Plant Physiology: Water, Carbon and Signals
Transport and water relations
Physiology questions test mechanisms, not just terms. Understand water potential (solute, pressure, matric components), osmosis, turgor, plasmolysis and the cohesion–tension theory of xylem transport. Know root pressure, guttation, transpiration and stomatal regulation by guard-cell turgor, potassium flux and abscisic acid. Learn the phloem pressure-flow hypothesis and the evidence from aphid stylet and tracer experiments.
Photosynthesis and respiration
Be able to compare C3, C4 and CAM pathways by anatomy, first carboxylation product, enzyme, and photorespiration behaviour. Know the light and dark reactions, photophosphorylation, the Calvin cycle, and the roles of RuBisCO and PEP carboxylase. For respiration, know glycolysis, the Krebs cycle and the electron transport chain, and the difference between aerobic and anaerobic pathways.
Growth, hormones and stress
Learn the roles of auxin, gibberellin, cytokinin, abscisic acid and ethylene, and classic experiments such as coleoptile curvature and the triple response. Understand photoperiodism, vernalisation, dormancy, seed germination and the phytochrome system. Know mineral nutrition, essential elements, deficiency symptoms and hydroponics.
Traps
Questions confuse the site of action: auxin in elongation, gibberellin in stem elongation and seed germination, cytokinin in cell division. Another trap is attributing photorespiration to C4 plants, or confusing the first stable product of C3 (3-PGA) with that of C4 (oxaloacetate).
Rule of thumb
For any physiological process, identify the driving force (gradient), the barrier (membrane), and the regulator (hormone or ion). This triad answers most mechanism questions.
Worked example
A plant wilts at midday despite wet soil. The likely cause is high leaf water potential deficit from transpiration exceeding uptake, not lack of soil water; stomatal closure follows ABA signalling.