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 and O₂ release for photosynthesis but are also responsible for 95% of water loss.
- Guard cells change shape to open or close stomata:
- Turgid guard cells → stomata open
- Flaccid guard cells → stomata close
- Stomatal density is influenced by species, development, and environment.
- Low CO₂ during leaf development → higher stomatal density
Mechanisms of Stomatal Opening and Closing
- K⁺ ions are key:
- Stomata open when guard cells accumulate K⁺ → water enters → turgor increases
- Stomata close when K⁺ exits → water leaves → turgor decreases
- Proton pumps (H⁺-ATPases) activated by light promote K⁺ uptake.
- Aquaporins regulate water flow in guard cells, influencing turgor pressure.
Stimuli That Control Stomata
Stomata typically open in the morning and close at night. Their opening is regulated by:
- Light: Blue light activates proton pumps for K⁺ entry.
- CO₂ depletion: Low internal CO₂ due to photosynthesis stimulates opening.
- Circadian rhythms: Internal clock maintains opening cycles even in darkness.
- Abscisic acid (ABA): Produced during drought stress, signals stomata to close.
Environmental Conditions and Transpiration
- Hot, dry, windy conditions increase transpiration.
- Wilting occurs when water loss exceeds absorption.
- Evaporative cooling keeps leaf temperatures up to 10°C cooler than the air.
Adaptations to Reduce Water Loss
- Xerophytes have adaptations such as:
- Short life cycles timed with rainfall
- Fleshy stems for water storage
- Reduced leaves like spines
- CAM photosynthesis: stomata open at night
- Additional features: thick cuticles, sunken stomata, trichomes
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.