Explain tropisms (photo-, gravi-, thigmotropism)

WHAT is a tropism?

Key contrast: a tropism is directional (bends toward/away from source), unlike a nastic movement (e.g. Mimosa leaf folding) which is non-directional and independent of where the stimulus comes from.

WHY do plants bend? (First principles)

A stem doesn't have muscles. The only way to bend is differential growth: if cells on the left side elongate more than cells on the right, the tissue curves toward the right. So the whole problem reduces to one question:

Which side grows faster, and why?

Answer: a plant growth hormone, auxin (chemically indole-3-acetic acid, IAA), promotes cell elongation in stems. Auxin is made at the shoot tip and moves down. A directional stimulus makes auxin pile up on one side. The side with more auxin elongates more, so the plant bends.

HOW phototropism works — Darwin → Went (derivation from experiments)

Worked example

Step 1 — Where is light sensed? (Darwin, 1880) Cover the tip of a grass coleoptile → no bending toward light. Cover the region below the tip → still bends. Why this step? It localises the photoreceptor to the tip, but the bending happens lower down → a signal must travel from tip to zone of elongation.

Step 2 — Is the signal chemical? (Boysen-Jensen) Put a permeable agar block between severed tip and stump → signal passes. Put an impermeable mica barrier on the lit side → no bending. Why this step? A diffusible chemical carries the message, not an electrical/mechanical one.

Step 3 — Isolate & re-apply. (Went, 1926) Collect the chemical into agar, place the agar block off-centre on a tip-less coleoptile in the dark → it bends away from the block's side. Why this step? Proves the chemical (auxin) causes elongation, and more auxin = more growth on that side.

Putting it together: blue light (sensed by phototropin photoreceptors) causes auxin to be redistributed away from the light, to the shaded side. The shaded side gets more auxin → elongates more → the shoot bends toward the light = positive phototropism.

Figure — Explain tropisms (photo-, gravi-, thigmotropism)

HOW gravitropism works

Roots: positively gravitropic (grow down). Here's the twist — roots are more sensitive to auxin, and in roots high auxin inhibits elongation. Extra auxin collects on the lower side → lower side grows less → root curves downward.
Shoots: negatively gravitropic (grow up). Extra auxin on the lower side → lower side grows more → shoot curves upward.

HOW thigmotropism works

Classic example: tendrils of pea, grape, passionflower coil around a support. On the side touching the object, cells are stimulated (mechanoreceptors, changes in auxin & ethylene) so that the non-contact side elongates faster, wrapping the tendril around the support = positive thigmotropism.
Climbing plants use this to reach light cheaply, without building a thick trunk.

Summary table

Tropism	Stimulus	Shoot	Root
Photo-	Light	positive (toward)	negative / none
Gravi-	Gravity	negative (up)	positive (down)
Thigmo-	Touch	(tendrils positive)	roots grow around objects

Recall Feynman: explain to a 12-year-old

A plant can't walk, so it moves by growing lopsided. It has a magic juice called auxin. If one side of the stem has more juice, that side stretches longer and the stem tips over the other way. Light makes the juice run to the shady side, so the plant leans into the sun. Gravity makes the juice sink down, so the shoot pushes up and the root dives down. And when a climbing plant feels a stick, it grows more on the outside and wraps around like a hug.

Flashcards

What is a tropism?

A directional growth response of a plant part to a unidirectional external stimulus.

Difference between positive and negative tropism?

Positive = growth toward the stimulus; negative = growth away from it.

Which hormone drives tropic bending?

Auxin (indole-3-acetic acid, IAA).

Where is light sensed in phototropism, and where is bending?

Sensed at the shoot tip (phototropin); bending occurs lower in the elongation zone.

In phototropism, which side accumulates auxin?

The shaded side, so it elongates more and the shoot bends toward light.

Why do shoots and roots respond oppositely to gravity?

Auxin collects on the lower side; in shoots high auxin promotes elongation (bends up), in roots it inhibits elongation (bends down).

What are statoliths?

Dense starch-filled plastids (amyloplasts) that settle by gravity and act as gravity sensors.

Define thigmotropism with an example.

Directional growth in response to touch; e.g. tendrils coiling around a support.

Which experiment proved the phototropic signal is a diffusible chemical?

Boysen-Jensen's agar (signal passes) vs mica (signal blocked) barrier experiment.

How is a tropism different from a nastic movement?

A tropism is directional (relative to stimulus source); a nastic movement (e.g. Mimosa folding) is non-directional.

Connections

Auxin and Plant Hormones
Cell Elongation and the Acid Growth Hypothesis
Nastic Movements vs Tropisms
Photoreceptors — Phototropins and Phytochromes
Root and Shoot Anatomy — Zones of Elongation
Plant Adaptations for Climbing

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, plants chal nahi sakte, na light ki taraf daud sakte hain — to woh grow karke direction change karte hain. Isi ko tropism kehte hain: ek directional stimulus ke response me directional growth. Steering ka kaam karta hai ek hormone — auxin (IAA). Auxin ek side pe zyada ho jaaye to us side ke cells zyada lambe (elongate) hote hain, aur plant doosri taraf jhuk jaata hai. Simple funda: jis side auxin zyada, shoot me wo side zyada badhti hai.

Phototropism me blue light ko tip sense karti hai (phototropin), aur auxin ko shaded side pe bhej deti hai. Shaded side lambi ho jaati hai, to shoot light ki taraf jhuk jaata hai — positive phototropism. Gravitropism me starch-bhare granules (statoliths) gravity se cell ke neeche baith jaate hain, jisse "neeche kaunsa side hai" pata chal jaata hai. Auxin lower side pe jama hota hai — shoot me lower side zyada badhti hai to shoot upar (negative), lekin root me high auxin growth ko rokta hai, isliye root neeche jaata hai (positive). Yehi twist bahut students bhulte hain.

Thigmotropism matlab touch pe response — tendrils (pea, angoor) jab support ko chhute hain to jo side touch nahi kar rahi wo zyada elongate hoti hai, isse tendril support ke around lipat jaati hai. Isse plant kam energy me climb karke light tak pahunch jaata hai.

Yaad rakho: auxin light se dark ki taraf bhaagta hai, aur gravity ke saath neeche baithta hai — aur root hamesha rebel hai (ulta reaction). Yeh samajh liya to poora chapter clear.

Test yourself — Plant Biology