Abscisic acid (ABA) accumulates during water stress and environmental changes, promoting abscission layer formation and leaf fall.

High auxin to low cytokinin ratio promotes callus formation, while the ratios determine the type of organogenesis.

Osmotic potential (Ψs) = -iCRT, where it depends on solute concentration (C) and absolute temperature (T).

Sieve tube members conduct sugars while companion cells provide ATP and metabolic support for active transport.

Water photolysis (2H2O → O2 + 4H+ + 4e-) releases electrons that are transferred through electron transport chains.

RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase) catalyzes CO2 fixation with RuBP in the Calvin cycle.

CAM plants like pineapple, cactus, and agave fix CO2 at night into organic acids and release during day for photosynthesis.

Root pressure is generated by active transport of mineral ions into the xylem sap, creating a concentration gradient that draws water in osmotically. This is distinct from transpiration pull and is a key concept in plant physiology.

Phytochrome exists in two interconvertible forms (Pr and Pfr) and absorbs maximally in red (~660 nm) and far-red (~730 nm) regions. It's crucial for seed germination and photoperiodism - important for UPSC agriculture-related questions.

In C4 plants, PEP carboxylase fixes CO2 into oxaloacetate (4-carbon compound), unlike C3 plants where RuBisCO produces 3-PGA. This adaptation reduces photorespiration and is efficient in hot climates.