Category Botany

   Ripening may be regarded as a special case of sequence. During ripening, a number of enzyme-assisted reactions take place inside the fruits. The list includes softening of tissues, hydrolysis, changes in pigmentation, flavour and respiration rate, and conversion of carbohydrates and organic acids into fruit sugars. These changes are induced by ethylene which is also called as ripening hormone.

            It has been found that during ripening, ethylene production goes up. In fleshly fruits like mango significant amount of ethylene may be present some time before ripening, but the fruit’s response to ethylene is inhibited till the fruit is harvested. In banana, a presumably effective ethylene concentration may be present in the unripe fruit, but the fruit is insensitive to that concentration at that stage. Only as it matures, it becomes sensitive and begins to ripen.

            Generally, an ethylene-forming mechanism and breaking of the insensitiveness to ethylene are attained only fruits reach a certain physiological age.

            When unripe fruits are kept inside a sack or tin of rice, the time needed to attain this critical physiological age is shortened. It could be that the fruit is totally cut off from light which promotes yellowing. (It is not known whether there is any increase in the temperature of the fruit.) The ethylene produced in the fruit also diffuses rapidly through the fruit’s tissues.

            If the fruits are placed in an airy place, this ethylene may be immediately lost in the air. When confined in rice or sack, its flow is restricted and there is always a layer of ethylene surrounding the fruit which accelerates ripening.

 

“Temperature changes can delay or hasten the ripening of banana. Banana is a tropical fruit, adapted to ripen quickly at a certain stage of its development and at a particular temperature and humidity. It continues to ripen after harvest, with more and more of its starch converted into sugars by the action of enzymes. When harvested, a banana contains about 20 per cent starch and only 1 percent sugar. By the time the fruit is ripe, the proportions are reversed.

            Banana also releases comparatively large quantities of ethylene gas to help itself ripen; the gas will even ripen other fruit put in a bag with a ripening banana.

            Bananas are usually harvested when still green, cutting off the supply of nutrients at the stem, and then shipped at a temperature low enough to slow the action of the enzymes of ripening. Later, the bananas are brought back up to a temperature and humidity that let the enzymes become active again.

            To high a temperature destroys the enzymes, and too low a temperature can break down the cell walls of the fruit so the contents mix and the fruit oxidizes browns and soften abnormally. The optimum temperature and humidity conditions for ripening are about 20 degree C and 90-95 per cent relative humidity. Storage temperatures should be about 13 degree C.

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The details can also be taken from NYT article: http://www.nytimes.com/1998/02/10/science/q-a-490229.html as originally this article has been published as link submitted above.

         

 

 

 Ripening of fruits is associated with the process of senescence or aging in plants. It involves change in colour, texture, flavour, sugar content and acidity, and is influenced by the ripening hormone ethylene.

            Mr. T. Nagendra Pillai of Guruvayoor, Kerala, writes: As ripening begins, there is a climacteric increase in respiration, which is followed by increased ethylene production. It triggers a series of biochemical changes such as lateral growth, loosening of cell walls resulting in more intercellular spaces, conversion of starch and organic acids into sugars, hydrolysis of stored materials, softening by enzymatic changes of pectin substances, decrease in chlorophyll content with corresponding increase in anthocyanins (colouring) pigments and emission of characteristic volatile oils. 

            Mr. S. Palaniappan of Pudukkottai, TN, adds that ethylene production is increased more than 100-fold during climatic rise.

            Mrs. P.S. Dheenadayalan, Cimbatore, says: Colour changes occur due to synthesis f carotenoids (yellow and red) and phenolic compounds like anthocyanins (red and blue).

            Changes in texture occur by limited degradation of cell walls followed by an increase in poly-galaturonase and pectin-esterase activity. In banana and apple, the enzyme phosphorylase and in mangoes, amylase, break the starch into glucose and sucrose leading to their sweet taste. Volatile compounds such as ethyl 2- methyl1 but rate (in apple) elicit a sweet smell.

            He adds, acidity of fruits is due to the presence of malic acid (in apple, apricot, banana, cherry and plum), citric acid (in gooseberry, tomato, and peaches), and malic acid and tartaric acid (in grapes).

