"Plants release water" — technically correct, but it only describes one side. Transpiration is not simply evaporation: it is the driving force that pulls water from root to leaf, carries calcium and magnesium to growing points, and regulates leaf temperature. When transpiration is weak, all of these processes slow with it. Conversely, excessive transpiration overloads the root and closes stomata — halting both photosynthesis and nutrient delivery together.
Quick Glossary
- Transpiration — evaporation of water through leaf stomata; the primary driving force of water movement from root to leaf within the plant
- Stomata — microscopic openings on the leaf formed by two guard cells; they open in response to light and adequate turgor for gas exchange and transpiration, and close under stress or darkness
- VPD (Vapour Pressure Deficit) — the difference between the vapour pressure inside the leaf (always close to 100% saturation) and that of the surrounding air; determines how strongly the air "pulls" water out of the leaf
Transpiration as a Pump: How Water Moves Through the Plant
A plant has no heart or pump. Water moves from root to leaf through a physical process: water evaporates from cell surfaces into the leaf cavity and exits through the stomata — and this "departure" of molecules at the surface pulls the entire chain of liquid upward from the root. The more active the transpiration, the stronger the water stream and the more dissolved elements move upward with it.
Calcium and magnesium are "passengers" in this stream. They move primarily with water and cannot actively travel against the current. When water flow is weak due to low transpiration, calcium reaches older lower leaves where the stream still exists — but does not reach young growing points where flow is minimal. This produces marginal necrosis on young leaves and tomato blossom end rot despite "normal" EC and pH.
What Regulates Transpiration Rate
VPD — the primary factor. The difference between humidity inside the leaf and outside determines the driving force. The inside of a leaf is always close to saturation. If the outside air is dry (high VPD) — the pull is strong, transpiration is intense. If the air is humid (low VPD) — the pull is weak.
Lighting. Stomata open in response to light — photosynthetically active radiation is the signal to open. Without light, stomata are closed and transpiration is minimal even in dry air. In the morning after a dark night, stomata are still partially closed, and a sudden temperature rise when lights switch on at full intensity can cause stress before opening is complete.
Temperature. Higher air temperature raises VPD at the same humidity — increasing transpiration. In addition, at higher temperatures molecular mobility increases and evaporation accelerates. The leaf cools itself through transpiration — leaf temperature is typically 1–3°C below air temperature during normal transpiration.
Stomatal state. Several conditions close stomata as a protective response: excessive VPD (the plant protects against excessive moisture loss), CO₂ above a critical level, pathogens or mechanical stress, and darkness. When stomata close, transpiration, CO₂ uptake, and photosynthesis all stop together.
What Transpiration Means for Greenhouse Climate
Plants are active participants in a room's microclimate — not passive consumers. In a densely planted greenhouse, they release significant moisture into the air through transpiration — and this moisture is part of the humidity balance that must be accounted for when calculating ventilation.
During active growth, 1 m² of leaf surface transpires 1–5 litres of water per day depending on VPD, light levels, and crop type. In a 50 m² greenhouse with a dense canopy — 50–250 litres of moisture enter the air daily. If ventilation does not remove this moisture, humidity rises, VPD drops, transpiration slows — and the calcium deficiency chain triggers itself.
This is why ventilation is not just for "fresh air" — it manages the moisture balance that the plants themselves produce.
Three Mistakes That Cost the Most
Treating calcium deficiency with fertiliser without checking transpiration. Marginal necrosis and tip burn are, in the overwhelming majority of cases, not a shortage of calcium in the solution but poor delivery due to low transpiration. Check VPD and air movement within the canopy first — then decide whether calcium supplementation is needed.
Raising temperature and VPD sharply after the dark period. Stomata open gradually after the plant "wakes up." A sudden VPD spike when lights come on at full intensity causes transpiration to rise before the root system has time to "ramp up." Result: temporary tip wilt in the morning even with a moist substrate. A gradual ramp-up in light intensity at the start of the photoperiod (sunrise simulation) reduces this stress.
Not accounting for transpiration when calculating ventilation. "I have 20 plants — they don't release much" — with a dense canopy and DLI above 20 mol/m²/day, they release a significant amount. Ventilation designed only for basic air exchange without accounting for the transpiration load will produce chronically elevated humidity within the canopy.
Signs That Transpiration Is Normal
- In the morning after lights on, guttation is visible at leaf tips — droplets of liquid emerging through hydathodes when root pressure is active and cell turgor is full
- Leaves remain firm and turgid throughout the day
- No calcium necrosis at growing points despite normal EC
- Leaf temperature measured with a thermal camera or IR thermometer is 1–3°C below air temperature — a sign of active transpiration and evaporative cooling