Transpiration, stomata and environmental effects
Organisation • Plant tissues, organs and systems
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Definition and mechanics of transpiration
Transpiration is the loss of water vapour from aerial parts of plants, mainly leaves, by evaporation from internal leaf cell surfaces into air spaces and diffusion through stomata. Evaporation from the spongy mesophyll creates a water vapour concentration gradient from inside the leaf to the external air, causing water to diffuse out and continuous water columns to be pulled up through xylem vessels by cohesion and tension . High leaf evaporation causes increased tension in xylem, which increases water uptake from root hair cells by osmosis. Xylem vessels are hollow, lignified tubes that allow unbroken upward flow; any factor that raises evaporation increases the transpiration stream until a limiting factor (soil water availability, xylem cavitation) reduces the flow.
Translocation in phloem
Translocation is the movement of dissolved sugars (mainly sucrose) through living phloem tissue from regions of production (sources) to regions of use or storage (sinks). Pressure-flow (mass flow) explains movement: active loading of sucrose at sources lowers water potential in phloem, causing water influx and high turgor pressure that pushes sap towards sinks where sugars unload and water exits . Phloem cells remain living and have companion cells that support active transport. Translocation operates independently of the transpiration stream but both systems interact because water movement into phloem can derive from xylem.
Stomata structure and guard-cell function
Stomata are microscopic pores mainly on the lower leaf surface; each stoma sits between two kidney-shaped guard cells that change shape to open or close the pore. Guard cells accumulate solutes (e.g., potassium ions) to lower their water potential, so water enters by osmosis and cells swell, causing the pore to open. Loss of solutes causes guard cells to lose turgor and the pore to close . Guard-cell opening permits CO2 uptake for photosynthesis and O2/ water vapour exchange. Guard-cell control balances the need for CO2 against the risk of excessive water loss; physiology and environment jointly determine stomatal aperture.
Effect of temperature on transpiration
Cause: Higher temperature increases kinetic energy of water molecules and raises saturation vapour pressure, increasing evaporation from leaf cell surfaces. Effect: Transpiration rate increases because the internal-to-external vapour concentration gradient becomes steeper and diffusion through stomata speeds up . Limiting factors: At very high temperatures, stomata may close to limit water loss, reducing transpiration despite high evaporation potential; soil water shortage also limits sustained transpiration.
Effect of humidity on transpiration
Cause: External air humidity sets the external water vapour concentration. High humidity reduces the internal–external concentration gradient. Effect: Transpiration rate decreases when air is humid because diffusion of water vapour out of the leaf slows; low humidity increases transpiration by steepening the gradient . Practical implication: Transpiration measurements must record humidity to compare rates; dew or mist reduces measured transpiration substantially.
Effect of air movement and light intensity
Cause: Air movement removes the thin layer of humid air surrounding the leaf (boundary layer), maintaining a steep vapour gradient. Effect: Increased wind speed raises transpiration rate because diffusion continues rapidly away from stomata. Conversely, still air allows a humid boundary layer to build and slows transpiration . Cause: Light intensity influences photosynthesis and stomatal opening. Effect: Bright light increases stomatal opening for CO2 uptake; stomatal opening increases transpiration. Darkness causes stomatal closure and reduces transpiration. Seasonal or diurnal light patterns therefore drive predictable daily transpiration cycles.
Key notes
Important points to keep in mind