Q.Define parthenocarpy. Name the plant hormone used to induce parthenocarpy.
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Auxins — The Plant's Growth Director
Imagine a plant growing toward sunlight. Somewhere inside, there must be a signal that tells the shaded side to stretch longer, bending the stem toward the light. That signal is auxin.
Auxins are the first plant hormones ever discovered, and they are the master regulators of cell elongation. The most common natural auxin is Indole-3-Acetic Acid (IAA). It is produced primarily in the shoot apical meristem — the growing tip of the stem — and moves downward, controlling how the plant grows and responds to its environment.
What auxins actually do
The core action of auxin is simple: it makes plant cells expand lengthwise. But because it moves directionally (from tip to base, and away from light), it creates patterns of growth that shape the entire plant.
Apical dominance is a classic example. The terminal bud produces auxin, which travels down the stem and suppresses the growth of lateral (side) buds. This keeps the plant growing tall and straight. If you cut off the tip, auxin production stops, and the side buds finally get their chance — the plant becomes bushier. Gardeners and farmers exploit this when they prune.
Rooting works differently. At low concentrations, auxin actually promotes root formation. That is why commercial rooting powders contain synthetic auxins like IBA (Indole-3-Butyric Acid) — dip a cutting in it, and roots form faster.
Prevention of abscission (leaf and fruit drop) is another role. Auxin keeps the abscission layer — a zone of weak cells at the base of the leaf stalk — from forming. As long as enough auxin flows from the leaf into the stem, the leaf stays attached. When auxin levels drop (due to age or stress), the abscission layer develops and the leaf falls.
Auxin (IAA) at low concentration promotes cell elongation and root initiation. The same auxin (IAA) at high concentration inhibits lateral bud growth and can even inhibit root growth. Concentration, not just presence, decides the outcome.
The mechanism — how auxin makes cells grow
Auxin activates a proton pump in the cell membrane, pumping H⁺ ions into the cell wall. This acidifies the wall, activating enzymes called expansins that loosen the cellulose fibres. The cell wall becomes more flexible, and water enters by osmosis, inflating the cell. This is called the acid growth hypothesis. …
Parthenocarpy is the development of a seedless fruit. Among the plant growth regulators, auxin is the hormone used to induce parthenocarpy, with tomato being the specific example given of a crop where auxin is used to bring about seedless fruit development. …
Parthenocarpy is the formation of fruit without seeds, and auxin is the plant growth regulator used to bring this about, as in tomatoes.
Among the many practical effects of auxins listed in relation to horticulture and agriculture, one specific effect is the induction of parthenocarpy — the development of a fruit that lacks seeds, since it forms without the normal process of fertilisation that would otherwise be needed to trigger fruit development and seed formation. Auxin is named as the growth regulator used to …
Method: Naming a Hormone-Induced Developmental Phenomenon
Use this method for any "define term X, name the hormone that induces it" question about a specific PGR effect.
Steps
Step 1: Give the definition as a precise, standalone statement
State what the named phenomenon IS, independent of any hormone — e.g. parthenocarpy is the development of a fruit without seeds, because it occurs without the normal fertilisation step.
Step 2: Match the phenomenon to the hormone known to cause it
Scan the concept material for which PGR is explicitly credited with producing that exact effect, rather than the hormone most generally associated with the plant/organ in question.
Step 3: Attach the specific example crop, if one is given …
- TG EAPCET 2025Set ap-2025-04-30-FN1 markMCQQ.Which hormone promotes flowering in pineapple and induce parthenocarpy in tomato? (A) Ethylene (B) Auxin (C) Gibberellin (D) Cytokinin
›Reveal solutionSolution
The hormone Auxin is known to promote flowering in pineapple and is a primary inducer of parthenocarpy (seedless fruit development) in tomatoes.
