Exercises
Define growth, differentiation, development, dedifferentiation, redifferentiation, determinate growth, meristem and growth rate.
Answer
Growth: An irreversible, permanent increase in the size, weight, volume, or number of cells of an organism over a period of time.
Differentiation: The process by which cells derived from the apical meristem acquire structural and functional specialisation, becoming different from each other and from the parent meristematic cells.
Development: All the changes that an organism undergoes from germination to senescence, encompassing growth, differentiation, and maturation.
Dedifferentiation: The regaining of the capacity to divide by already differentiated cells (e.g., parenchyma cells of cortex becoming cambium-like during wound healing or regeneration).
Redifferentiation: The process by which dedifferentiated cells once again lose their ability to divide and differentiate into specific cells/tissues to perform functions.
Determinate growth: Growth that stops after reaching a certain size or stage; characteristic of animals and leaves of plants.
Meristem: A group of actively dividing cells in plants (e.g., apical and lateral meristems) that are capable of continuous cell division and are responsible for growth.
Growth rate: The increase in growth per unit time; it may be arithmetic (constant absolute increase) or geometric (increase proportional to existing size).
Why is not any one parameter good enough to demonstrate growth throughout the life of a flowering plant?
Answer
Growth in plants is a complex process that manifests in multiple ways: increase in cell number (mitosis), cell volume, fresh weight, dry weight, length, surface area, etc. No single parameter captures all aspects of growth equally well. For example, fresh weight increases with water absorption and may not reflect actual growth; dry weight is better but is lost in seeds during germination. Length and surface area apply mainly to certain organs. In a seed, the embryo is in a dormant state, and measuring growth by weight would be misleading as the seed may not be visibly growing. Hence, a combination of parameters gives a more accurate picture of growth at different stages.
Describe briefly: (a) Arithmetic growth (b) Geometric growth (c) Sigmoid growth curve (d) Absolute and relative growth rates
Answer
(a) Arithmetic growth: In arithmetic growth, one of the daughter cells resulting from mitotic division continues to divide while the other differentiates. Growth occurs at a constant rate; if plotted against time, it gives a linear (straight-line) curve. Example: elongation of a root at a constant rate. Lt = L0 + rt, where Lt = length at time t, L0 = initial length, r = growth rate per unit time.
(b) Geometric growth: In early stages, plants (and most organisms) show geometric (exponential) growth — both daughter cells continue to divide. Growth is rapid and doubles at each division: W1 = W0 × ert, where r is relative growth rate. When plotted, gives a J-shaped (exponential) curve until resources become limiting.
(c) Sigmoid growth curve: When geometric/exponential growth slows due to nutrient depletion or space limitations, the growth curve becomes S-shaped (sigmoid). It has three phases: (i) lag phase (slow initial growth), (ii) log/exponential phase (rapid growth), (iii) stationary phase (growth rate slows, reaches a plateau). This is the typical growth curve seen in natural populations and organisms.
(d) Absolute growth rate is the measurement of total growth per unit time (e.g., cm/day). Relative growth rate is the growth per unit time expressed per unit of initial measure (e.g., g/g/day), indicating the efficiency of growth. Relative growth rates allow comparison between organs or species of different sizes.
List five main groups of natural plant growth regulators. Write a note on discovery, physiological functions and agricultural/horticultural applications of any one of them.
Answer
The five main groups of plant growth regulators (PGRs) are: (1) Auxins, (2) Gibberellins, (3) Cytokinins, (4) Abscisic acid (ABA), and (5) Ethylene.
Gibberellins — Discovery: Gibberellins were discovered by Japanese scientist E. Kurosawa (1926) while studying "Bakanae" (foolish seedling) disease of rice caused by the fungus Gibberella fujikuroi. The chemical was isolated and characterised as gibberellic acid (GA3). Over 100 types of gibberellins are now known.
Physiological functions: (i) Promote stem elongation (internodal elongation) by stimulating cell elongation and division; (ii) Break seed dormancy and promote germination; (iii) Promote flowering in long-day plants and biennials (bolting); (iv) Delay senescence and promote fruit set; (v) Regulate stamen development (in pumpkin); (vi) Promote elongation and growth of fruits.
Agricultural/horticultural applications: (i) Used to increase the length and quality of grape bunches (seedless grapes); (ii) Speed up malting in brewing industry; (iii) Delay fruit ripening for long-distance transport; (iv) Promote flowering and fruit setting in citrus crops; (v) Increase yield in sugar cane by elongating the stem (increasing sugar content).
Why is abscisic acid also known as stress hormone?
Answer
Abscisic acid (ABA) is called the stress hormone because it is produced in plants in response to various environmental stresses and helps the plant cope with them. Under water stress (drought), ABA levels rise sharply and cause: (i) rapid closure of stomata (to prevent water loss); (ii) promotion of root growth (to access deeper water sources); (iii) induction of seed dormancy (to survive unfavourable conditions); (iv) abscission of leaves (to reduce transpiration). Under cold/heat stress, ABA also accumulates. Because it mediates these stress responses, ABA is called the stress hormone.
'Both growth and differentiation in higher plants are open.' Comment.
Answer
The term "open" refers to the indeterminate nature of growth and differentiation in higher plants. Unlike animals, plants retain meristematic regions (shoot apical meristem, root apical meristem, lateral meristems) throughout their life, allowing indefinite growth. Growth and organ formation (leaves, branches, flowers) can continue as long as the plant is alive. Differentiation is also "open" in the sense that already differentiated cells retain the capacity to dedifferentiate and redifferentiate, enabling regeneration and plasticity. This is in contrast to animals where most differentiation is determinate and irreversible.
