Part 2 — Phases of Plant Growth & Growth Rate

Part 2 — Phases of Plant Growth & Growth Rate

26. Which of the following is NOT one of the classical phases of plant growth?
    A. Cell division
    B. Cell elongation (expansion)
    C. Cell death (senescence as primary growth phase)
    D. Maturation/differentiation
    Answer: C
    Explanations:

• A: Incorrect. Cell division (in meristems) is a classical growth phase.

• B: Incorrect. Cell enlargement/elongation follows division.

• C: Correct. Senescence is an eventual stage but not a classical active growth phase.

• D: Incorrect. Maturation/differentiation is the final growth phase.

27. The sigmoid growth curve in plants indicates:
    A. Constant growth rate throughout life
    B. Three phases: lag, exponential (log), and stationary/maturation
    C. Only exponential growth without limit
    D. Immediate senescence after germination
    Answer: B
    Explanations:

• A: Incorrect. Growth rate changes with time, not constant.

• B: Correct. Sigmoid curve shows lag (slow), log (rapid), and plateau (maturation).

• C: Incorrect. Exponential occurs only in log phase and isn’t indefinite.

• D: Incorrect. Senescence occurs later, not immediate.

28. Relative Growth Rate (RGR) is defined as:
    A. Absolute growth per plant per unit time
    B. Growth per unit biomass per unit time
    C. Total height of plant divided by age
    D. Number of leaves per plant
    Answer: B
    Explanations:

• A: Incorrect. That’s absolute growth rate.

• B: Correct. RGR = (1/W)(dW/dt), growth relative to existing mass.

• C: Incorrect. Not the formal RGR definition.

• D: Incorrect. Not related to RGR metric.

29. Absolute growth rate differs from relative growth rate because:
    A. Absolute is per unit biomass; relative is per plant.
    B. Absolute measures actual increase (e.g., dry weight/day); relative standardizes to biomass.
    C. They are identical.
    D. Relative growth rate ignores time.
    Answer: B
    Explanations:

• A: Incorrect. That reverses definitions.

• B: Correct. Absolute growth is raw increase (g/day); RGR is normalized per g biomass.

• C: Incorrect. They differ conceptually and numerically.

• D: Incorrect. RGR includes time (per unit time).

30. Which phase involves most cell differentiation?
    A. Lag phase
    B. Exponential (log) phase
    C. Maturation phase
    D. Dormant phase
    Answer: C
    Explanations:

• A: Incorrect. Lag is preparatory with limited differentiation.

• B: Incorrect. Log phase emphasizes division and expansion more than final differentiation.

• C: Correct. Maturation phase sees cells completing differentiation and specialisation.

• D: Incorrect. Dormancy is inactive, not differentiation-focused.

31. Which is a correct statement about plant growth rate?
    A. RGR is higher in mature trees than seedlings.
    B. Young seedlings often show higher RGR than older plants.
    C. RGR is constant across species and ages.
    D. Absolute growth never changes with age.
    Answer: B
    Explanations:

• A: Incorrect. Mature trees usually have lower RGR than fast-growing seedlings.

• B: Correct. Seedlings often show high RGR due to smaller biomass base.

• C: Incorrect. RGR varies with species, environment, and age.

• D: Incorrect. Absolute growth commonly changes with developmental stage.

32. Which measurement is commonly used to estimate plant growth in experiments?
    A. Leaf color only
    B. Dry weight biomass
    C. Root hair length only
    D. Seed coat thickness
    Answer: B
    Explanations:

• A: Incorrect. Leaf color is qualitative, not a standard growth metric.

• B: Correct. Dry weight (after oven drying) is a standard quantitative growth measurement.

• C: Incorrect. Root hair length is specific and not overall growth measure.

• D: Incorrect. Seed coat thickness unrelated to ongoing growth.

33. During rapid growth (log phase), which cellular activities are dominant?
    A. Maturation and secondary wall deposition only
    B. High rates of cell division and cell enlargement
    C. Only programmed cell death
    D. Only photosynthesis with no division
    Answer: B
    Explanations:

• A: Incorrect. Secondary wall deposition occurs later in specialized cells.

• B: Correct. Log phase combines active mitosis and cell expansion leading to rapid size increase.

• C: Incorrect. Programmed cell death doesn’t define growth phase.

• D: Incorrect. Photosynthesis may contribute energy, but division and expansion are central.

34. Which curve best represents cumulative plant growth over time?
    A. Linear straight line
    B. Sigmoid (S-shaped) curve
    C. Exponential forever increasing without plateau
    D. Parabolic opening left
    Answer: B
    Explanations:

• A: Incorrect. Growth is not usually constant linear long-term.

