The trap that costs marks here: confusing photoperiodism with vernalization, or swapping the plant categories (short-day, long-day, day-neutral) with the wrong examples. NEET questions on this topic rely on precise recall of which plant belongs to which photoperiodic category and what vernalization actually requires.
Photoperiodism is the physiological response of plants to the relative length of day and night (photoperiod) that governs the timing of flowering. The phenomenon was first described by Garner and Allison (1920). The critical factor is actually the duration of the dark period (scotophase), not the light period — a frequent source of confusion.
Plants are classified by their photoperiodic response:
The photoreceptor involved is phytochrome — specifically the interconversion between Pr (red-absorbing, inactive) and Pfr (far-red-absorbing, active). A flash of red light during a long dark period can prevent flowering in SDPs because it converts Pr → Pfr, effectively "breaking" the dark period.
Vernalization is the promotion of flowering by exposure to low temperature (typically 0–5°C) for a defined period. Winter wheat and biennial plants like carrots and cabbage require vernalization. The hormone vernalin was proposed (though not conclusively isolated) as the stimulus. Vernalization acts on the shoot apex.
Key distinction for NEET: photoperiodism = light-dark duration response; vernalization = cold-temperature requirement. Both influence flowering time, but through entirely different environmental cues (NCERT Class 11 Biology Chapter 15, page 293).
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
Photoperiodism in plants was first studied by:
Which of the following is a short-day plant?
The critical factor that determines flowering in photoperiodic plants is:
A flash of red light given during the dark period to a short-day plant will:
Vernalization is the process of promoting flowering by:
The site of perception of vernalization stimulus in a plant is:
If a long-day plant is kept under short-day conditions but the dark period is interrupted by a brief flash of red light followed immediately by a flash of far-red light, the plant will:
Which of the following correctly distinguishes photoperiodism from vernalization?
Given
A researcher grows *Xanthium* (a short-day plant) under controlled conditions. The plant is exposed to 8 hours of light and 16 hours of darkness. During the 10th hour of darkness, a 5-minute flash of red light is given.
Required
Will the plant flower? Justify using the phytochrome model.
Concept
Short-day plants require an uninterrupted dark period exceeding a critical length to flower. The photoreceptor phytochrome exists in two interconvertible forms: Pr (red-absorbing, biologically inactive for this response) and Pfr (far-red-absorbing, biologically active). Red light converts Pr → Pfr.
Relevant principle
In SDPs, continuous darkness allows Pfr (accumulated during the light period) to slowly revert to Pr. If the dark period is uninterrupted and long enough, Pfr levels drop below a threshold, permitting flowering. A red-light flash during the dark period converts Pr back to Pfr, resetting the "clock" and effectively breaking the continuous dark period.
Substitution / application
- Dark period provided: 16 hours (sufficient for *Xanthium*). - Red-light flash at hour 10: converts Pr → Pfr. - After the flash, the remaining dark period is only 6 hours — likely below the critical dark-period threshold. - Pfr levels are now elevated, which inhibits flowering in SDPs.
Reasoning
The red-light interruption at the 10th hour creates two short dark segments (10 h and 6 h) instead of one continuous 16-hour dark period. Neither segment, from the plant's perspective after phytochrome resetting, satisfies the critical uninterrupted dark-period requirement.
Final answer
The plant will **not flower**. The red-light flash broke the continuous dark period by converting Pr → Pfr, preventing the SDP flowering response.
Common trap
Confusing the effect of red vs. far-red light: if the flash had been far-red light, it would convert Pfr → Pr and would NOT inhibit SDP flowering (the dark period would remain effectively unbroken for the phytochrome response). NEET questions often test this red/far-red reversal.
Similar NEET-style question
"A short-day plant is kept under 8 h light / 16 h dark. A red-light flash is given at the 8th hour of darkness, followed immediately by a far-red flash. Will the plant flower? Explain." (Answer: Yes — far-red reverses the red effect, restoring Pr dominance, so the dark period is effectively unbroken.) ---
Photoperiodism: response to relative duration of light/dark. Short-day plants (Xanthium — flowering when night > critical), long-day plants (Henbane), day-neutral. Phytochrome: red/far-red receptor. Vernalization: cold treatment to induce flowering (winter wheat).
