RowQ
The Vault
RowQ
The Vault
CBSE Class 11 Biology · 10 questions · 24 marks
A seed becomes a tree through growth that never truly stops, guided by chemical messengers rather than a nervous system. This chapter defines growth as an open-ended, indeterminate process localised at meristems, follows the sigmoid curve that plots it over time, and introduces the five classic groups of plant hormones whose balance decides whether a bud sprouts, a fruit ripens or a leaf falls.
The plant hormone responsible for the phenomenon of apical dominance, where the terminal bud suppresses the growth of lateral buds, is:
Answer
Auxin is correct — auxin produced by the actively growing apical bud moves downward and inhibits the growth of lateral buds, and removing the apical bud (as in pruning) releases the lateral buds from this inhibition.
Gibberellins were first discovered in connection with which phenomenon?
Answer
'Foolish seedling' disease of rice caused by a fungus is correct — gibberellins were isolated from Gibberella fujikuroi, the fungus that causes rice seedlings to grow abnormally tall and spindly, which led to the discovery of this hormone class.
Which hormone is chiefly responsible for triggering the ripening of climacteric fruits and is used commercially to ripen fruit for the market?
Answer
Ethylene is correct — this gaseous hormone triggers the biochemical changes of ripening, including softening, colour change and sugar accumulation, and fruit sellers often use ethylene gas or ethylene-releasing compounds to ripen fruit rapidly and uniformly.
The growth curve obtained by plotting the increase in size of a plant organ against time is typically:
Answer
A sigmoid (S-shaped) curve is correct — growth starts slowly in the lag phase, accelerates through a rapid exponential (log) phase, and then slows and levels off as the organ approaches its mature size, giving the characteristic S shape.
Assertion (A): Abscisic acid is often called the stress hormone of plants. Reason (R): ABA promotes rapid stomatal closure under water-deficit conditions, helping the plant conserve water.
Answer
Both A and R are true and R is the correct explanation of A — ABA accumulates rapidly under water stress and acts on guard cells to close stomata within minutes, limiting water loss through transpiration, which is precisely the protective, stress-response role that earns it the name stress hormone.
Distinguish between short-day plants and long-day plants with respect to photoperiodism.
Answer
Short-day plants flower only when the length of the uninterrupted dark period they receive exceeds a certain critical duration, so they typically flower in late winter or early spring or in autumn when nights are long; chrysanthemum is a common example. Long-day plants flower only when the dark period is shorter than a critical duration, so they typically flower in the long days of late spring and summer; spinach is a common example. In both cases it is now understood that the length of the uninterrupted night, not the length of the day, is the factor actually being measured by the plant.
Describe the three phases of growth at the cellular level in a root or shoot.
Answer
The meristematic phase occurs at the tip of the root or shoot, where cells are small, densely cytoplasmic, thin-walled and actively dividing by mitosis, continuously adding new cells to the growing organ. Just behind this region lies the phase of elongation, where the newly formed cells stop dividing and instead enlarge considerably, mainly by taking up water into a growing central vacuole, which is the phase primarily responsible for the increase in length of the organ. Further back still is the phase of maturation, where the cells that have finished elongating undergo differentiation, developing the specific structural and biochemical features appropriate to the tissue they will become, such as thickened walls in xylem vessels, and reach their final, permanent size and shape.
Describe the discovery, sources and major physiological effects of auxins and cytokinins.
Answer
Auxins were the first plant hormone to be discovered, arising from Charles and Francis Darwin's observations on the bending of canary grass coleoptiles toward light, later traced by F. W. Went to a chemical substance diffusing from the coleoptile tip, which he named auxin and which was later identified chemically as indole-3-acetic acid (IAA). Auxins are synthesised mainly at shoot apices and young leaves and move down the plant. Their major effects include promoting cell elongation in shoots by loosening the cell wall, inducing apical dominance in which the growing shoot tip suppresses the growth of lateral buds below it, stimulating the initiation of adventitious roots on stem cuttings (which is exploited commercially in rooting hormones for propagation), and preventing premature fruit and leaf drop; at higher concentrations synthetic auxins such as 2,4-D act selectively as weedicides, killing broad-leaved dicot weeds in cereal fields while sparing the monocot crop. Cytokinins were discovered when it was found that coconut milk, and later an artificial compound called kinetin obtained from herring sperm DNA, could induce cell division in plant tissue cultures even in the presence of auxin. Naturally occurring cytokinins are synthesised mainly in regions of active cell division such as root tips, and they are transported upward through the xylem. Their major effects include promoting cytokinesis and cell division, delaying the senescence (ageing and yellowing) of leaves, a phenomenon called the Richmond-Lang effect, helping to overcome apical dominance by promoting the growth of lateral buds, and controlling the pattern of organ formation in tissue culture, where a high ratio of cytokinin to auxin favours the formation of shoots while a high ratio of auxin to cytokinin favours root formation.
