Explain photoperiodism and flowering

WHAT is photoperiodism?

WHY does it matter? It synchronises flowering with the season, ensuring pollinators, favourable weather, and time to set seed.

HOW does the plant measure the night? — Phytochrome

$\text{Pr} \xrightarrow[\text{red (660 nm)}]{} \text{Pfr} \qquad \text{Pfr} \xrightarrow[\text{far-red (730 nm)}]{} \text{Pr}$

HOW it tracks time:

In daylight (rich in red), Pr converts to Pfr → so by dusk the leaf is full of Pfr.
During the night, Pfr slowly reverts back to Pr (dark reversion) and is also degraded.
The amount of Pfr left at dawn encodes how long the night was. A long night → little Pfr left; a short night → lots of Pfr left.

Figure — Explain photoperiodism and flowering

WHERE is light sensed, and HOW is the signal sent? — Florigen

The signalling chain (derivation of the pathway from first principles):

Right photoperiod → correct phytochrome state in the leaf.
This regulates the CONSTANS (CO) protein.
CO switches on the FT gene → FT protein (florigen).
FT moves via phloem to the apex, partners with FD, and activates floral identity genes (e.g. APETALA1).
Apical meristem becomes a floral meristem → flower forms.

Worked examples

Recall Feynman: explain it to a 12-year-old

Imagine a plant holding a tiny hourglass that only runs when it's dark. During the day, sunlight fills the top of the hourglass with a special "day-sand" called Pfr. At night the sand slowly trickles away. When morning comes, the plant feels how much sand is left: lots of sand = the night was short (summer); almost no sand = the night was long (winter). Some plants flower when nights are long, some when nights are short. If you sneak in and shine a flashlight in the middle of the night, you refill the hourglass — and the confused plant thinks the night was short! The leaf figures all this out, then sends a secret message (florigen/FT) through the plant's "straws" (phloem) up to the tip, telling it: "Time to make a flower!"

Active-recall flashcards

#flashcards/biology

Photoperiodism is a response to what specifically?

The relative lengths of day and night (the photoperiod).

What does a plant actually measure to time flowering — day length or night length?

The length of the continuous dark (night) period.

Define critical photoperiod.

The threshold day length above/below which a plant switches to flowering.

A short-day plant flowers when the day is ___ the critical value.

below (i.e. nights are long).

Give two examples of short-day plants.

Chrysanthemum, rice, soybean, tobacco (any two).

Give two examples of long-day plants.

Wheat, spinach, radish, henbane (any two).

What is a day-neutral plant? Example?

Flowers regardless of day length; e.g. tomato, maize, cucumber.

Name the light-sensing pigment for photoperiodism.

Phytochrome.

Which phytochrome form is active and which wavelength converts to it?

Pfr (active), formed by red ~660 nm light.

Which wavelength converts Pfr back to Pr?

Far-red ~730 nm.

What is a "night break" and its effect on an SDP?

A red-light flash during the dark period; it prevents flowering in an SDP.

Why does red-then-far-red flash restore flowering in an SDP?

Far-red reverses Pfr→Pr; the last wavelength decides, so the plant reads a long night again.

Where is photoperiod perceived and where do flowers form?

Perceived in the leaf; flowers form at the shoot apex/bud.

What is florigen and its molecular identity?

The mobile flowering signal; the FT (FLOWERING LOCUS T) protein.

How does florigen travel?

Through the phloem from leaf to shoot apex.

Define vernalisation.

Promotion of flowering by prolonged exposure to cold.

Connections

Phytochrome and Photoreceptors
Plant Hormones
Vernalisation
Phloem Transport (Translocation)
Shoot Apical Meristem and Floral Development
Circadian Rhythms in Plants
Seed Germination and Light

Concept Map

Hinglish (regional understanding)

Intuition Hinglish mein samjho

Dekho, photoperiodism ka matlab hai — plant ko pata chal jaata hai ki season kaunsa hai, sirf din aur raat ki lambai dekh kar. Lekin twist yeh hai: plant actually din nahi, raat (dark period) measure karta hai. Isliye "short-day plant" ko sahi mein long-night plant samajhna chahiye. Agar raat lambi hai to short-day plant (jaise rice, chrysanthemum) flower karta hai, aur agar raat chhoti hai to long-day plant (jaise wheat, spinach) flower karta hai.

Yeh raat ki lambai plant naapta hai ek pigment se — phytochrome. Iske do roop hain: Pr (red light 660 nm absorb karta hai, inactive) aur Pfr (far-red 730 nm absorb karta hai, active). Din mein sunlight se Pr → Pfr ban jaata hai, aur raat mein Pfr dheere-dheere wapas Pr ban jaata hai. Toh subah kitna Pfr bacha, usse plant guess karta hai ki raat kitni lambi thi. Agar aap raat ke beech mein red flash maar do (night break), to Pfr wapas ban jaata hai aur plant ko lagta hai raat chhoti thi — isliye short-day plant flower nahi karega. Aur agar red ke baad far-red flash maaro, to Pfr wapas Pr ho jaata hai, effect cancel — jo last wavelength doge wahi decide karta hai.

Ek aur important baat: light ka signal patti (leaf) perceive karti hai, lekin flower banta hai shoot ke top (bud) par. Toh ek mobile signal chahiye jo leaf se bud tak jaaye — ye hai florigen, jo asal mein FT protein hai, aur phloem ke through travel karta hai. Isse phool banane wale genes on ho jaate hain.

Yeh sab yaad rakhna important hai kyunki isse crops ka flowering aur yield control hota hai — agriculture mein rice, wheat kab flower karega ye poora photoperiod par depend karta hai. Exam mein "night break experiment" aur "Pr/Pfr reversibility" bahut favourite question hai!