-- NCERT Class 11 Biology, Ch. 15, p. 293These are the exact patterns that cause wrong answers in NEET. Each trap includes when it triggers and how to avoid it.
Category: Similar Terms
C3: 3-PGA (3-carbon) is first stable product. C4: oxaloacetate (4-carbon) is first stable product.
Question asks first stable product of CO2 fixation in C3/C4 plant.
Pathway name = carbons in first product. C3 → 3C; C4 → 4C.
Category: Negative Marking
Light reactions: water splits at PS II (P680) FIRST, electrons flow PS II → PS I (P700). PS II numbered AFTER PS I in discovery, but reaction order is PS II → PS I.
Question on Z-scheme order, water photolysis, electron source.
Discovery order ≠ reaction order. Photolysis at PS II; PS II reduces PS I.
Root cause: concept gap
Mnemonic: C3 → 3-PGA (3-carbon); C4 → OAA (4-carbon). Hatch-Slack pathway is C4.
Root cause: concept gap
Glycolysis is in CYTOPLASM (cytosol). Krebs cycle in mitochondrial matrix; ETC in inner membrane.
Root cause: term confusion
RuBisCO = ribulose-1,5-bisphosphate CARBOXYLASE/OXYGENASE. Carboxylase activity drives Calvin; oxygenase causes photorespiration.
38 questions from NEET 2020, 2021, 2022, 2023, 2024, 2025. Answers verified against NTA official keys.
Which of the following statements about RuBisCO is true?
Auxin is used by gardeners to prepare weed-free lawns. But no damage is caused to grass as auxin
Identify the step in tricarboxylic acid cycle, which does not involve oxidation of substrate.
The reaction centre in PS II has an absorption maxima at
Which micronutrient is required for splitting of water molecule during photosynthesis?
Which hormone promotes internode/petiole elongation in deep water rice?
Which of the following combinations is required for chemiosmosis?
Which one of the following plants does not show plasticity?
What amount of energy is released from glucose during lactic acid fermentation?
The first stable product of CO fixation in sorghum is 2
Which of the following statements is correct ? cambial ring
flÎÁh ∑§Ë ¬˝Á∑˝§ÿÊ •Áœ∑§Ã◊ Á∑§‚ ŒÊÒ⁄UÊŸ „Ê ÃË „Ò? 114. The process of growth is maximum during :
Recurring question shapes from past papers. Each pattern shows why wrong options look tempting.
photosynthesis pathway product confusion
C3 and C4 pathways share the Calvin cycle; students who cannot anchor the first stable product in each pathway invert the carbon counts: 3-PGA (3-carbon) is the C3 first stable product, OAA (4-carbon) is the C4 first stable product. RuBisCO has dual carboxylase and oxygenase activity; students who recall only one role select incorrect statements about its function, including denying that it causes photorespiration.
ps i ps ii assignment reversal
PS I and PS II are numbered in order of discovery, not order of action in the Z-scheme. PS II acts first (water photolysis, O2 release, P680 reaction centre at 680 nm); PS I acts second (P700 reaction centre at 700 nm, NADPH production). Students who use Roman numeral order assign water photolysis to PS I and attribute the 700 nm absorption maximum to PS II.
respiration compartment swap
Glycolysis feeds the mitochondrial Krebs cycle, creating a mental link: students place glycolysis in the mitochondrial matrix. The anchor -- glycolysis occurs in the cytosol because it predates mitochondria and operates in anaerobes -- is lost under time pressure.
rubisco function confusion
RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) has dual activity: carboxylase drives CO2 fixation in the Calvin cycle; oxygenase drives photorespiration. Questions give multiple statements about RuBisCO and ask which set is correct; students who have not anchored the dual-function name omit or deny the oxygenase role, selecting option sets that describe it as carboxylase only.
assertion reason unchecked reason
A/R questions in plant physiology present a true Assertion (e.g., ATP is used at two steps in glycolysis) alongside a Reason that sounds mechanistically plausible but may be factually imprecise or not the correct explanation. Students accept the Reason without independently checking whether its specific biochemical claim is accurate.
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