Explain how gibberellins, ethylene and abscisic acid each influence plant development, with one specific physiological example of each.
Answer
Gibberellins are a large group of hormones, first identified from the fungus Gibberella fujikuroi, that are best known for causing pronounced elongation of the stem. A striking example is 'bolting', where genetically dwarf varieties of cabbage or rosette plants such as beet, when treated with gibberellin, rapidly elongate their internodes and develop a tall flowering stalk almost overnight, a response so dramatic it is used as a bioassay for the hormone. Gibberellins also promote the germination of dormant seeds and buds by inducing the synthesis of hydrolysing enzymes such as amylase in the aleurone layer of cereal grains, and they can substitute for the cold or light requirement that some seeds otherwise need before they will germinate. Ethylene is unusual among plant hormones in being a simple gas that diffuses readily through plant tissue and the surrounding air. Its best-known effect is the induction of fruit ripening: as a fruit begins to ripen it produces a burst of ethylene that triggers the softening of the cell wall, breakdown of chlorophyll, conversion of starches to sugars and development of characteristic aroma and colour, and this response is exploited commercially by exposing unripe fruit to ethylene gas to ripen it rapidly for market. Ethylene also promotes the senescence and abscission of leaves and flowers and influences sex expression in cucurbits, increasing the proportion of female flowers when applied to cucumber vines. Abscisic acid acts largely as a growth inhibitor and stress hormone, generally opposing the effects of the growth-promoting hormones. Under conditions of water deficit, ABA accumulates rapidly in leaves and acts on the guard cells surrounding stomata, causing them to lose turgor and close within minutes, which is the fastest known hormonal response in plants and helps the plant conserve water by curbing transpiration. ABA is also responsible for inducing and maintaining dormancy in many seeds and buds, preventing premature germination or sprouting until conditions such as temperature and moisture become favourable, so that the seed's dormancy is broken only when ABA levels fall and the balance shifts toward growth-promoting hormones such as gibberellin.
A grower notices that mango trees in his orchard have grown very tall with a single dominant central shoot and few side branches. To make the trees bushier and easier to harvest, he prunes off the tip of each main shoot. Within two weeks, several lateral buds along each pruned shoot begin actively growing into new branches. Separately, he treats a batch of unripe green mangoes with a gas to speed up their ripening for market, and within days they turn yellow and soften. (a) Name the hormone responsible for the original single dominant shoot pattern, and the phenomenon it caused. (b) Explain, in hormonal terms, why removing the shoot tip caused the lateral buds to start growing. (c) Name the gas used to ripen the mangoes and describe one physiological change it triggers during ripening. (d) If the grower instead wanted to delay ripening during transport, suggest what type of hormonal treatment might help, with reasoning.
Answer
(a) The hormone responsible is auxin, produced by the actively growing apical bud at the shoot tip, and the phenomenon it caused is apical dominance, the suppression of lateral bud growth by the dominant terminal shoot. (b) Pruning removed the source of auxin at the shoot tip, so the high concentration of auxin that had been suppressing the lateral buds was no longer being supplied to them. With this inhibitory influence removed, the lateral buds were released from dormancy and began to grow actively into new branches, restoring a more branched growth habit. (c) The gas used is ethylene. During ripening it triggers breakdown of the green pigment chlorophyll (revealing yellow carotenoid pigments), softening of the fruit through cell wall degradation, and conversion of starches into sugars, giving the fruit its ripe colour, texture and sweetness. (d) To delay ripening, the grower could apply or maintain conditions favouring abscisic acid, or more practically reduce ethylene exposure and lower storage temperature, since ABA and cool conditions generally slow down the metabolic changes of ripening; keeping ethylene gas away from stored fruit and refrigerating it during transport are standard practical measures based on this principle of opposing the ripening-promoting hormone.
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