Plant Physiology Calculators

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Plant physiology is the study of how plants function — encompassing photosynthesis, water and nutrient transport, respiration, hormone-regulated growth, responses to environmental stimuli, and the molecular mechanisms underlying development. Plants face unique physiological challenges as sessile, autotrophic organisms: they must harvest light energy, transport water against gravity, obtain mineral nutrients from soil, and respond to biotic and abiotic stresses. Plant physiology has direct applications in agriculture, horticulture, and environmental biology.

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Photosynthesis

Two-stage process converting light to chemical energy:

  • Light reactions (thylakoid membranes): Water oxidized (O₂ released); light drives electron transport producing ATP and NADPH; photosystems I and II transfer electrons.
  • Calvin cycle (stroma): CO₂ fixed by RuBisCO; ATP and NADPH reduce carbon to G3P (triose phosphate); precursor to glucose, sucrose, starch, amino acids.

Water Transport (Cohesion-Tension Theory)

Water moves from soil → roots → xylem → leaves along a water potential gradient (Ψ). Transpiration creates tension in xylem; water cohesion and adhesion in narrow vessels allow tension to pull water columns upward. Ψ = Ψs + Ψp (osmotic + pressure potential).

Mineral Nutrition

Essential macronutrients (N, P, K, Ca, Mg, S) and micronutrients (Fe, Mn, Zn, Cu, B, Mo). Nitrogen absorbed as NO₃⁻ or NH₄⁺; phosphorus as H₂PO₄⁻. Mycorrhizal fungi extend root absorptive area and dramatically increase P uptake.

Plant Hormones

  • Auxin (IAA): Cell elongation, phototropism, apical dominance, root initiation
  • Gibberellin: Stem elongation, seed germination, fruit development
  • Cytokinin: Cell division, shoot initiation, delays senescence
  • Abscisic acid (ABA): Stomatal closure in drought, seed dormancy, stress response
  • Ethylene: Fruit ripening, leaf abscission, response to wounding

Stomatal Regulation

Guard cells regulate stomatal aperture. ABA triggers K⁺ efflux → turgor loss → stomata close. Blue light triggers H⁺-ATPase → K⁺ influx → stomata open. Trade-off: open stomata allow CO₂ entry but increase water loss via transpiration.

Glossary

Transpiration
Evaporation of water from plant leaves through stomata. Drives water movement from roots to leaves via cohesion-tension. Accounts for ~90–95% of plant water loss. Regulated by guard cells via stomatal aperture control.
Auxin (IAA)
A plant hormone (indole-3-acetic acid) promoting cell elongation, mediating phototropism and gravitropism, causing apical dominance by suppressing lateral buds, and stimulating root initiation. Produced primarily in shoot apical meristems.
ABA (Abscisic Acid)
A plant stress hormone that triggers stomatal closure during drought (K⁺ efflux from guard cells), induces seed dormancy, promotes bud dormancy in winter, and coordinates abscission. Synthesized in response to water deficit and other stresses.

Frequently Asked Questions

Water moves along a water potential gradient: soil (high Ψ) → roots → xylem → leaves (low Ψ, due to transpiration). Transpiration from leaves creates tension (negative pressure) in xylem — water is pulled up by cohesion-tension. Water molecules in xylem are held together by hydrogen bonding (cohesion) and adhere to xylem walls (adhesion), allowing tension to be transmitted from leaves to roots without breaking the water column.

Auxin (IAA): promotes cell elongation; mediates phototropism and gravitropism; causes apical dominance. Gibberellin: promotes stem elongation and seed germination; used commercially to increase fruit size. Cytokinin: promotes cell division; maintains shoot meristems; delays leaf senescence. ABA: closes stomata in drought; induces seed dormancy; coordinates stress responses. Ethylene: promotes fruit ripening; triggers leaf and fruit abscission; produced in response to wounding and flooding.

C3 plants (wheat, rice, soybeans): CO₂ fixed directly by RuBisCO into 3-carbon 3-PGA. At high temperatures and low CO₂, photorespiration wastes 20–50% of fixed carbon. C4 plants (corn, sugarcane): CO₂ pre-fixed as 4-carbon malate in mesophyll cells, then concentrated around RuBisCO in bundle sheath cells — suppressing photorespiration. C4 plants are more efficient in hot, bright, water-limited conditions but have higher structural costs.

Guard cells control aperture through osmotic changes. Opening: blue light activates H⁺-ATPase → proton efflux → K⁺ influx into guard cells → water follows → cells swell and bow outward → pore opens. Closing: ABA activates K⁺ efflux and Ca²⁺ release → guard cells lose turgor → stomata close. High CO₂ inside leaves also promotes closure. Open stomata allow CO₂ entry for photosynthesis but increase water loss via transpiration.