Ncert Questions
Question 1. Define growth, differentiation, development, dedifferentiation, redifferentiation , determinate growth, meristem and growth rate.
Answer: Growth is an increase in the amount of protoplasm, usually accompanied by an irreversible increase in size and weight involving the division, enlargement and the differentiation of cells. Differentiation. The process of becoming different in structure and function is termed differentiation. The differentation of cells involves a series of modifications such as changes in shape, size extent of secondary wall and protoplasmic contents. Development. Development is the sequence of processes in the overall life history of a cell or an organism including growth, differentiation, maturation and senescence. Dedifferentiation Plants show another interesting phenomenon. The living differentiated cells that by now have lost the capacity to deicide and regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation. Redifferentiation, interfascicular cambium and cork cambium these meristems / tissues are able to produce cells that once again lose the capacity to divide but mature to perform specific function, i.e get redifferentiated and this phenomenon is called redifferentiation. Determinate Growth. It is a type of growth in which growth follows a precise pattern such that each part has its characteristic position and an unalterable fate. Meristem. Specific areas in higher plants, which take part in the formation of new cells, are called meristems. The cell of such meristems has the capacity to divide and self- perpetuate. Meristems are responsible for the primary growth of the plant and principally contribute to the elongation of the plants. Growth rate. The increased growth per unit time is termed as growth rate. Growth rate can be expressed mathematically.
Question 2. Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer: Growth at a cellular level is principally a consequence of increase in the amount of protoplasm. Since increase in protoplasm is difficult to measure directly, one generally measures some quantity, which is more or less proportion to it. Growth is, therefore, measured by a variety of parameters. Increase in fresh weight;Increase in dry weight;Increase in length, area and volume;No increase in cell number.
Question 3. List five main groups of natural plant growth regulators. Write a note of discovery, physiological functions and agricultural / horticultural applications of any one of them.
Answer: The five main groups of natural plant growth regulators are:AuxinsGibberellinsCytokininsEthyleneAbscisic acid Discovery, physiological functions and applications of gibberellins is as follows:- Discovery of Gibberellins:Gibberellins are kind of promotery PGR. There are more than 100 gibberellins reported from different organisms such as fungi and higher plants. They are denoted as GA1, GA2, GA3 and so on. However GA3 was one of the first gibberellins to be discovered and remains the most intensively studied form. The ‘bakane’, (foolish seedling) a disease of rice seedlings was caused by a fungal pathogen Gibberalla fujikuroi E. Kurosawa reported the appearance of symptoms of the disease in uninfected rice seedlings when they were treated with sterile filterate of the fungus. The active substances were later identified as gibberellic acid. Physiological effects of gibberellins are: 1. Elongation of stem and expansion of leaf. 2. Reversal of dwarfism, particularly in corn. 3. Parthenocarpy. 4. Substituting cold treatment. 5. Seed germination as gibberellins stimulate production of digestive enzymes. 6. Breaking of seed and bud dormancy. 7. Bolting occurs at start of the reproductive phase. It can be artificially induced in cabbage by application of gibberellins 8. Cell division of cambium. Practical uses of application of Gibberellins. 1. To break seed dormancy. 2. To induce parthenocarpy. Commercial application such as spraying of Thompson seedless grapes results in larger fruits with longer stalks. 3. To increase yields of melons and useful curcurbits. 4. It causes elongation of sugarcane and also increases sugarcane contents.
Question 4. What do you understand about photoperiodism and vernalisation? Describe their significance.
Answer: Photoperiodism. The ability of plant to detect and respond to the length of daily period of light or (relative length of day and night) to which the plant is exposed is called photoperiodism. It is a physiological change occurring in plants in response to relative length of day and night. The term photoperiodism. It is a physiological change occurring in plants in response to relative length of day and night. The term photoperiodism was used by Garner and Allard (1920) for the responses of plant to photoperiods expressed in the form of flowering. On the basis of photoperiod, there are three classes of plants. 1. Short day plant (SDP) e.g. Nicotiana tobaccum, Xanthium, Rice, Dahlia, Cosmos, Soyabean. 2. Long day plant (LDP) e.g. Henbane (Hyoscyamus niger), wheat, oat, sugar, beat, Radish, Lettuce. 3. Neutral or intermediate day plants or Short daylong plant (S-LDP) or Day neutral plants e.g. Tomato, Sunflower etc. Vernalization. The term vernalization was coined by Lysenko (1928) for promotion of flowering by a previous cold treatment. The method of inducing early flowering by pre treatment of seeds with a certain low temperature is known as vernalization. For flowering in winter varieties, winter cold treatment is necessary. In nature, plant requiring cold treatment is necessary usually behave as biennials. They germinate and grow vegetatively in first season and produce flowers during second season after getting the cold treatment. Some plants respond to the vernalization at seed or seedling stage e.g. secage cereale. It is now definitely known by various grafting experiments that growing point is the sire, which receives the cold stimulus. It is responsible for the production of a hormone like substance called as vernalin. It is basically a type of gibberellin.
Question 5. Why is abscisic acid also known as stress hormone?
Answer: Abscisic acid carries out the physiological function of increasing the tolerance of plants to various kinds of stresses. Hence it is also called stress hormone. Drought, water logging and other adverse conditions stimulate its production.
Question 6. "Both growth and differentiation in higher plants are ‘open’" Comment.
Answer: In higher plants, when growth occurs new cells are always being added to the plant body by the activity of the meristem. Such a type of growth is referred to as open form of growth. Even differentiation in plants is open because cells or tissues arising out of the same meristem have different structures at maturity. The final structure at maturity of a cell / tissue is determined by the location of the cell within.
Question 7. "Both a short day plant and a long day plant produce flower simultaneously in a given place"- Explain.
Answer: Flowering in certain plants at a given place depends not only on a combination of height and dark exposures but also their relative duration. For flowering both the duration of light and the duration of darkness are equally important. Some plants require the exposure to light for a period exceeding a well defined critical duration; such plants are short day plants. Certain other plants must be exposed to light for a period less than the critical duration before flowering is initiated & such plants are long day plants.
Question 8. Which one of the plant growth regulators would you use if you are asked to:
(a) induce rooting in a twig,
(b) quickly ripen a fruit,
(c) delay leaf senescence,
(d) induce growth in axillary buds,
(e) ‘bolt’ a rosette plant
(f) induce immediate stomatal closure in leaves.
Answer: (a) auxins(b) ethylenne(c) cytokinins(d) cytokinins (e) gibberellins(f) abscisic acid.
Question 9. Would a defoliated plant respond to photoperiodic cycle? Why?
Answer: A defoliated plant does not respond to photoperiodic cycle at all. This is because the site of perception of light or dark stimuli are the leaves. Leaves have the hormone florigen that makes them capable of responding to stimulus of photoperiodicity. So in the absence of any foliage over the plant, there is no response towards photoperiodic cycle.
Question 10. What would be expected to happen if:(a) GA3 is applied to rice seedlings, (b) Dividing cells stop differentiating(c) A rotten fruit gets mixed with unripe fruits, You forget to add cytokinin to the culture medium.
Answer: (a) Application of GA3 to rice seedlings shall cause an intense increase in cell growth of stem- young leaves and other aerial parts. (b) If dividing cells stop differentiating then there is the possibility of formation of a large mass of cells in the form of tumor that may cause cancerous condition. (c) If we forget to add cytokinin to the culture medium then the cell division process may halt as cytokinin is a plant growth regulator that is responsible for the induction of cytokinensis.