• B: Correct. Plants usually exhibit a sigmoid curve with plateau at maturity.

• C: Incorrect. Exponential occurs temporarily but not indefinitely due to limiting factors.

• D: Incorrect. Parabola is not the typical growth profile.

35. The concept of ‘compensatory growth’ refers to:
    A. Growth compensation by one organ after loss of another (e.g., increased growth after pruning)
    B. Always reduced growth following any damage
    C. Equal cell death and division rates only
    D. Seed dormancy maintenance
    Answer: A
    Explanations:

• A: Correct. Plants can redirect resources to compensate for lost tissues (e.g., regrowth).

• B: Incorrect. Not always reduced; compensation is increased growth in some parts.

• C: Incorrect. Compensatory growth is not about equal rates only.

• D: Incorrect. Not related to dormancy.

36. Which measurement would you use to calculate net assimilation rate (NAR)?
    A. Dry weight gain per unit leaf area per unit time
    B. Root length only
    C. Seed number per plant
    D. Stem diameter only
    Answer: A
    Explanations:

• A: Correct. NAR = increase in plant dry weight per unit leaf area per unit time (assimilation efficiency).

• B: Incorrect. Root length alone doesn’t give assimilation info.

• C: Incorrect. Seed number is reproductive output, not assimilation rate.

• D: Incorrect. Stem diameter is growth parameter but not assimilation rate.

37. If absolute growth = 5 g/day and plant mass = 50 g, RGR ≈:
    A. 0.1 day⁻¹
    B. 10 day⁻¹
    C. 0.01 day⁻¹
    D. 1 day⁻¹
    Answer: A
    Explanations:

• A: Correct. RGR = (5 g/day) / (50 g) = 0.1 day⁻¹.

• B: Incorrect. 10 would be 5/0.5, wrong scale.

• C: Incorrect. 0.01 would be 0.5 g/day for 50 g.

• D: Incorrect. 1 day⁻¹ implies 50 g/day absolute growth.

38. Which environmental factor most directly affects growth rate by altering metabolic enzyme activity?
    A. Soil texture only
    B. Temperature
    C. Flower color
    D. Gravitational constant
    Answer: B
    Explanations:

• A: Incorrect. Soil texture affects water/nutrients indirectly but not directly enzyme kinetics.

• B: Correct. Temperature affects enzyme activity rates and hence growth.

• C: Incorrect. Flower color is a trait, not a metabolic driver.

• D: Incorrect. Gravity is constant and not a main regulator of biochemical enzyme rates in plants.

39. Which is true about determinate vs indeterminate growth?
    A. Determinate growth continues throughout life without limit.
    B. Indeterminate growth stops after a fixed size.
    C. Determinate growth stops after reaching a genetically pre-determined size (e.g., some flowers).
    D. Both are identical in plants.
    Answer: C
    Explanations:

• A: Incorrect. That’s indeterminate growth.

• B: Incorrect. That’s determinate growth description reversed.

• C: Correct. Determinate growth ends at an organ-specific size; indeterminate keeps growing (e.g., stems).

• D: Incorrect. They are different growth habits.

40. Which hormone is most closely associated with cell elongation contributing to growth rate?
    A. Cytokinin
    B. ABA
    C. Auxin (IAA)
    D. Salicylic acid
    Answer: C
    Explanations:

• A: Incorrect. Cytokinins promote division, delay senescence; less direct on elongation.

• B: Incorrect. ABA generally inhibits growth under stress.

• C: Correct. Auxin promotes cell elongation via acid-growth mechanisms.

• D: Incorrect. Salicylic acid is mainly defense-related.

41. The ‘lag phase’ in plant growth curve typically represents:
    A. Period of rapid division and expansion
    B. Establishment and slow growth after germination while metabolic systems activate
    C. Immediate flowering after germination
    D. Senescence and death
    Answer: B
    Explanations:

• A: Incorrect. That’s the log (exponential) phase.

• B: Correct. Lag phase shows preparation: metabolism initiating, not rapid outward growth.

• C: Incorrect. Flowering is much later.

• D: Incorrect. Senescence occurs after maturation.

42. Which practice directly increases the relative growth rate of a crop in early stages?
    A. High planting density always reduces RGR
    B. Providing fertilizer and optimum light for photosynthesis
    C. Removing all leaves
    D. Intentionally causing drought stress
    Answer: B
    Explanations:

• A: Incorrect. High density can reduce RGR via competition; not always positive.

