Transpiration

Transpiration: Loss of water vapor from aerial parts of plant, mainly through stomata in leaves.

Significance: (1) Creates transpiration pull for water absorption, (2) Cools the plant, (3) Maintains turgidity, (4) Transports minerals, (5) Maintains water balance.

Sites: 90-95% through stomata (stomatal transpiration), 3-5% through cuticle (cuticular), 1-2% through lenticels (lenticular).

Necessary Evil: Essential for water transport but causes water loss (97-99% absorbed water is lost).

Difference from Evaporation: Transpiration is biological (living cells), evaporation is physical. Transpiration is regulated, evaporation is not.

Stomata: Tiny pores on leaf surface surrounded by two guard cells. Regulate gas exchange and transpiration.

Guard Cells: Kidney-shaped cells (in dicots) with thick inner wall and thin outer wall. Contain chloroplasts.

Stomatal Pore: Opening between guard cells. Opens and closes based on turgidity of guard cells.

Distribution: More on lower epidermis (hypostomatous). Absent in submerged parts. Sunken in xerophytes.

Opening Mechanism: Guard cells turgid → stomata open (day). Guard cells flaccid → stomata close (night).

Diagram Importance: Surface view and sectional view of stomata showing guard cells is crucial for ICSE.

Opening (Day): (1) Photosynthesis in guard cells produces glucose, (2) Glucose → Starch conversion stops, (3) Glucose increases osmotic pressure, (4) Water enters guard cells by osmosis, (5) Guard cells become turgid, (6) Thin outer walls stretch, thick inner walls don't, (7) Stomatal pore opens.

Closing (Night): (1) No photosynthesis, (2) Glucose → Starch conversion, (3) Osmotic pressure decreases, (4) Water leaves guard cells, (5) Guard cells become flaccid, (6) Stomatal pore closes.

Role of K⁺ ions: Potassium ions actively pumped into guard cells during day, increasing osmotic pressure.

pH Changes: High pH (day) favors opening, low pH (night) favors closing.

Stomatal Transpiration: Through stomata. Accounts for 90-95% of total transpiration. Most important and regulated.

Cuticular Transpiration: Through waxy cuticle on epidermis. 3-5% of total. Unregulated, continuous.

Lenticular Transpiration: Through lenticels in woody stems. 1-2% of total. Occurs in woody plants.

Comparison: Stomatal is maximum and regulated; cuticular is minimum and unregulated.

External Factors: (1) Light - increases transpiration (stomata open), (2) Temperature - high temp increases (more evaporation), (3) Humidity - high humidity decreases (less gradient), (4) Wind - moderate wind increases (removes saturated air), strong wind decreases (stomata close), (5) Atmospheric pressure - low pressure increases.

Internal Factors: (1) Leaf area - large area increases, (2) Leaf structure - thick cuticle decreases, (3) Number of stomata - more stomata increases, (4) Root-shoot ratio - affects water availability.

Water Availability: Adequate soil water increases transpiration. Water stress closes stomata.

Xerophytes: Plants adapted to dry conditions. Have features to reduce water loss.

Thick Cuticle: Reduces cuticular transpiration. Example: Cactus, Aloe vera.

Sunken Stomata: Stomata in pits, reducing air movement. Example: Nerium (oleander).

Reduced Leaf Area: Small or modified leaves (spines). Example: Cactus, Acacia.

Rolled Leaves: Reduces exposed surface. Example: Grasses in dry areas.

Hairy Leaves: Trichomes trap moisture. Example: Many desert plants.

Succulent Stems: Store water. Example: Cactus, Euphorbia.

CAM Metabolism: Stomata open at night, closed during day. Example: Pineapple, Agave.

Advantages: (1) Creates transpiration pull for water absorption and transport, (2) Transports minerals from roots to leaves, (3) Cools the plant (evaporative cooling), (4) Maintains cell turgidity, (5) Provides water for photosynthesis.

Disadvantages: (1) Wasteful - 97-99% absorbed water lost, (2) Can cause wilting in water stress, (3) Energy expenditure to absorb more water, (4) Limits plant distribution in dry areas.

Wilting: Loss of turgidity causing drooping. Temporary (recovers at night) or permanent (severe water stress).

Cobalt Chloride Paper Test: Blue paper turns pink in presence of water. Shows transpiration from leaf surfaces.

Potometer: Measures rate of water uptake (approximates transpiration rate). Leafy shoot in water-filled tube.

Bell Jar Experiment: Plant in bell jar shows water droplets on inner surface, proving transpiration.

Ganong's Potometer: Measures transpiration rate under different conditions (light, wind, humidity).

Importance: Experiments demonstrate transpiration and factors affecting it.