Transpiration and stomatal regulation
Why plants lose so much water, how stomata open and close, what controls transpiration, transpiration ratio and VPD sums, and antitranspirants.
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A maize plant in full sun can lose several litres of water in a day, and a hectare of crop can transpire 40,000 to 60,000 litres on a hot day. Only about 1 per cent of the water a plant takes up is used to build tissue; almost all the rest evaporates from the leaves. This loss, transpiration, is the unavoidable cost of photosynthesis: the pores that let carbon dioxide in also let water vapour out.
How plants control this trade-off through their stomata is one of the most examined topics in plant physiology, and it underlies irrigation, drought tolerance and water use efficiency in agronomy.
Kinds of transpiration
- Stomatal transpiration, through the stomatal pores: usually about 90 per cent or more of the total when stomata are open.
- Cuticular transpiration, through the waxy cuticle of leaves and stems: small in most crops, but important when stomata are closed, and higher in thin-cuticled or young leaves.
- Lenticular transpiration, through lenticels in bark and fruit: minor.
Guttation, the loss of liquid water droplets from hydathodes at leaf margins, usually at night in humid conditions with moist soil, is driven by root pressure and is not transpiration.
Why transpiration matters
- It creates the tension that pulls water and dissolved minerals from the roots to the leaves, as explained by the cohesion–tension theory.
- It cools leaves: evaporating water carries away heat, keeping leaves several degrees cooler than they would be in strong sun.
- It is the price of carbon gain: stomata must open for CO2 to enter.
Too much transpiration, when roots cannot keep up, leads to water stress, wilting, stomatal closure and lost photosynthesis.
Stomata: structure and numbers
A stoma (plural stomata) is a pore bordered by two guard cells, often flanked by subsidiary cells. Dicots usually have kidney-shaped guard cells; grasses have dumbbell-shaped guard cells, which open faster. Leaves have from about 50 to over 500 stomata per mm2, and many dicot leaves have more on the lower surface (hypostomatous leaves), while grasses often have them on both sides (amphistomatous). Floating leaves, such as water lilies, have them only on the upper surface.
Even fully open, the pores occupy only about 1 to 3 per cent of the leaf surface, yet diffusion through them can approach that from an open water surface of similar area. H. T. Brown and F. Escombe explained this in 1900: diffusion through small pores is proportional to their perimeter rather than their area, so many small, well-spaced pores are very efficient.
How stomata open and close
The modern explanation is the potassium ion (K+) theory, developed from the work of S. Imamura (1943) and M. Fujino (1967):
- Light, especially blue light sensed by phototropins, activates a proton pump (H+-ATPase) in the guard cell membrane. Red light acts partly through photosynthesis in the guard cells and partly by lowering internal CO2.
- Pumping protons out makes the inside of the cell more negative, opening inward potassium channels; K+ flows in.
- Chloride enters, and malate is made from starch, balancing the charge.
- These solutes lower the guard cells' water potential; water enters by osmosis, and turgor rises.
- Because cellulose microfibrils are arranged radially around the guard cells and their inner walls are thicker, swelling guard cells bow apart and the pore opens.
Closing reverses this. Under drought, roots and leaves produce the hormone abscisic acid (ABA), which, through a signalling pathway, activates anion channels and outward K+ channels; ions and water leave and the guard cells go limp. The older starch–sugar hypothesis, which linked opening to the conversion of starch into sugar, has been largely replaced by the K+ theory, though sugars contribute to guard cell solutes later in the day.
CAM plants, such as pineapple and agave, reverse the usual rhythm: their stomata open at night, when air is cooler and more humid, saving water.
Factors affecting transpiration
| Factor | Effect |
|---|---|
| Light | opens stomata; also heats leaves |
| Vapour pressure deficit (dry air) | increases the driving force; very high VPD partly closes stomata |
| Temperature | raises the leaf's vapour pressure and so the deficit |
| Wind | removes the humid boundary layer around the leaf, up to a point |
| Soil water | dry soil lowers leaf water potential and triggers ABA and closure |
| CO2 concentration | high CO2 partly closes stomata |
| Leaf traits | cuticle, hairs, sunken stomata, leaf rolling and leaf area |
How much water does a crop transpire?
Transpiration rate can be written as E = gs × VPD ⁄ P, where gs is stomatal conductance, VPD the vapour pressure deficit between leaf and air, and P air pressure.
