Instant Notes: Plant Biology

Stomata of dicots are typically kidney-shaped, while those of grasses are

dumb-bell shaped. In both types, microfibrilsof the cell wall (Topic B2) are

arranged radially, causing enlargement of the pore when the guard cells swell

(Fig. 2).

Stomatal aperture is tightly regulated by the plant, in a system, which integrates

carbon dioxiderequirement with lightandwater stress. Stomatal aperture

often varies according to a circadian(day/night)rhythm. Low CO 2 concentra-

tions in the guard cells result in opening, while high CO 2 concentrations result

in closure. Stomata in most species open at dawn and remain open in daylight,

given adequate water availability; this may in part be due to changing CO 2

concentrations as a result of photosynthesis and respiration.

Stomatal guard cells are also very sensitive to water stress. Localized turgor

loss results in wilting of the guard cells (hydro-passive closure) and stomatal

closure. Water stress elsewhere in the plant results in the production of abscisic

acid(ABA, Topic F2) which results in stomatal closure. The mechanism of ABA

sensing probably involves ABA receptor proteins(Topic F3) and may involve

the action of calcium as an intracellular messenger that alters membrane ion

channel activity, giving altered turgor and stomatal closure.

Guard cellstake up K+and increase in turgorin favorable conditions, resulting

in swelling of the cells and opening of the pore. Potassium is rapidly lost from the

guard cells during stomatal closure. The driving force for potassium uptake is

provided by the plasma membrane proton pump(Topic I3). This is stimulated

during stomatal opening and the resulting membrane hyperpolarizationis

believed to open K+channels in the plasma membrane which permit a passive K+

influx. This, together with influx of Cl–and organic anions such as malate

provides the increase in turgor. When stomata close, other plasma membrane ion

channels open resulting in a rapid efflux of anions and a drop in turgor.

Xerophytes show a number of adaptations to water stress, including sunken

stomata, thickened cuticles and succulence. A key adaptation is the presence of

crassulacean acid metabolism (CAM; Topic H3). CAM plants show a specialized

rhythm of stomatal action that minimizes water loss. Coupled with their unique

metabolism their stomata only open at night, when they fix CO 2 as malate in the

vacuole. The stomata then remain closed during the day, when evaporative

losses would be greatest.

Stomatal
adaptations of
xerophytes

Stomata: action
and regulation

I2 – Water retention and stomata 123

Guard cell

Radial microfibrils

Stomatal pore

Subsidiary cell

(a) (b)

Fig. 2. Stomata of (a) a dicot and (b) a grass.