Plant physiology Study Guide

📖 Core Concepts Plant Physiology – Study of how plants function, covering photosynthesis, respiration, nutrition, hormones, tropisms, photoperiodism, stress responses, germination, stomatal control, and transpiration. Hormones – Small chemical signals synthesized anywhere in the plant; they coordinate growth, development, and stress responses. Photomorphogenesis – Light‑driven structural development mediated by photoreceptors (phytochrome for red/far‑red, cryptochrome for blue/UV‑A). Photoperiodism – Flowering response determined by day‑length class (long‑day, short‑day, day‑neutral) via phytochrome’s red/far‑red ratio detection. Pigments – Chlorophylls (a & b) absorb red & blue light; carotenoids are accessory pigments (β‑carotene, lutein, lycopene); anthocyanins give red‑purple‑blue colors and can reflect light to chlorophyll. Essential Elements – Macronutrients (C, N, P, K, Ca, Mg, S) needed in bulk; micronutrients (Fe, Mn, Zn, Cu, B, Mo, Cl) needed in trace amounts. Transport – Vascular tissue moves minerals (roots → leaves) and carbohydrates (leaves → roots). Tropisms – Directional growth responses: phototropism (light), geotropism/gravitropism (gravity) caused by differential cell elongation. --- 📌 Must Remember Auxin → cell elongation, apical dominance, root initiation. Gibberellin → stem elongation, seed germination, flowering. Cytokinin → cell division, delay leaf senescence. Abscisic acid (ABA) → seed dormancy, stomatal closure, stress tolerance. Ethylene → fruit ripening, leaf abscission, mechanical stress response. Phytochrome detects R:FR ratio; high red → “day” signal, high far‑red → “night” signal. Long‑day plants need ≥ critical daylight to flower; short‑day plants need ≥ critical night (continuous darkness). Carotenoids are accessory pigments that protect chlorophyll and expand usable light spectrum. Anthocyanin color is pH‑dependent (red → acidic, blue → alkaline). Tropism mechanism – faster cell elongation on the shaded side (phototropism) or opposite gravity vector (geotropism). --- 🔄 Key Processes Photoperiodic Flowering Decision Light → phytochrome → converts to active Pfr (red) or inactive Pr (far‑red). Ratio of Pfr/Pr assessed each night. Long‑day: high Pfr → flowering; short‑day: low Pfr → flowering. Auxin‑mediated Phototropism Light → asymmetric auxin distribution (higher on shaded side). Auxin stimulates cell elongation → organ bends toward light. Stomatal Closure (ABA pathway) Drought → increased ABA synthesis in roots → ABA travels to guard cells → ion efflux → loss of turgor → stomata close. Carotenoid Synthesis (simplified) Isopentenyl diphosphate → geranylgeranyl diphosphate → phytoene → lycopene → β‑carotene / lutein. Vascular Transport Xylem (upward, water & minerals) driven by transpiration pull & root pressure. Phloem (bidirectional, sugars) via pressure‑flow: source loading → high osmotic pressure → bulk flow → sink unloading. --- 🔍 Key Comparisons Auxin vs. Gibberellin – Auxin: cell elongation & apical dominance; Gibberellin: stem elongation and seed germination. Phytochrome vs. Cryptochrome – Phytochrome: red/far‑red detection, controls flowering & seed germination; Cryptochrome: blue/UV‑A detection, regulates circadian rhythms & leaf expansion. Long‑day vs. Short‑day Plants – Long‑day: flower when day > critical length; Short‑day: flower when night > critical length (continuous darkness required). Carotenoids vs. Anthocyanins – Carotenoids: lipid‑soluble, accessory pigments, protect chlorophyll; Anthocyanins: water‑soluble flavonoids, pH‑dependent colors, may reflect light. Xylem vs. Phloem – Xylem moves water/minerals upward only; Phloem moves photosynthates (sugars) from source to sink in both directions. --- ⚠️ Common Misunderstandings “All plants need the same amount of each nutrient.” → Macronutrients vs. micronutrients differ dramatically in required quantities. “Ethylene only ripens fruit.” → Also triggers leaf abscission and responses to mechanical stress. “Photosynthesis only uses chlorophyll.” – Carotenoids and other pigments broaden the light spectrum used. “Phototropism is caused by light‑induced growth on the illuminated side.” – Growth actually occurs on the shaded side, bending the organ toward the light. “ABA closes stomata only during drought.” – It also mediates seed dormancy and general stress responses. --- 🧠 Mental Models / Intuition “Red = day, far‑red = night” – Remember phytochrome as a binary switch: red light flips it on (Pfr), far‑red flips it off (Pr). “Hormone traffic jam” – Think of each hormone as a courier with a specific delivery address (e.g., auxin → elongating cells, ABA → guard cells). “Water‑up, sugar‑down” – Visualize xylem as a one‑way upward elevator for water/minerals; phloem as a two‑way conveyor belt for sugars. “Shade‑avoidance = stretch‑out” – When a plant senses low R:FR (shade), it stretches (elongates) to outgrow competitors. --- 🚩 Exceptions & Edge Cases Day‑neutral plants – Flower independent of photoperiod; often rely on temperature (vernalization) cues. Anthocyanin color shift – In some species, anthocyanins appear red even at neutral pH due to metal‑complex formation. ABA accumulation – Can also rise in response to high salinity, not just drought. Carotenoid function – Besides light harvesting, they protect chlorophyll from photo‑oxidative damage; deficiency can cause photobleaching. --- 📍 When to Use Which Identify flowering cue → Use photoperiod class (long‑day/short‑day) before considering temperature or hormonal treatments. Diagnose wilting → Check xylem water status (transpiration deficit) before suspecting phloem carbohydrate shortage. Control fruit ripening → Apply ethylene (or inhibitors) rather than altering temperature alone. Promote seed germination → Apply gibberellins if dormancy is hormone‑regulated; use red light flash for phytochrome‑sensitive species. Combat drought stress → Target ABA pathway (e.g., exogenous ABA) rather than auxin‑based growth promoters. --- 👀 Patterns to Recognize Red‑far‑red ratio spikes → Indicates shade → Expect stem elongation (shade‑avoidance). High auxin on one side → Predict bending toward light (phototropism). Elevated ABA + closed stomata → Sign of water stress. Presence of carotenoids + chlorophyll → Typical of photosynthetically active green tissues. Sudden ethylene surge → Anticipate fruit ripening or leaf drop. --- 🗂️ Exam Traps Confusing auxin with gibberellin – A choice linking auxin to seed germination is wrong; that’s gibberellin. Assuming all short‑day plants need short days – Many are actually long‑night (require uninterrupted darkness), so “short day” wording can be misleading. Choosing “chlorophyll absorbs green” – Chlorophyll reflects green; a common distractor. Attributing stomatal closure solely to drought – ABA‑mediated closure also occurs under high salinity and cold; answer choices limited to “drought” are incomplete. Mixing up xylem and phloem directionality – Remember: xylem = up only, phloem = source→sink (bidirectional). ---