Stomata (singular stoma) are pores in the leaf epidermis, each flanked by two guard cells that swell to open the pore and shrink to close it. Open stomata let carbon dioxide in for photosynthesis and let water vapour out (transpiration). In cannabis most stomata are on the lower leaf surface.
In practice
Stomatal behaviour links the room's climate to growth: - Light opens them; darkness largely closes them. - Dry air (high VPD, roughly above 1.5-1.6 kPa) makes them partly close to save water, so the plant fixes less carbon. - Drought or root stress triggers abscisic acid, which closes them. - Elevated CO₂ (for example 800-1,200 ppm) lets plants keep stomata a bit more closed for the same carbon gain.
This is why a VPD in range, typically 0.8-1.2 kPa in veg and 1.0-1.5 kPa in flower, helps growth: the plant can keep stomata open without losing water too fast.
Foliar sprays work partly through stomata and cuticle, which is why spraying the leaf underside and at lights-on is common practice.
Don't confuse with
- Trichomes: outgrowths, not pores.
- Lenticels: breathing pores on bark and older stems.
- Hydathodes: water pores at leaf margins.