On a hot afternoon at 30 °C and 40 per cent relative humidity, the saturation vapour pressure is about 4.24 kPa, so the air holds 0.40 × 4.24 = 1.70 kPa and, for a leaf at air temperature, VPD = 4.24 − 1.70 ≈ 2.5 kPa.
With gs = 0.3 mol per m2 per second and P = 101 kPa: E = 0.3 × 2.5 ÷ 101 ≈ 0.0074 mol per m2 per second, or 7.4 mmol per m2 of leaf per second, about 0.48 kg of water per m2 of leaf per hour.
Transpiration ratio and water use efficiency
The transpiration ratio is the mass of water transpired per unit mass of dry matter produced; its inverse is a measure of water use efficiency. Classic pot experiments by L. J. Briggs and H. L. Shantz in Colorado (1913–1917) found values of roughly 250 to 350 for C4 crops such as maize, sorghum and millet, and about 500 to 900 for C3 crops such as wheat, rice, legumes and alfalfa.
Water for a wheat crop
A wheat crop produces 10 t/ha of above-ground dry matter with a transpiration ratio of 500.
- Water transpired = 10,000 kg × 500 = 5,000,000 kg = 5,000 m3 per hectare.
- Spread over 10,000 m2, that is 0.5 m, or 500 mm of water, plus soil evaporation.
A C4 crop with a ratio of 300 would need about 300 mm for the same dry matter.
Antitranspirants
Chemicals to reduce transpiration have been tested for decades:
- Film-forming types (waxes, silicone and latex emulsions) coat the leaf.
- Reflecting types, such as kaolin clay sprays, lower leaf temperature.
- Stomata-closing types, such as ABA and its analogues (older ones, like phenylmercuric acetate, are now banned because of toxicity).
Because closing stomata also cuts CO2 uptake, antitranspirants usually reduce growth, so they are used mainly for transplants, cut flowers, nursery stock and short drought periods rather than whole field crops.
Key points
- Transpiration is stomatal (about 90 per cent or more), cuticular and lenticular; guttation is liquid loss via hydathodes and is not transpiration.
- It pulls water up the plant and cools leaves, at the cost of water.
- Stomata open when blue light drives a proton pump, K+, Cl− and malate enter, and water follows; ABA closes them under drought.
- Transpiration rises with light, temperature, VPD and wind, and falls with dry soil and high CO2.
- Transpiration ratio: about 250–350 for C4 and 500–900 for C3 crops (Briggs and Shantz).
- Antitranspirants save water but also reduce photosynthesis.
What the exams ask
- School-leaving and entrance papers: types and significance of transpiration, stomatal structure, factors affecting transpiration, and guttation.
- Degree and research-fellowship entrances: mechanism of stomatal opening (K+ theory, starch–sugar hypothesis), ABA signalling, Brown and Escombe's small-pore law, transpiration ratio and antitranspirants.
- Professional and research examinations: stomatal conductance and VPD, water use efficiency, and breeding for drought tolerance.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Why do leaves wilt at midday even in moist soil?
On hot, dry afternoons transpiration can exceed the rate at which roots and stems can deliver water. Leaves lose turgor temporarily and recover in the evening; this is temporary wilting, distinct from permanent wilting in dry soil.
Do plants transpire at night?
Usually little, because stomata close in darkness, but many species keep them partly open and lose some water at night. CAM plants do most of their gas exchange at night.
Why do drought-tolerant plants have sunken stomata and hairs?
Both trap a layer of still, humid air over the pores, which reduces the vapour pressure gradient and so cuts water loss.
Sources
- Taiz, L., Zeiger, E., Møller, I. M. and Murphy, A. Plant Physiology and Development, 6th edition. Sinauer Associates, 2015. Chapters 4 and 10.
- Brown, H. T. and Escombe, F. "Static diffusion of gases and liquids in relation to the assimilation of carbon and translocation in plants." Philosophical Transactions of the Royal Society B 193, 223–291 (1900).
- Briggs, L. J. and Shantz, H. L. "The water requirement of plants." USDA Bureau of Plant Industry Bulletin 284 (1913).
- Hetherington, A. M. and Woodward, F. I. "The role of stomata in sensing and driving environmental change." Nature 424, 901–908 (2003).
- Assmann, S. M. and Jegla, T. "Guard cell sensory systems: recent insights on stomatal responses to light, abscisic acid, and CO2." Current Opinion in Plant Biology 33, 157–167 (2016).
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