Plant Physiology Study Notes: Auxin, ABA, Tropisms, and Photosynthesis

Posted on Nov 5, 2026 in Biotechnology

Plant Hormones and Growth Responses

1. Auxin (IAA) Precursor

Auxin (IAA) is naturally synthesized from the precursor amino acid tryptophan. Synthetic versions such as NAA, IBA, and 2,4-D are classified as plant growth regulators rather than hormones, because they are not made by the plant.

2. Auxin and Cell Elongation

Auxin increases the concentration of \(H^{+}\) ions in primary cell walls, which activates enzymes that loosen cellulose fibers. Increased turgor pressure then causes the plasticized cell wall to expand.

3. Polar Transport of IAA

  • In shoots, transport occurs preferentially in a basipetal direction (downward from the apex).
  • In roots, transport occurs preferentially in an acropetal direction (toward the root tip apex).
  • Both require metabolic energy.

4. Physiological Functions of Abscisic Acid (ABA)

ABA induces stomatal closure during drought stress to decrease transpiration, inhibits fruit ripening, and maintains seed dormancy by inhibiting early cell growth and germination.

Tropisms and Photoperiodism

5. Matching Tropisms to Environmental Stimuli

  • Heliotropism: Sunlight
  • Thigmotropism: Touch or mechanical contact (e.g., Venus flytrap or tendril winding)
  • Gravitropism: Gravity (positive in downward roots, negative in upward stems)

6. Photomorphogenesis vs. Photoperiodism

  • Photomorphogenesis: Nondirectional, light-triggered development and structural changes, such as seed germination and de-etiolation when a seedling first emerges from soil.
  • Photoperiodism: The plant’s response to changes in the length of the night, which controls seasonal activities like leaf growth and flowering.

7. Photoreceptors Controlling Photoperiodism

Photoperiodism is controlled by phytochromes, which absorb red light. Light changes the physical shape of the phytochrome molecule, which triggers altered internal cellular signaling cascades.

Photosynthesis and Nutrients

8. Central Metal in Chlorophyll

The central metal coordinating element in chlorophyll is magnesium (Mg2+), which acts as the central atom. It belongs to Group 3 (nutrients that remain in ionic form or act as enzyme cofactors).

9. Roles of Manganese and Chloride in Light Reactions

Manganese (Mn2+) and chloride (Cl–) ions work together in the Oxygen-Evolving Complex (OEC) of PSII to liberate oxygen gas (O2) from water molecules. Manganese also serves as a critical electron transporter between water and PSII.

10. Noncyclic Electron Flow from PSII to PSI

  1. Light excites P680 in PSII; the electron is removed by the primary electron acceptor, pheophytin (Pheo).
  2. Pheophytin transfers the electron to plastoquinone (PQ), which moves within the membrane and becomes reduced to PQH2.
  3. PQH2 delivers electrons one at a time to cytochrome b6/Fe-S protein and then onto cytochrome f.
  4. The electron is transferred to the mobile, copper-containing protein plastocyanin (PC) located in the thylakoid lumen.
  5. PC moves along the membrane to deliver the electron to P700 in PSI, replacing the electron that P700 lost to light excitation.

11. Phases of the Calvin Cycle

  1. Phase 1: Carboxylation (Carbon Fixation) by Rubisco
  2. Phase 2: Reduction (with ATP and NADPH)
  3. Phase 3: RuBP Regeneration

Nutrient Mobility and Plant Adaptations

12. Deficiency Symptoms in Older Leaves

If a deficiency symptom appears in older leaves first, the element is highly mobile; the plant relocates the nutrient from mature tissues to protect new growth. Examples include nitrogen, potassium, magnesium, and phosphorus.

13. C3 Limitations in Hot Environments and C4/CAM Solutions

Increased temperatures cause Rubisco to bind oxygen instead of \(CO_{2}\), leading to photorespiration (C2 photosynthesis), which wastes fixed carbon and energy.

C4 and CAM plants use phosphoenolpyruvate (PEP) to regenerate carbon fixation, allowing them to fix carbon more efficiently without losing energy to photorespiration.

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