Plant Water Balance: Stomata, ABA and Xerophytes

A plant answer earns marks when it links a structure to water movement. Name the feature, then explain how it changes evaporation, diffusion, storage or absorption.

When water stress increases, ABA causes potassium ions (K⁺) to leave guard cells. Water follows by osmosis, the cells lose turgor and the stomatal pore closes, so less water vapour diffuses out of the leaf.

On a 40 degree morning in the Great Victoria Desert, an arid-zone plant faces an immediate crisis: its stomata must open to absorb carbon dioxide for photosynthesis, but every minute they stay open, water evaporates into the dry air. A 2019 CSIRO survey (Nolan et al.) measured these plants holding leaf water potential within plus or minus 0.2 MPa.

Each stoma is flanked by two guard cells with unevenly thickened walls; the inner wall facing the pore is thicker than the outer wall. When the cells take up water and swell, that asymmetry makes them bow outward and pull the pore open. When they lose water and go flaccid, they straighten and the pore closes.

The swelling is driven by ions, not by pumping water. In light, guard cells actively transport potassium ions (K⁺) in, lowering their water potential, so water follows by osmosis and turgor rises. Under drought stress the hormone abscisic acid (ABA) triggers K⁺ efflux; water follows out, turgor falls and the stoma closes within minutes.

Plant water balance showing transpiration, stomata and root pressure

Stomatal control manages two variables at once. Open stomata admit carbon dioxide for photosynthesis but leak water vapour; closed stomata conserve water but starve the leaf of carbon dioxide. A plant that keeps its stomata open on a hot day wilts within hours, while one that keeps them shut through a drought survives but cannot grow.

Many Australian drought-adapted plants resolve the trade-off by opening stomata only in the cooler morning hours, when evaporation is slowest, then closing them for the rest of the day. That timing maximises the carbon dioxide gained per unit of water lost, a water-use-efficiency strategy measured in field studies.

CAM plants such as cacti and agaves push this further: they open stomata only at night, fix the carbon dioxide into organic acids, then close during the hot day and release the stored carbon dioxide to the Calvin cycle. Growth is slow, but water loss per gram of carbon fixed is the lowest of any plant group.

Xerophytes are plants structurally adapted for water conservation. Unlike stomatal closure, which is an active physiological response, these are permanent features that passively reduce water loss at all times, so the active mechanisms do not have to work as hard. Beyond the big three above, three more mechanisms appear regularly in exams.

Fine hairs, called trichomes, trap a layer of still humid air against the leaf surface, shallowing the diffusion gradient for water vapour, and they reflect some solar radiation, which lowers leaf temperature. The silver-grey sheen of many wattles and saltbushes is exactly this adaptation, visible from metres away.

Leaf orientation and colour work on temperature rather than diffusion. Steeply angled leaves, as in mallee eucalypts, intercept less direct midday radiation per unit area, and pale or silvery surfaces reflect more of what does arrive. A cooler leaf has a lower internal vapour pressure, so the gradient driving transpiration out is smaller.

Other xerophytes store or source water instead of saving it. Succulents such as pigface hold water in fleshy leaves or stems, spinifex rolls its leaves into cylinders that enclose humid air around the stomata, and many desert shrubs invest in deep or widespread root systems that keep water supply matched to demand.

Plants are not the only organisms managing water against an osmotic gradient. For aquatic animals, water moves by osmosis toward the side with more solute, so whether a fish is gaining or losing water depends entirely on whether its blood is more or less concentrated than the water around it.

A marine bony fish has blood at about 350 mOsm/kg bathed in seawater of about 1000 mOsm/kg, so water leaves its body continuously. It drinks seawater to replace the loss, excretes the excess sodium and chloride through specialised gill cells by active transport, and produces small volumes of concentrated urine.

A freshwater fish faces the mirror-image problem: its blood, around 300 mOsm/kg, is far saltier than the surrounding water, so water floods in by osmosis. It never drinks, its gills actively absorb sodium and chloride ions to replace salts lost by diffusion, and its kidneys produce large volumes of very dilute urine.

Both strategies are negative feedback holding blood osmolarity inside a tolerance range, the same control logic as the ADH pathway from L04, with kidneys and gills as effectors. Euryhaline species such as salmon switch between the two modes as they migrate, reversing the direction of their gill ion transport under hormonal control.

Transpiration itself is not a homeostatic response; it is the physical process the plant has to manage. Four environmental variables set its rate, and each one acts either on the water vapour concentration gradient between the leaf interior and the outside air, or on the aperture of the stomata.

Higher temperature raises the vapour pressure inside the leaf, steepening the outward gradient. Lower humidity dries the air, steepening it from the other side. Wind strips away the humid boundary layer clinging to the leaf surface, keeping the gradient at its maximum. Higher light intensity activates the guard cell K⁺ pumps, widening the pores.

A potometer measures the rate of water uptake by a cut shoot, which closely approximates transpiration because most absorbed water is transpired. An air bubble in a capillary tube moves as the shoot draws water, and the distance travelled per minute under each condition is the dependent variable, with all other factors controlled.

In a typical school result, a shoot in still air moves the bubble about 4 mm per minute, while the same shoot in front of a fan manages over 9. The limitation worth quoting is that the potometer measures uptake, not transpiration directly: a small fraction of the water is used in photosynthesis or retained for turgor.

Writing these out by hand is the fastest way to lock the mechanisms in. Keep the arrows; they force you to remember the order, which is what exam questions test.