Leaf fall occurs in tree when there is an increase in the concentration of

Appeared in: UPPSC 2017

Explanation

Ethylene is a gaseous plant hormone that acts as a key regulator of the aging process in plants, known as senescence.
An increased concentration of ethylene promotes abscission, which is the natural detachment of parts of a plant, typically dead leaves and ripe fruit.
Ethylene activates genes that produce enzymes like cellulase and pectinase.
These enzymes break down the cell walls in the abscission layer at the base of the leaf petiole, weakening the structure and causing the leaf to fall.

Option A: Gibberellins are primarily associated with promoting growth, such as stem elongation, seed germination, and flowering.
Option B: Auxins, particularly when present in high concentrations in young leaves, actually inhibit or prevent leaf fall. It is the decrease in auxin levels that makes the leaf susceptible to ethylene's effects.
Option D: Cytokinins have the opposite effect; they are known to delay senescence (aging) and work to keep leaves green and healthy. They counteract the effects of ethylene.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Plant Hormones and Abscission as background academic context rather than a clinical decision trigger.
Biological Principle: Understanding hormonal regulation is a fundamental concept in biology, applicable to both plants and animals. Just as ethylene controls processes in plants, hormones in the human body regulate growth, metabolism, and reproduction.
Application in Agriculture: Knowledge of ethylene's function is used commercially to control fruit ripening. For example, bananas are often shipped green and then exposed to ethylene gas to ripen them before sale.
What if?: If the question asked which hormone prevents leaf fall, the answer would be Auxin (in young leaves) or Cytokinin (by delaying aging). This highlights the antagonistic and synergistic relationships between different hormones.

How to Approach the Question

First, identify the key process in the question: 'Leaf fall'.
Recall the biological term for leaf fall, which is 'abscission'.
Systematically review the primary functions of each plant hormone listed in the options.
Match the function of promoting abscission and senescence with the correct hormone.
Eliminate hormones with functions related to growth (Gibberellins, Auxins) or anti-aging (Cytokinins).
Conclude that Ethylene is the hormone that increases to cause leaf fall.

Concept Tested & Keywords

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