Category Science

           Plant tissue culture (PTC) is the art and science of multiplying plants or plant parts (such as organs, tissues, cells, pollens, sores and embryos) under controlled conditions of light, temperature and humidity in an optimal nutrient medium under aseptic conditions in a glass vessel.

            Even a single cell has the potency to perform all the metabolic activities to form an independent plant. This phenomenon is known as totipotency. This is successfully used in tissue culture. Infinite number of plants can be produced from single explants in a span of time, irrespective of natural conditions. The seedlings will be genetically ‘true copies’ of the mother plant. That means, genetic purity can be maintained as far as required, in every seeding, which is almost impossible in conventional means of propagation.

            PTC has been successfully tried in almost all plant varieties. Some plants like orchids produce millions of non-endosperm us seeds in a single fruit, which cannot be cultivated in natural conditions. When grown through PTC, they have more than 75 percent germination. In the medium in which the plants are cultivated all the required micro and macro nutrients and vitamins are added. Since the growth inside the glass vessel is heterophic, a carbon source in the form of sucrose is added to the medium. PTC has been commercialized and is a lucrative business.

            In PTC, a very small tissue from a parent plant called as explants is placed in a test tube in a nutrient medium. The tissue may be taken from any part of the plant, that is, root, stem, leaf, anther or embryo. This is because all plant cells possess totipotency meaning a single cell can give rise to an entire plant.

            The nutrient medium used in tissue culture consists of sucrose apart from mineral salts and vitamins. Plant hormones such as Auxins are used to help growth and cell division. The solidifying agent, agar makes the medium semi solid otherwise the culture is done suspension. The inoculated tubes are kept in an incubator to maintain sterile conditions and controlled temperature and light. After 2-3 weeks of incubation an irregular mass of cells called callus develops, which on sub culturing gives rise to small plantlets. These are potted and maintained in a green house and subsequently transferred to the field. PTC is aimed at engineering crop plants for good traits.

Abscission is a physiological process whereby plants shed a part, such as leaf, flower or fruit, and retard their vegetative growth. It is a survival mechanism adopted by plants to live through adverse conditions and is promoted by a plant hormone called abscisin (also abscisic acid) produced by leaves and fruits. Extreme temperatures limit the metabolic activities such as respiration, of plants. Such a reduction consequently necessitates only a low level of photosynthetic activity.

The reduction in the requirement of energy can be to such a level which could be got from the photosynthesis activity of a few green cells, present in the terminal regions, after all the leaves fall.

Sometimes, in winter, ice crystals begin to form in the extra- cellular spaces and the viscosity of the cell protoplasm increases. To counter this, compatible osmotic such as betaine (an alkaloid) begins to build up. This process, osmo-regulation, helps plants to overcome the stress due to frost.

   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.