Plant Physiology

C3 plants fix CO₂ directly via RuBisCO into 3-carbon compounds (3-PGA) — they lose up to 40% of fixed carbon through photorespiration. Examples: wheat, rice, soybean, most trees. C4 plants use the Hatch-Slack pathway — CO₂ is first fixed in mesophyll cells into 4-carbon oxaloacetate, then concentrated around bundle sheath cells, suppressing photorespiration. Examples: maize, sugarcane, sorghum, bajra. CAM plants (crassulacean acid metabolism) fix CO₂ at night, store it as malic acid, and use it for photosynthesis during the day — adapted for arid conditions. Examples: pineapple, agave, cacti.

Kranz anatomy is the distinctive leaf structure in C4 plants — a ring of enlarged, chloroplast-rich bundle sheath cells surrounding each vascular bundle, with mesophyll cells arranged radially around them. This dual-cell architecture spatially separates initial CO₂ fixation (mesophyll) from the Calvin cycle (bundle sheath), creating a CO₂-concentrating mechanism that suppresses photorespiration. All C4 plants (maize, sugarcane, sorghum) have Kranz anatomy.

The five classical PGRs: Auxin (IAA) — cell elongation, apical dominance, root initiation, fruit development. Gibberellin (GA₃) — stem elongation, seed germination, bolting, breaking dormancy. Cytokinin (zeatin) — cell division, delay of senescence, lateral bud release. Abscisic acid (ABA) — stomatal closure under drought, seed dormancy, stress signalling (called 'stress hormone'). Ethylene — fruit ripening, leaf abscission, epinasty, triple response in seedlings.

Photorespiration is the oxygenase reaction of RuBisCO — when O₂ concentration is high relative to CO₂, RuBisCO fixes O₂ instead of CO₂, producing phosphoglycolate which must be recycled through a costly pathway releasing CO₂ and consuming ATP. C3 plants lose 25–40% of their photosynthetically fixed carbon through photorespiration. C4 plants virtually eliminate it by concentrating CO₂ around RuBisCO in bundle sheath cells. This is why C4 crops (maize, sugarcane) are more productive than C3 crops (wheat, rice) in hot, sunny conditions.

Vernalization is the promotion of flowering by exposure to prolonged cold temperatures (0–7°C for 4–8 weeks) — required by winter wheat, rye, and biennials. The cold stimulus is perceived in the shoot apical meristem and leads to epigenetic changes (histone methylation of FLC gene) that permit flowering. Photoperiodism is the response of flowering to day-length (actually night-length): Short-day plants (rice, soybean) flower when nights are longer than a critical period; long-day plants (wheat) flower when nights are shorter than a critical period; day-neutral plants (tomato, cotton) are insensitive to photoperiod.

ICAR JRF (Plant Physiology discipline — entire sections) and IBPS AFO (3–5 questions per exam on PGRs, photosynthesis types, transpiration) have the highest weightage. NABARD Grade A and Pre-PG entrance exams (IARI, BHU) also test it heavily. Most frequently asked: C3 vs C4 differences, PGR functions (especially ABA and ethylene), photorespiration, and photoperiodism classification.