What are Stomata?
Stomata are microscopic pores — typically 10–80 μm long — on the surface of leaves. Each stoma is flanked by two guard cells that swell or shrink to open or close the pore. Through these tiny openings, the tree exchanges gases with the atmosphere.
How They Work
Opening
- Guard cells absorb potassium ions and water.
- Cells swell, pulling the pore open.
- CO₂ enters for photosynthesis; O₂ exits.
- Water vapor escapes (transpiration).
Closing
- Guard cells release potassium and water.
- Cells shrink, closing the pore.
- Gas exchange and water loss stop.
Regulation
Stomata respond to multiple signals:
- Light: Open in daylight, close in darkness (in most species).
- CO₂: High internal CO₂ triggers closure.
- Humidity: Dry air promotes closure to conserve water.
- Drought stress: Abscisic acid (ABA) hormone forces emergency closure.
- Temperature: Extreme heat causes closure.
Stomatal Density
Different species and habitats produce different numbers of stomata per leaf area:
- Sun leaves: More stomata — higher gas exchange capacity.
- Shade leaves: Fewer stomata — lower demand.
- Dry-forest species: Fewer or sunken stomata to reduce water loss.
- Rainforest species: More stomata — water is abundant.
Costa Rican Examples
Guanacaste (Enterolobium cyclocarpum)
Closes stomata rapidly during the dry season, conserving water and triggering leaf abscission.
Teca (Tectona grandis)
Large leaves with high stomatal density — transpires heavily, requiring well-drained soils with adequate rainfall.
Why It Matters
- Climate change: Stomatal density in fossil leaves records ancient atmospheric CO₂ levels.
- Water use efficiency: Breeding for optimal stomatal regulation improves crop drought tolerance.
- Air quality: Stomata also absorb pollutants, making urban trees natural air filters.