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

Plant Physiology, Climate Dynamics, and Phytogeography

The interplay between plant physiological responses to light, atmospheric composition, and geographical distribution forms the core of botanical ecology. Understanding photoperiodism, global warming mechanisms, and phytogeographical zonation is essential for addressing contemporary environmental challenges.

Photoperiodism and Flowering Mechanisms

Photoperiodism, defined by Garner and Allard (1920), is the physiological response of plants to the relative lengths of day and night. Florigen, a term coined by Chailakhyan (1937), refers to the hypothetical mobile signal produced in leaves that induces flowering in the shoot apical meristem. Modern molecular biology has identified this signal as the FT (Flowering Locus T) protein.

In Short-Day Plants (SDPs) like Chrysanthemum and rice, flowering is induced when the night period exceeds a critical length. Conversely, Long-Day Plants (LDPs) such as wheat and barley flower when the night period is shorter than the critical length. The mechanism is mediated by phytochrome, a photoreversible pigment. In SDPs, prolonged darkness allows the accumulation of FT protein in the phloem, which translocates to the meristem. There, FT interacts with the bZIP transcription factor TD3 to activate the FLOWERING LOCUS T (FT) gene, overriding the floral repressor TFL1. It is crucial to note that TFL1 acts as a local floral repressor within the meristem, inhibiting flowering until the FT signal is sufficient; it is not a mobile anti-florigen. In LDPs, short nights prevent the accumulation of FT, allowing TFL1 to remain active and suppress flowering. This photoperiodic adaptation dictates the latitudinal distribution of crops and wild species.

Greenhouse Gases and Global Warming

Global temperature rise is driven by the greenhouse effect, where gases trap outgoing longwave radiation. Carbon dioxide (CO2) is the primary contributor to total anthropogenic radiative forcing, absorbing infrared radiation in the 15 µm band. Methane (CH4), with a Global Warming Potential (GWP) of 28-30 times that of CO2 over 100 years, absorbs strongly at 7.7 µm, a region outside the atmospheric infrared window where absorption is relatively weak. Nitrous oxide (N2O) and Chlorofluorocarbons (CFCs) also contribute significantly, with CFCs having a GWP exceeding 4,000. Water vapour acts as a feedback mechanism, amplifying warming.

Adverse effects in India include the retreat of Himalayan glaciers, threatening water security for the Ganga and Indus basins. Monsoon variability is increasing, leading to erratic rainfall and crop failures. Coastal regions like the Sundarbans face submergence due to sea-level rise. Mitigation strategies include adhering to India’s Intended Nationally Determined Contributions (INDCs) under the Paris Agreement, aiming for 45% non-fossil fuel capacity by 2030. Agroforestry and soil carbon sequestration are vital for agricultural mitigation.

Phytogeographical Regions of India

India’s phytogeography is classified into ten regions by D. Chatterjee, distinct from the forest-type classification by Champion and Seth. These regions include the Tropical Evergreen Forests of the Western Ghats, Tropical Moist Deciduous Forests of the Eastern Ghats, and the Tropical Dry Deciduous Forests of the Deccan Plateau. The Himalayan regions feature Temperate Coniferous Forests and Alpine Meadows.

Species composition is influenced by climatic factors such as temperature and rainfall gradients, which determine biome boundaries. Edaphic factors, including soil pH and nutrient availability, shape local diversity. Topography creates microclimates, leading to high endemism in the Western Ghats and Eastern Himalayas. Anthropogenic pressures, such as deforestation and fragmentation, further alter species composition. The link between photoperiodic adaptation and latitudinal distribution is evident, as species in higher latitudes (Himalayas) exhibit different flowering cues compared to tropical lowlands.

Conclusion

The integration of plant physiology, climate science, and phytogeography reveals a complex system where molecular mechanisms like FT protein transport are linked to macro-scale ecological shifts. Addressing global warming requires precise understanding of these interactions to preserve India’s rich biodiversity and agricultural stability.