin changes colour from green to red, yellow, or blue. The degradation of starch by amylase produces simple sugar. The breakdown of pectin, thanks to pectinase, results in a softer fruit. Acids are broken down so that the fruit changes from sour to neutral. Besides, these enzymes break down large organic molecules into smaller ones that can be volatile (evaporate into the air) and we can detect as an aroma.
However these phenomena will be observed only in climacteric fruits such as mango, apple, banana, guava, pineapple etc and these fruits are able to continue ripening after being picked, a process accelerated by ethylene gas. Non-climacteric fruits such as watermelon, strawberries and oranges, do not ripen after harvest.
U. SURENDRAN
Scientist, Centre for Water Resources Development & Management,Kozhikode, Kerala