The Maestro of Plant Growth: Understanding Phytohormones
Plants, much like animals, have their own intricate chemical messengers called phytohormones (or plant hormones). These tiny molecules, produced in minute quantities, orchestrate nearly every aspect of a plant's life cycle – from germination and growth to flowering, fruiting, and senescence. What makes them fascinating is their versatility: a single hormone can have vastly different effects depending on its concentration, the specific plant species, the developmental stage, and its interaction with other hormones. Our task is to find one such versatile hormone that performs two specific roles: promoting flowering in pineapple and inducing parthenocarpy in tomato.
Let's break down the question and explore the candidates.
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Deconstructing the Question's Demands
We are looking for a single plant hormone that fulfills two distinct criteria:
- Promotes flowering in pineapple: Pineapple (a bromeliad) has unique flowering requirements.
- Induces parthenocarpy in tomato: Parthenocarpy is the development of fruit without prior fertilization, resulting in seedless fruits.
This requires us to identify a hormone with a dual role, which immediately narrows down our options.
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Evaluating Auxin: The Growth Regulator Extraordinaire
Auxins are a class of plant hormones primarily known for their role in cell elongation, apical dominance, and root formation. However, their influence extends much further, making them a strong candidate for our question.
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Promoting Flowering in Pineapple:
While ethylene is famously known for inducing flowering in pineapple (and is often used commercially for this purpose), auxins also play a crucial role. Synthetic auxins, such as NAA (naphthaleneacetic acid), are widely used in pineapple cultivation to synchronize flowering. At specific concentrations, auxins can trigger the flowering process in pineapples. This effect is often mediated by the auxin's ability to stimulate the plant's own production of ethylene, which then acts as the direct flowering signal. So, auxin acts as an upstream signal or a direct inducer depending on the context and concentration.
Watch outIt's a common misconception that only ethylene promotes flowering in pineapple. While ethylene is a potent and direct inducer, auxins are also effectively used, often by stimulating endogenous ethylene production, to induce or synchronize flowering in pineapple.
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Inducing Parthenocarpy in Tomato:
This is one of auxin's most well-established roles.
Parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules, resulting in seedless fruits.
In many plants, including tomatoes, the developing seeds produce auxins, which are essential for fruit growth. If fertilization doesn't occur, there are no developing seeds to produce these auxins, and the fruit typically aborts. However, by applying exogenous auxins (either naturally occurring or synthetic), the ovary can be stimulated to develop into a fruit even without fertilization, leading to seedless tomatoes. This is a common horticultural practice. …
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- TG EAPCET 2021Set ap-2021-08-09-FN1 markMCQQ.The orientation to light of a non-motile organism or part of its body is called (A) Phototaxis (B) Photokinesis (C) Phototropism (D) Photoperiodism
›Reveal solutionSolution
The question asks for the term describing orientation to light in a non-motile organism or body part. The correct answer is phototropism, which is growth-based orientation in plants and other sessile organisms.
The key distinction here is between movement and growth. In biology, when we talk about an organism's response to light, the specific term depends on whether the organism can move from place to place (motile) or is fixed in place (non-motile).
For motile organisms like bacteria, algae, or animals, light-directed movement is called phototaxis — think of a moth flying toward a flame or a single-celled alga swimming toward brighter water. For non-motile organisms, especially plants, the response to light involves differential growth rather than locomotion. A plant cannot pack up and move to a sunnier spot; instead, it grows toward the light by elongating cells on the shaded side more than those on the lit side. That growth-based orientation is phototropism.
Let's walk through each option to see why only one fits.
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Phototaxis — This is the directional movement of a motile organism (or a freely moving cell) toward or away from light. The organism physically changes location. Since the question specifies a non-motile organism or part, this cannot apply. A rooted plant does not exhibit taxis.
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Photokinesis — This refers to a change in the speed of movement (not direction) in response to light intensity. It is a non-directional response — the organism moves faster or slower but not toward or away from the light source. Again, this requires motility, so it is irrelevant for a non-motile organism. …
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