• B: Correct. Adequate nutrients and light boost assimilation and RGR in early stages.

• C: Incorrect. Removing leaves reduces photosynthetic area and lowers growth.

• D: Incorrect. Drought reduces growth and RGR.

43. Secondary growth results in:
    A. Length increase only
    B. Increase in girth/width due to vascular cambium activity
    C. Reduction of woody tissues
    D. Flower production
    Answer: B
    Explanations:

• A: Incorrect. Primary growth adds length; secondary adds girth.

• B: Correct. Vascular cambium and cork cambium produce secondary xylem and phloem increasing thickness.

• C: Incorrect. Secondary growth creates more woody tissues.

• D: Incorrect. Flowering is reproductive, not associated specifically with secondary growth.

44. Which plant part exhibits indeterminate growth?
    A. Leaf blade of many species
    B. Root apical meristem (in many species)
    C. Petal of flower
    D. Seed embryo after maturation
    Answer: B
    Explanations:

• A: Incorrect. Leaves are determinate in most species.

• B: Correct. Roots often grow indeterminately (continuous root apical meristem activity).

• C: Incorrect. Petals are determinate organs.

• D: Incorrect. Seed embryo development ends upon seed maturity (determinate).

45. Growth that results from increase in cell number is called:
    A. Cell expansion
    B. Cell division (proliferation)
    C. Senescence
    D. Differentiation
    Answer: B
    Explanations:

• A: Incorrect. Expansion is size increase without new cells.

• B: Correct. Increasing cell numbers arises from mitotic divisions.

• C: Incorrect. Senescence is aging and decline.

• D: Incorrect. Differentiation is specialization of existing cells.

46. Primary growth in plants is due to activity of:
    A. Vascular cambium
    B. Apical meristems
    C. Cork cambium
    D. Lenticels
    Answer: B
    Explanations:

• A: Incorrect. Vascular cambium gives secondary growth.

• B: Correct. Apical meristems at shoot and root tips cause primary (length) growth.

• C: Incorrect. Cork cambium is secondary lateral meristem.

• D: Incorrect. Lenticels are gas exchange structures, not meristems.

47. Which is used to express growth increment in many plant physiology studies?
    A. Leaf color chart only
    B. Increase in dry weight per unit time
    C. Number of flowers only
    D. Root tip color
    Answer: B
    Explanations:

• A: Incorrect. Not quantitative standard metric.

• B: Correct. Dry weight increase per time is a common, quantitative measure of growth.

• C: Incorrect. Flower number is reproductive metric, not general growth increment.

• D: Incorrect. Root tip color irrelevant.

48. If a plant’s RGR declines with age, likely cause is:
    A. Decreased photosynthetic efficiency per leaf area or increased respiration relative to growth
    B. Increased cell division rate indefinitely
    C. Constant resource availability
    D. Immediate death
    Answer: A
    Explanations:

• A: Correct. As plant grows, leaf aging, shading, and higher maintenance respiration often reduce RGR.

• B: Incorrect. Division doesn’t increase indefinitely; RGR decline implies reduced per-mass growth.

• C: Incorrect. Constant resources wouldn’t by itself cause RGR decline.

• D: Incorrect. Decline to zero is not implied.

49. In experiments to measure growth, which transformation reduces size-dependent variation?
    A. Using absolute growth only
    B. Calculating relative growth rate (RGR)
    C. Ignoring plant size
    D. Measuring only shoot height without mass
    Answer: B
    Explanations:

• A: Incorrect. Absolute growth favors larger plants; size-dependent bias remains.

• B: Correct. RGR normalizes growth to existing mass, reducing size bias.

• C: Incorrect. Ignoring size loses important context.

• D: Incorrect. Height alone may not reflect biomass changes.

50. Which statement best describes ephemeral tissues compared to permanent tissues?
    A. Ephemeral tissues live longer and are permanent.
    B. Permanent tissues undergo rapid division and are short-lived.
    C. Ephemeral tissues (e.g., epidermis, young leaves) are short-lived and replaced; permanent do not divide frequently.
    D. Permanent tissues are always photosynthetic.
    Answer: C
    Explanations:

• A: Incorrect. Ephemeral are short-lived, not longer-lived.

• B: Incorrect. Permanent tissues are usually differentiated and non-dividing.

• C: Correct. Ephemeral tissues function briefly and get replaced; permanent tissues are stable, specialized.

• D: Incorrect. Many permanent tissues are non-photosynthetic (xylem, sclerenchyma).

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