Explain tropisms (photo-, gravi-, thigmotropism)

4.9.8 · Biology › Plant Biology

WHAT is a tropism?

Key contrast: tropism directional hoti hai (source ki taraf ya source se door bend karti hai), unlike nastic movement (jaise Mimosa leaf ka fold hona) jo non-directional hoti hai aur stimulus kahaan se aa raha hai us par depend nahi karti.

WHY do plants bend? (First principles)

Ek stem mein muscles nahi hote. Bend karne ka ek hi tarika hai — differential growth: agar left side ke cells right side ke cells se zyada elongate karein, toh tissue right ki taraf curve karega. Toh poora problem ek hi sawaal tak aa jaata hai:

Kaun si side zyada fast grow karti hai, aur kyun?

Jawaab: ek plant growth hormone, auxin (chemically indole-3-acetic acid, IAA), stems mein cell elongation promote karta hai. Auxin shoot tip par banta hai aur neeche ki taraf move karta hai. Ek directional stimulus auxin ko ek side par ikatha kar deta hai. Jis side mein zyada auxin hoga woh zyada elongate hogi, isliye plant bend ho jaati hai.

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

Worked example

Step 1 — Light kahan sense hoti hai? (Darwin, 1880) Grass coleoptile ki tip ko cover karo → light ki taraf koi bending nahi. Tip ke neeche wale hisse ko cover karo → phir bhi bend karta hai. Yeh step kyun? Yeh photoreceptor ko tip mein localise karta hai, lekin bending neeche hoti hai → tip se elongation zone tak koi signal zaroor travel karta hoga.

Step 2 — Kya signal chemical hai? (Boysen-Jensen) Katee hui tip aur stump ke beech ek permeable agar block rakho → signal pass ho jaata hai. Lit side par ek impermeable mica barrier rakho → koi bending nahi. Yeh step kyun? Ek diffusible chemical message le jaata hai, na ki koi electrical/mechanical signal.

Step 3 — Isolate & re-apply. (Went, 1926) Chemical ko agar mein collect karo, agar block ko off-centre ek tip-less coleoptile par andheron mein rakho → woh block ki side se door bend karta hai. Yeh step kyun? Prove karta hai ki chemical (auxin) elongation cause karta hai, aur zyada auxin = us side zyada growth.

Sab milaakar: blue light (phototropin photoreceptors dwara sense ki gayi) auxin ko light se door, shaded side ki taraf redistribute karti hai. Shaded side ko zyada auxin milta hai → woh zyada elongate hoti hai → shoot light ki taraf bend ho jaati hai = positive phototropism.

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

HOW gravitropism works

Roots: positively gravitropic (neeche grow karti hain). Yahan ek twist hai — roots auxin ke liye zyada sensitive hoti hain, aur roots mein high auxin elongation ko inhibit karta hai. Extra auxin lower side par ikatha hota hai → lower side kam grow karti hai → root neeche curve karti hai.
Shoots: negatively gravitropic (upar grow karte hain). Lower side par extra auxin → lower side zyada grow karti hai → shoot upar curve karta hai.

HOW thigmotropism works

Classic example: pea, grape, passionflower ke tendrils ek support ke around coil ho jaate hain. Jo side object ko touch kar rahi hai, us side ke cells stimulate hote hain (mechanoreceptors, auxin & ethylene mein changes) jisse non-contact side zyada fast elongate hoti hai, aur tendril support ke around wrap ho jaata hai = positive thigmotropism.
Climbing plants iska use karte hain light tak pahunchne ke liye, bina thick trunk banaye, saste mein.

Summary table

Tropism	Stimulus	Shoot	Root
Photo-	Light	positive (taraf)	negative / none
Gravi-	Gravity	negative (upar)	positive (neeche)
Thigmo-	Touch	(tendrils positive)	roots objects ke around grow karti hain

Recall Feynman: explain to a 12-year-old

Plant chal nahi sakti, toh woh ek taraf zyada grow karke move karti hai. Uske paas ek magic juice hota hai jise auxin kehte hain. Agar stem ki ek side mein zyada juice ho, toh woh side zyada stretch hoti hai aur stem doosri taraf jhuk jaata hai. Light juice ko shady side pe bhaagti hai, toh plant sun ki taraf lean karti hai. Gravity juice ko neeche sink karti hai, toh shoot upar push karta hai aur root neeche dive karti hai. Aur jab ek climbing plant ek lakdi feel karti hai, woh bahar ki side par zyada grow karti hai aur hug ki tarah wrap ho jaati hai.

Flashcards

Tropism kya hota hai?

Ek directional growth response hoti hai plant ke kisi hisse ki, ek unidirectional external stimulus ke jawab mein.

Positive aur negative tropism mein kya fark hai?

Positive = stimulus ki taraf growth; negative = stimulus se door growth.

Tropic bending mein kaun sa hormone kaam karta hai?

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

Phototropism mein light kahan sense hoti hai, aur bending kahan hoti hai?

Shoot tip par sense hoti hai (phototropin); bending neeche elongation zone mein hoti hai.

Phototropism mein kaun si side auxin accumulate karti hai?

Shaded side, toh woh zyada elongate hoti hai aur shoot light ki taraf bend ho jaata hai.

Shoots aur roots gravity ke response mein opposite kyun react karte hain?

Auxin lower side par collect hota hai; shoots mein high auxin elongation promote karta hai (upar bend), roots mein inhibit karta hai (neeche bend).

Statoliths kya hote hain?

Dense starch-filled plastids (amyloplasts) jo gravity se settle hote hain aur gravity sensors ki tarah kaam karte hain.

Thigmotropism ko ek example ke saath define karo.

Touch ke response mein directional growth; jaise tendrils ka support ke around coil karna.

Kaunse experiment ne prove kiya ki phototropic signal ek diffusible chemical hai?

Boysen-Jensen ka agar (signal pass hota hai) vs mica (signal block hota hai) barrier experiment.

Tropism ek nastic movement se kaise alag hai?

Tropism directional hoti hai (stimulus source ke relative); nastic movement (jaise Mimosa ka fold hona) non-directional hoti hai.

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