(a) Describe C₄ cycle of photosynthesis and differentiate amongst C₃, C₄ and CAM plants. 10+10=20 (b) Define Vernalization…

Part (a): C₄ Cycle and Differentiation

The C₄ cycle, or Hatch-Slack pathway, is a photosynthetic mechanism that minimizes photorespiration by spatially separating initial CO₂ fixation from the Calvin cycle. It occurs in plants with Kranz anatomy, characterized by a ring of large, chloroplast-rich bundle-sheath cells surrounding the vascular bundles, encircled by mesophyll cells. In mesophyll cells, phosphoenolpyruvate (PEP) carboxylase, which has a high affinity for HCO₃⁻, fixes carbon into oxaloacetate (OAA). OAA is reduced to malate (in NAD-ME subtype) or converted to pyruvate (in NADP-ME subtype). This C₄ acid is transported to bundle-sheath cells, where it is decarboxylated, releasing CO₂. This concentrated CO₂ is then fixed by RuBisCO in the Calvin cycle. Note that while NADP-ME plants like maize have agranal bundle-sheath chloroplasts, NAD-ME and PCK subtypes possess granal chloroplasts, and suberized walls are not universal across all C₄ types.

The differentiation among C₃, C₄, and CAM plants is as follows:

• CO₂ Acceptor: C₃ uses RuBisCO; C₄ uses PEP carboxylase initially; CAM uses PEP carboxylase at night.
• First Stable Product: C₃ produces 3-phosphoglycerate (3-PGA); C₄ produces OAA/Malate; CAM produces OAA/Malate.
• Photorespiration: High in C₃; negligible in C₄ and CAM.
• Water Use Efficiency (WUE): Expressed as g DM/g H₂O, WUE increases from C₃ (lowest) to C₄ to CAM (highest) due to reduced stomatal conductance and CO₂ concentration mechanisms.
• Stomatal Behavior: C₃ opens during the day; C₄ opens during the day but with lower conductance; CAM opens at night to fix CO₂.
• Distribution: C₃ dominates temperate regions; C₄ thrives in hot, dry, high-light environments (e.g., tropical savannas); CAM is common in arid regions (e.g., cacti, pineapple).

Part (b): Vernalization

Vernalization is the induction of flowering by exposure to prolonged low temperatures, typically between 0°C and 10°C, which is essential for many winter annuals and biennials.

Mechanism: The molecular mechanism involves the epigenetic regulation of the FLOWERING LOCUS C (FLC) gene, a major repressor of flowering. In un-vernalized plants, FLC expression is high, inhibiting the FT (Flowering Locus T) gene. Exposure to cold induces the expression of VIN3 (Vernalization Insensitive 3) and VRN (Vernalization Responsive) genes. These proteins facilitate the recruitment of Polycomb Repressive Complex 2 (PRC2) to the FLC locus, leading to histone methylation (H3K27me3) and chromatin compaction. This silences FLC expression. With the repressor removed, the FT gene is activated in the phloem companion cells of leaves. FT protein moves to the shoot apical meristem, where it interacts with FD protein to activate floral meristem identity genes, initiating flowering.

Importance: In agriculture, vernalization is crucial for breeding and crop management. In wheat, it ensures that plants do not flower prematurely in winter, allowing them to accumulate biomass. In sugar beet, vernalization is required to trigger bolting and seed production. Understanding this mechanism allows breeders to develop spring-sown varieties that do not require a cold period, expanding the geographical range of cultivation. It also helps in synchronizing flowering for pollination and harvest.

Part (c): Biological Nitrogen Fixation

Biological Nitrogen Fixation (BNF) is the enzymatic reduction of atmospheric nitrogen (N₂) to ammonia (NH₃) by nitrogenase. It is divided into symbiotic (e.g., Rhizobium in legumes) and asymbiotic (e.g., Azotobacter, Clostridium) types.

Root Nodule Formation: The process begins with the recognition of flavonoids released by legume roots by Rhizobium bacteria. This triggers the expression of nod genes in bacteria, producing Nod factors. These factors induce root hair curling and the formation of an infection thread, a tubular structure through which bacteria enter the root cortex. The bacteria are released into the cytoplasm of cortical cells, where they differentiate into bacteroids. Simultaneously, the plant initiates nodule organogenesis, forming a vascular connection to supply nutrients to the nodule.

Nitrogenase Complex: Nitrogenase is a two-component enzyme system: the Fe protein (Gn) and the Mo-Fe protein (Gp). The reaction is: N₂ + 8H⁺ + 8e⁻ + 16 Mg-ATP → 2NH₃ + H₂ + 16 Mg-ADP + 16 Pi. The Fe protein transfers electrons to the Mo-Fe protein, where N₂ is reduced. The process is strictly anaerobic; oxygen inhibits nitrogenase. In symbiotic nodules, leghemoglobin, a heme protein, binds oxygen, maintaining a low O₂ concentration that protects nitrogenase while allowing respiration. The nif genes encode the nitrogenase components, while nod genes control nodule formation. This process is vital for sustainable agriculture, reducing the need for synthetic fertilizers.