(a) Define the terms photoperiodism and florigen. Describe the mechanism of response in short day and long day plants giving…

(a) Photoperiodism, Florigen and the SDP and LDP Mechanisms

Photoperiodism (Garner and Allard, 1920) is the developmental response of a plant to the relative lengths of day and night. The night length is often the measured variable: a night break cancels a long night. Critical day length is the threshold that must be exceeded (LDP) or not exceeded (SDP) for flowering.

Florigen is the leaf-generated, phloem-mobile flowering stimulus hypothesised by Chailakhyan (1937). The protein product of FLOWERING LOCUS T (FT) — Hd3a in rice — is florigen. It travels from leaf to shoot apical meristem and, with the bZIP partner FD (FLOWERING LOCUS D), activates floral meristem-identity genes (AP1, SOC1).

Short-day plants (qualitative: Xanthium strumarium, Glycine max, Chrysanthemum indicum; quantitative: Oryza sativa). Flower when the uninterrupted night is longer than the critical night. Phytochrome Pfr at dusk decays in the dark; a long night lets the inductive programme run (in rice, Hd1 represses Hd3a in long days and permits it in short days). A brief red night-break forms Pfr and cancels induction.

Long-day plants (Arabidopsis thaliana, Spinacia oleracea, Hordeum vulgare, Triticum aestivum, Hyoscyamus niger). Flower when the day exceeds the critical length (night shorter than critical). CONSTANS protein is stabilised only when its expression coincides with light; long days produce that coincidence → FT is transcribed → FT protein moves to the meristem. Short nights therefore induce FT in LDPs.

Day-neutral plants (Solanum lycopersicum, many Phaseolus cultivars) ignore photoperiod. The same FT–FD module is used in breeding to move flowering time by latitude.

(b)(i) How Greenhouse Gases Raise Global Temperature

Process. Incoming short-wave solar radiation warms the surface; the surface emits long-wave infrared. Greenhouse gases absorb specified IR bands and re-radiate, so tropopause-level outgoing long-wave radiation falls and the surface–troposphere system warms until balance is restored. The process is distinct from stratospheric ozone depletion.

Gases

• CO₂: principal anthropogenic forcing; strong 15 µm band; from fossil fuel and land-use change; long atmospheric life.
• CH₄: rice paddies, ruminants, fossil leaks, wetlands; GWP about 28–30 over 100 years; band near 7.7 µm.
• N₂O: fertilised soils, industry; high GWP, long life.
• F-gases / remaining CFCs and HFCs: very high GWP, small mass.
• Water vapour: strongest natural GHG, but a feedback, not a primary anthropogenic driver.

(b)(ii) Adverse Effects of Global Warming and Mitigation Effects. Glacier and ice-sheet loss; sea-level rise (Sundarbans, deltas); stronger heat extremes; monsoon irregularity and crop-heat stress; coral bleaching; range shifts and extinction risk; possible permafrost carbon release.

Mitigation. Cut CO₂ (renewables, efficiency, electrification), CH₄ (paddy water management, leak control, diet) and N₂O (precision nitrogen). Protect and restore forests and soils (agroforestry, soil carbon). Paris Agreement NDCs set the political frame (India's updated NDC targets 45% lower emissions intensity of GDP by 2030 from 2005 and about 50% non-fossil power capacity, with net zero by 2070), but they work only if energy and land systems actually change. Adaptation (heat-tolerant cultivars, water storage) is necessary but does not replace mitigation.

(c)(i) Phytogeographical Regions of India

A phytogeographical region is an area with a recognisable flora under a common climate. It differs from Champion and Seth's forest-type classification. D. Chatterjee (1962) divided the Indian region into nine floristic regions:

1. Western Himalaya — Kashmir to Kumaon; altitudinal belts: Shorea/Dalbergia foothills → Pinus roxburghii, Quercus, Cedrus deodara → Abies, Betula, Rhododendron alpine.
2. Eastern Himalaya — wetter, richer; Magnolia, orchids, tree Rhododendron, Abies densa.
3. Indus plain — arid Punjab–Rajasthan–Kutch; Prosopis, Acacia, Capparis, Calligonum.
4. Gangetic plain — alluvial; moist to dry deciduous remnants, Shorea, Dalbergia, Sundarbans mangroves (Heritiera) in the delta.
5. Central India — Vindhya–Satpura; Tectona grandis, Shorea robusta, bamboo.
6. Deccan — rain-shadow plateau; Hardwickia, Santalum album, Acacia, Terminalia.
7. Malabar (Western coast / Ghats) — tropical evergreen; dipterocarps, Vateria, high endemism.
8. Assam — Brahmaputra valley and hills; evergreen and bamboo, orchids.
9. Andaman and Nicobar — island rainforest and littoral flora; high island endemism.

(c)(ii) Environmental Factors Influencing Species Composition

• Climate: rainfall amount and season (Malabar vs Deccan rain-shadow); temperature and frost line (Himalayan belts).
• Physiography: orographic lift, rain-shadow, aspect, altitude.
• Soil / edaphic: laterite, alluvium, saline delta, desert sand — each filters the flora.
• Biotic and historical: fire, grazing, Pleistocene refugia, and now fragmentation and climate change shifting belts upslope.