Rucete ✏ Campbell Biology In a Nutshell
Unit 6 PLANT FORM AND FUNCTION — Concept 36.4 The Rate of Transpiration Is Regulated by Stomata
Transpiration, the loss of water vapor from a plant’s surface, is crucial for nutrient transport but must be carefully regulated to prevent dehydration. This balance is achieved primarily through stomata, small pores on the leaf surface that open and close to manage both water loss and gas exchange.
Stomata: The Gatekeepers of Gas and Water Exchange
Stomata allow CO₂ intake for photosynthesis and O₂ release, but 95% of water loss also occurs through them.
Guard cells flanking each stoma control pore size by changing shape:
When turgid, they bow outward → stomata open.
When flaccid, they collapse → stomata close.
Stomatal density varies by species and environment; it's under both genetic and environmental control.
Example: Low atmospheric CO₂ during leaf development → increased stomatal density.
Mechanisms of Stomatal Opening and Closing
K⁺ ions (potassium) play a key role:
Stomata open when guard cells accumulate K⁺ → lowers water potential → water enters → turgor pressure increases.
Stomata close when K⁺ is lost → water leaves → turgor drops.
Proton pumps (H⁺-ATPases) are activated by light and create membrane potentials that facilitate K⁺ uptake.
Aquaporins help regulate water flow in and out of guard cells, impacting their turgidity.
Stimuli That Control Stomata
Stomata usually open in the morning and close at night. Their behavior is influenced by:
Light: Blue light triggers proton pumps, promoting K⁺ uptake.
CO₂ depletion: Low internal CO₂ (due to photosynthesis) stimulates opening.
Internal biological clock: Stomatal rhythms persist even in continuous darkness, controlled by circadian rhythms.
Abscisic acid (ABA): Produced under drought stress, it signals stomata to close to conserve water, though it limits photosynthesis.
Environmental Conditions and Transpiration
Hot, dry, windy weather increases evaporation and thus transpiration.
Wilting occurs when water loss exceeds uptake, reducing cell turgor.
Evaporative cooling lowers leaf temperature, protecting enzymes from heat damage (up to 10°C cooler than ambient air).
Adaptations to Reduce Water Loss
Xerophytes (desert plants) have evolved:
Short life cycles timed with rain.
Fleshy stems to store water.
Reduced leaves (like spines in cacti).
CAM photosynthesis: Stomata open at night for CO₂ uptake, minimizing water loss.
Structural features like thick cuticles, sunken stomata, and trichomes also reduce water loss.
In a Nutshell
Transpiration is vital but risky—it supports nutrient flow and cooling but can cause dehydration. Stomata manage this balance through turgor changes in guard cells triggered by environmental cues. Drought, light, internal clocks, and CO₂ levels all play roles. Desert plants show remarkable adaptations to keep water in while still supporting photosynthesis.