Plant Transport, Transpiration, Translocation & Hormones – Full Study Notes

Xylem Transport & Transpiration (Cohesion–Tension Mechanism)

• Definition of transpiration
– Loss of water vapour from leaf air spaces or moist cell walls to the atmosphere.
– Occurs mainly through open stomata.
• Sequence of events on a hot, sunny day

1. Water molecule evaporates from mesophyll surface → creates an empty space (negative pressure/vacuum).
2. Cohesion (H-bonding between water molecules) keeps the column continuous.
3. Adhesion (H-bonding to xylem wall) prevents column from falling due to gravity.
4. Resulting tension "pulls" water upward from roots to shoots without metabolic energy.
   • Key physical properties
   – Water is polar ⇒ strong cohesion/adhesion.
   – Transpiration pull = bulk flow driven by a water-potential gradient $\Delta\Psi$ (most negative in leaf air spaces, least negative in soil).
   – Typical values: leaf $\Psi\approx-1.5\,\text{MPa}$, soil $\Psi\approx-0.3\,\text{MPa}$.
   • Root pressure
   – At night stomata close; minerals actively pumped into xylem ↓ $\Psi_s$ → water enters by osmosis → weak positive pressure (root pressure).
   – Can cause guttation (exudation of droplets at leaf tips).
   – Magnitude small; transpiration pull is the dominant force in tall plants.
   • Environmental factors affecting transpiration rate
   – High temperature ↑ evaporation.
   – Low relative humidity ↑ gradient.
   – Wind removes boundary-layer vapour.
   – Light (via stomatal opening) increases rate.
   • Stomatal regulation
   – Guard-cell turgor governed by K⁺ uptake ↔ efflux.
   – Stimuli: blue light, low internal CO₂, circadian clock, ABA under drought.
   • Cooling effect
   – High latent heat of vaporization dissipates heat, protecting enzymes from denaturation.
   • Failure conditions
   – Drought/freezing → air bubbles (embolism) break water column, plant wilts/dies.

Xerophytic (Dry-Adapted) Modifications

• Thick cuticle; reduced/rolled leaves.
• Stomata sunk in pits with trichomes (hairs) to trap humid air.
• CAM photosynthesis: stomata open at night, CO₂ stored as malate; photosynthesis completed by day.
• Succulent water-storage tissues; stem photosynthesis (leafless phylloclades).

Xylem Structure Recap

• Dead, lignified tracheids/vessel elements.
• Secondary walls reinforced with helices/rings of lignin → withstand large negative pressure.
• Vascular arrangement
– Dicot stem: xylem/phloem in a ring.
– Monocot stem: scattered vascular bundles.

Phloem Transport (Translocation) – Pressure-Flow Model

• Definition: movement of photosynthate (mainly sucrose) from source to sink through sieve tubes.
• Step-by-step

1. Loading: Sucrose actively transported (proton-sucrose cotransport) into sieve-tube members at the source.
2. ↓ $\Psi_s$ ⇒ water diffuses in from adjacent xylem.
3. Generates positive hydrostatic pressure ($\approx+0.2\,\text{MPa}$) → bulk flow toward sink.
4. Unloading at sink (storage tissue/root/fruit) raises $\Psi_s$ of sap; water exits back to xylem.
   • Cycle repeats continuously; flow direction depends on relative source–sink status.
   • Key contrasts with xylem
   – Phloem: living cells, positive pressure, bidirectional.
   – Xylem: dead cells, negative pressure, unidirectional (root → shoot).
   • Supporting cells
   – Sieve elements (blue in diagram) lack nuclei; companion cells (yellow) provide ATP, proteins.
   – Parenchyma (green) fills bundle; sclerenchyma fibres (red) strengthen.
   • Other notes
   – Transfer cells: wall ingrowths ↑ membrane area for solute passage.
   – Aphids pierce phloem to sample sugary sap; pathway exploited by plant viruses via plasmodesmata.

Water Potential Formula Reminder

• $\Psi = \Psi<em>s + \Psi</em>p + \Psi<em>g + \Psi</em>m$
– For most cell-to-cell comparisons $\Psi<em>g,\Psi</em>m\approx0$, giving $\Psi\approx\Psi<em>s+\Psi</em>p$.

Root & Specialised Root Types (from sample questions)

• Storage roots – e.g., carrot, beet (tuberous enlargement).
• Root nodules – houses N₂-fixing bacteria (legumes).
• Buttress roots – vertical flanges supporting tall rainforest trees.
• Aerial/pneumatophores – aerating roots for mangroves; protrude above water for O₂ uptake.
• Photosynthetic roots – green, exposed roots (e.g., orchids).
• Prop/diffuse roots – additional anchorage (e.g., maize prop roots).

Plant Hormones – Overview

• Growth promoters

1. Auxins – cell elongation at apical meristem; apical dominance; root initiation; work synergistically with cytokinins.
2. Gibberellins (GA) – stem elongation, seed germination, bolting, fruit growth.
3. Cytokinins – cytokinesis, shoot formation, delay senescence.
   • Growth inhibitors
4. Abscisic Acid (ABA) – drought signal; stomatal closure; dormancy induction; accumulation of storage proteins in seeds.
5. Ethylene (gas) – fruit ripening, leaf abscission, triple response in seedlings.
   • Other signalling molecules
   – Brassinosteroids, jasmonates, salicylic acid, strigolactones, karrikins (fire-derived germination cue) – minor exam focus.

Seed Germination & GA / ABA Antagonism

• Seed anatomy
– Testa (seed coat), plumule (embryonic shoot), radicle (embryonic root), micropyle (water entry), hilum (scar of ovule attachment).
• Germination steps

1. Water uptake via micropyle.
2. Activates GA synthesis in embryo.
3. GA → aleurone layer → $\rightarrow$ transcription of $\alpha$-amylase.
4. $\alpha$-amylase hydrolyses starch → maltose → glucose supporting embryo growth until true leaves photosynthesise.
5. ABA counteracts GA; high ABA keeps seeds dormant.

Additional Practical / Ethical / Exam Notes

• Transpiration–cohesion–adhesion is passive ⇒ no ATP cost; contrasts with active transport in phloem loading.
• Manipulating hormones (spraying GA, ethylene) used in agriculture to control height, synchronize fruit ripening, delay senescence.
• Excessive water loss or xylem blockage during drought/freezing is a major cause of crop failure → relevance to climate change resilience.
• Exam visuals: distinguish dicot vs monocot vascular patterns, identify xerophyte features, recognise specialised roots and stems.