Transpiration as a Physical and Biological Process
Separate the physical diffusion of water vapor from the biological controls that regulate stomata, leaf temperature, and whole-canopy water loss.
Educational reference · evidence, sources, and limits shown below
Separate the physical diffusion of water vapor from the biological controls that regulate stomata, leaf temperature, and whole-canopy water loss.
Terms to know
- Transpiration
- Loss of water vapor from plant tissues, primarily through stomata.
- Stomatal conductance
- Measure of how readily gases pass through stomatal pores.
- Boundary-layer conductance
- Conductance of the thin air layer adjacent to the leaf surface.
- Latent heat
- Energy carried away when liquid water evaporates.
Core science
Transpiration begins with evaporation from wet cell-wall surfaces into internal leaf air spaces, which are commonly treated as nearly saturated. Water vapor then diffuses through stomata and the leaf boundary layer into drier surrounding air. The rate depends on vapor-pressure difference and on stomatal and boundary-layer resistances, not on RH alone.
Stomata respond to light, internal carbon dioxide, abscisic acid, leaf water status, temperature, circadian regulation, and other signals. Opening permits CO2 entry for photosynthesis but also increases water loss. Closing conserves water, yet can reduce carbon assimilation and leaf cooling. These responses are dynamic and may differ among leaves within one canopy.
Evaporation consumes energy and can cool a transpiring leaf below surrounding air. When roots cannot keep pace, stomata tend to close and leaf temperature may rise. Water use therefore emerges from the interaction of radiation, air temperature, humidity, airflow, leaf area, root supply, and plant regulation.
Why this matters in cultivation
- Do not interpret a high irrigation total as proof of productive transpiration. Leaks, evaporation, drainage, and luxury application must be separated from actual plant water use. A crop can also show low water use because of low light, high humidity, cold roots, damaged roots, or stomatal closure.
- Trend water input, drainage, substrate storage change, and plant mass when possible. The resulting water balance is more useful than a single room VPD value for detecting changes in crop demand.
Measure and record
Environment
Air and leaf temperature, RH, VPD method, PPFD, photoperiod position, and fan state.
Water balance
Applied water, collected drainage, substrate mass or water-content change, and interval.
Canopy
Leaf area proxy, canopy density, cultivar or clone, and developmental stage.
Plant signals
Leaf angle, temperature difference from air, wilting, and recovery.
System changes
Lighting, HVAC, fan, irrigation, pruning, or spacing changes before the response.
Common misconceptions
Correction: Physical evaporation is regulated by living stomata and the plant hydraulic system.
Correction: Excess demand can cause hydraulic stress; very low demand can also alter transport and disease risk.
Correction: VPD is one driver; conductance, leaf area, energy load, and root supply also matter.
Evidence limits
Whole-crop transpiration cannot be calculated accurately from VPD alone. Robust estimates require canopy conductance, energy balance, leaf area, and system-specific validation.
Related encyclopedia topics
- THC-ENC-068-071, THC-ENC-081, THC-ENC-085-091, and irrigation-water-balance records.
Source notes
- Nobel P.S. (2009). Physicochemical and Environmental Plant Physiology, 4th ed. Academic Press.
- Buckley T.N. (2017). Modeling stomatal conductance. Plant Physiology 174:572-582.
- THC Cannabis Plant Science Source Packet v1.1 (project source, May 2026).
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.