'Both a short day plant and a long day plant can flower simultaneously in a given place.' Explain.
Answer
Photoperiodism governs flowering: short day plants (SDPs) flower when the night length exceeds a critical minimum (short days / long nights), while long day plants (LDPs) flower when night length is shorter than a critical maximum (long days / short nights). SDPs and LDPs can flower simultaneously in the same place if the critical night/day lengths of both are satisfied at the same time of year. For example, an SDP that requires nights longer than 12 hours and an LDP that requires nights shorter than 14 hours can both flower when day length is, say, 13 hours — the LDP's requirement (day > 12h) and SDP's requirement (night > 10h) may overlap in a particular season at a given location.
Q8. Which plant growth regulator would you use if you are asked to
(a) induce rooting in a twig
Ans. Auxin (IAA/IBA) — promotes adventitious root formation in stem cuttings.
(b) quickly ripen a fruit
Ans. Ethylene — promotes fruit ripening (used as ethephon, which releases ethylene).
(c) delay leaf senescence
Ans. Cytokinin — delays senescence by promoting protein synthesis and retarding degradation.
(d) induce growth in axillary buds
Ans. Cytokinin — relieves apical dominance and promotes lateral bud/axillary bud growth.
(e) 'bolt' a rosette plant
Ans. Gibberellin — causes rapid elongation (bolting) of the internode in rosette plants.
(f) induce immediate stomatal closure in leaves
Ans. Abscisic acid (ABA) — rapidly induces stomatal closure by causing efflux of K+ and water from guard cells.
Would a defoliated plant respond to photoperiodic cycle? Why?
Answer
No, a defoliated plant would not respond to the photoperiodic cycle. The leaves are the organs that perceive the light/dark cycle (photoperiod). The light-sensitive pigment phytochrome is present in leaves and perceives the duration of light and darkness. When leaves are absent, no photoperiodic signal can be generated or transmitted to the shoot apex (where flowers are induced). The flower-inducing stimulus (florigen/flowering hormone) is produced in the leaf and transported to the apex; without leaves, this signal is absent, and the plant cannot flower in response to appropriate photoperiods.
Q10. What would be expected to happen if
(a) GA3 is applied to rice seedlings
Ans. GA3 would cause excessive elongation of the rice seedlings — they would become abnormally tall, thin, and pale green (mimicking the "Bakanae" or foolish seedling disease). The internodes would elongate dramatically.
(b) dividing cells stop differentiating
Ans. If dividing cells stopped differentiating, the plant would not be able to form specialised tissues and organs. Growth would become chaotic (similar to tumour growth/callus), forming an unorganised mass of meristematic cells without functional leaves, stem, roots, or flowers. The plant would not survive normally.
(c) a rotten fruit gets mixed with unripe fruits
Ans. The rotten (overripe) fruit releases ethylene gas, which would accelerate the ripening of the unripe fruits around it. This is the basis of the traditional practice of keeping a ripe fruit (like a banana) with unripe fruits to hasten their ripening.
(d) you forget to add cytokinin to the culture medium
Ans. In tissue culture media, cytokinin promotes cell division and shoot differentiation. Without cytokinin, the cells/callus would not differentiate into shoots; instead, the culture might continue to grow as an undifferentiated callus or remain stunted, with no shoot organogenesis. The root-to-shoot balance would shift toward root formation (since auxin would dominate).
Important Extra Questions
What is vernalisation?
Answer
Vernalisation is the phenomenon by which prolonged exposure to low temperature (cold treatment) during early growth accelerates or enables flowering in certain plants (biennials and winter annuals). For example, winter wheat requires a cold period before it can flower in spring. Vernalisation ensures the plant does not flower prematurely and synchronises flowering with favourable conditions.
What is the role of ethylene in fruit ripening?
Answer
Ethylene promotes fruit ripening by stimulating the synthesis of enzymes that break down cell wall components (pectin, cellulose), converting starch to sugars, changing fruit colour (chlorophyll degradation, synthesis of anthocyanins), and softening the fruit. It also promotes the production of more ethylene (autocatalytic), ensuring uniform and rapid ripening.
Differentiate between photoperiodism and vernalisation.
Answer
Photoperiodism is the response of a plant to the relative duration of light and darkness (day length), regulating flowering. Vernalisation is the requirement of a cold period (low temperature) for a plant to become competent to flower. Photoperiodism involves a light stimulus perceived by the leaf; vernalisation involves a temperature stimulus perceived by the meristem/growing point.
What are senescence and abscission?
Answer
Senescence is the natural process of ageing in plant organs (leaves, flowers, fruits) leading to deterioration and eventual death, involving breakdown of chlorophyll, proteins, and nucleic acids. Abscission is the shedding of leaves, flowers, or fruits from the plant, mediated by the formation of an abscission zone at the base of the organ and regulated by the balance of auxin and ethylene.
What is apical dominance and how is it related to auxin?
Answer
Apical dominance is the suppression of lateral (axillary) bud growth by the actively growing apical bud. The apical meristem produces auxin (IAA), which moves downward and maintains high auxin concentrations in the lateral buds, inhibiting their growth. Removal of the apical bud reduces auxin levels, allowing lateral buds to grow, resulting in bushier plants. Cytokinin can also overcome apical dominance by promoting lateral bud growth.