            Ripening is a pre-requisite for the development of embryos after fertilization for better dispersal of seeds for survival.

     Apple contains an enzyme known as polyphenol oxidase (it is a copper containing enzyme).

            When the fruit is cut, this enzyme becomes reactive as it comes into contact with air. It reacts with the sugar present in the fruit and results in the formation of brown colour on the cut surface. If cut apple is dipped in an ascorbic acid solution browning of the cut surface can be prevented as the acid inhibits activity of the enzyme.

            Apple contains iron in the form of ferrous ions. These ferrous ions easily oxidize into ferric ions. This ion in the ferric state is brown in colour. When the apple is cut open the ferrous ions on the cut surface are exposed to the air. Air oxidizes them and the resulting ferric ions turn the surface brown.

The flavonoids are fifteen-carbon compounds that are generally distributed throughout the plant kingdom.

The most common basic flavonoid skeletron, shown below, is usually modified in such a way that more double bonds are present, causing the compounds to absorb visible light and thus giving them colour.

The two carbon rings at the left and right ends of the molecule are designed the A and B rings respectively.

Three widely distributed groups of flavonoids are: anthocyanins, flavonols, and flavones. The anthocyanins are coloured pigments most commonly seen in the red, purple and blue flowers. They are also present in various other plant parts, such as certain fruits, stems, leaves, and even roots.

Most fruits and flowers owe their colours to anthocyanins, although some, such as tomato fruits and several yellow flowers, are coloured by carotenoids.

Several different anthocyanins exist in plants, and often more than one is present in a particular flower. These molecules differ only in the number of hydroxy1 groups attached to the B ring of the basic flavonoid structure.

The exact colour of the anthocyanins depends first upon the substituent groups present on the B ring.

When methy1 groups are present they cause a reddening effect. Secondly, the anthocyanins are sometimes associated with other phenoic types of compounds, and this seems to cause them to become bluer. Finally, the pH of the cell sap has a strong controlling influence upon their colour.

The flavonols and flavones are closely related to the anthocyanins, except that they differ in the central oxygen-containing ring structure of the flavonoid. Naturally occurring flavonols and flavones are hydroxylated in various positions on both A and B rings.

Most of the flavones and flavonols are yellowish or ivory coloured pigments and, like the anthocyanins, they often contribute to the yellow, cream, ivory and white colour of flowers.

            Sometimes they do not appear coloured to the human eye, but they are apparent to bees or other insects that are attracted to flowers containing them. This is because the eyes of the insects are sensitive t ultraviolet wavelengths that give these compounds their colours.

                        

Nastic movement is responsible for blossoming of flowers. Usually this movement takes place in a flat plant part oriented relative to the plant body and produced by diffuse stimuli causing disproportionate growth or increased turgor pressue in the tissues of one surface. It normally occurs in leaves and petals which are bilaterally symmetrical.

          Changes in the environment send a signal to the plant and result in a differential growth between the upper and lower surfaces of petals resulting in blossoming of flowers and different conditions.

            In case of jasmine, this response occurs due to stimulus caused by the change over from brightness to darkness.

            As there is more growth on the upper sides (epinastic movement) of the petals, the flower opens. If there is more growth on the lower side of petals the flower closes (hyponastic movement).

Certain flowers such as sunflower are attracted to the sun strongly. They begin the day facing east and then follow the sun. This is because of a phenomenon called phototropism.

         Phototropism is a growth-mediated response of a plant to stimulation by visible light. The response is stimulated by a hormone called auxin present in the stem.

   Auxins promote lengthwise growth of plants. The auxin, beta-indylacetic acid (IAA), is formed either from the amino acid, tryptophan, or from the breakdown of carbohydrates known as glycosides.

They promote growth by acting on the chemical bonds of carbohydrates on the cell wall. In positively phototropic plants when one side of the plant is shaded, greater quantities of auxin are produced on the darker side. This causes that side of the plant to grow fast. In the case of sunflower, the phenomenon is pronounced so as to make the flower turn towards the sun.