Control of Water Transport by Guard Cells

Bulk water movement through xylem is influenced and powered primarily by water loss to the atmosphere. Although water loss through the cuticle is important, transstomatal transpiration is usually more significant whenever stomatal pores are open. Open stomata always represent a trade-off between carbon dioxide absorption and water loss. Whenever water supply in the soil is adequate, water loss is actually advantageous—the water movement is the primary means of carrying minerals upward from roots to shoots, and the evaporative cooling that results from transpiration can prevent heat stress in leaves and young stems. However, if the soil is too dry to supply water, transpiration represents an immediate, potentially lethal threat due to desiccation. Numerous mechanisms have evolved that control stomatal opening and closing. Each mechanism is keyed to a particulat environmental factor, and their interaction results in great sensitivity to potential stresses in the habitat.

If the leaf has an adequate moisture content, light and carbon dioxide are the normal controlling factors. For most healthy, turgid plants, light most often controls guard cd water relations. Blue light is the important, triggering wavelength, and the action spectrum of opening closely matches the absorption spectrum of a flavin or flavoprotein pigment. It is not yet known how absorption of light by the pigment leads to potassium pumping.

The presence of light also leads to the photosynthetic fixing of carbon dioxide; the decrease in internal carbon dioxide concentration may also lead to stomatal opening. Artificial manipulation of the amount of carbon dioxide available can stimulate guard cells to open or close in light or dark. At night, with no photosynthesis, carbon dioxide levels are high and presumably contribute to stomatal closing.

All these mechanisms in healthy plants are completely overridden by a much more powerful mechanism that is triggered by water stress. As leaves begin to dehydrate, they release the hormone abscisic acid. This hormone immediately causes guard cells to lose potassium and close the stomatal pore. Closure occurs even in blue light and low concentrations of carbon dioxide, factors that would otherwise favor opening. Water stress- induced closure often occurs in the early afternoon on a warm, dry day if root uptake and xylem conduction cannot keep up with transpiration. Under these circumstances, lack of carbon dioxide stops photosynthesis even though light is available.

In plants with crassulacean acid metabolism, stomatal opening and closing are re- versed when the plant is water-stressed. Stomata open at night and close in the morning. Temperature is particularly important for these plants; if night temperatures are too high, stomata may remain closed for days or weeks at a time. The low night temperatures typical of the desert are essential for stomatal opening and carbon dioxide absorption. Under conditions of mild temperatures and abundant moisture in the tissues, such as after a spring rainfall, CAM plants convert to C₃ metabolism, opening their stomata in the morning and picking up carbon dioxide with RuBP carboxylase directly. When the soil dries j after several days, they revert to CAM and night opening of stomata.