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

(a) C4 cycle and comparison of C3, C4 and CAM plants C4 (Hatch–Slack) cycle: in mesophyll cells, PEP carboxylase (no oxygenase activity) fixes CO2 (as HCO3−) onto phosphoenolpyruvate (PEP) to give oxaloacetate (OAA), converted to malate or aspartate. These move through plasmodesmata to bundle sheath cells, where decarboxylation releases CO2 at high concentration around RuBisCO; the Calvin cycle then forms sugars. The residual pyruvate returns to mesophyll and is regenerated to PEP by pyruvate phosphate dikinase using 2 ATP. Cost: 5 ATP per CO2. Comparison: C3 (wheat, rice) has a single cell type, RuBisCO fixes CO2 directly into 3-PGA, high photorespiration, optimum 15–25 °C. C4 (maize, sugarcane, sorghum) shows Kranz anatomy, spatial separation of fixation (mesophyll, PEP carboxylase, OAA) and Calvin cycle (bundle sheath), negligible photorespiration, optimum 30–40 °C, high water- and nitrogen-use efficiency. CAM (pineapple, Opuntia, Kalanchoe) separates the steps in time: stomata open at night, CO2 is fixed to malate stored in vacuoles, and by day with stomata closed malate is decarboxylated for the Calvin cycle; it suits arid habitats with very low water loss and slow growth.

(b) Vernalization Vernalization is the promotion of flowering by prolonged exposure to low temperature (about 0–10 °C for weeks) in cold-requiring plants, such as winter wheat, rye and biennials like cabbage, carrot, beet and Brassica napus. The cold stimulus is perceived by the shoot apical meristem, and is retained after return to warmth through mitotic cell divisions. Gassner first showed it (1918) and Lysenko popularised the term; Melchers and Lang proposed a hypothetical hormone “vernalin”, and gibberellins can replace cold in some biennials. At molecular level, cold epigenetically silences the floral repressor FLC (through Polycomb-group proteins and histone methylation), permitting flowering. High temperature immediately afterwards causes devernalization. Importance: it lets winter varieties be sown as spring crops after treating seed (spring-sown winter wheat), shortens the generation time in breeding and synchronises flowering with season. It differs from photoperiodism, which responds to day length, and from stratification, which breaks seed dormancy.

(c) Biological nitrogen fixation Biological nitrogen fixation is the reduction of atmospheric N2 to NH3 by prokaryotes (Rhizobium, Azotobacter, Anabaena, Frankia). In legumes, Rhizobium leguminosarum and relatives form root nodules. Roots secrete flavonoids; bacteria release Nod factors, causing root hair curling. An infection thread carries bacteria to the cortex, where cells divide to form a nodule, and the bacteria become bacteroids enclosed in a peribacteroid membrane. Leghaemoglobin gives the nodule its pink colour and binds oxygen, protecting oxygen-sensitive nitrogenase while supplying respiration. Nitrogenase has two parts: the Fe-protein (dinitrogenase reductase) and the MoFe-protein (dinitrogenase). Reaction: N2 + 8H+ + 8e− + 16 ATP → 2NH3 + H2 + 16 ADP + 16 Pi. The ammonia is assimilated into glutamine and glutamate and exported as amides (e.g., asparagine) or ureides.