The Role of the Epidermal Cells in the Stomatal Movements
Authors: Stárlfelt M. G. 
Published: April 2006

The water deficit of the leaves, the osmotic values of the stomatal cells and epidermal cells at incipiment plasmolysis, as well as the width of the stomatal apparatus and pore opening, were measured every hour from 6‐17 o'clock under natural environmental conditions. During the noon hours, the intensity of light in clear weather ranged from 40,000‐55,000 lux in the open position, and from 15,000‐20,000 lux in the shade. The temperature was usually 15–20°C.

The experimental object was Vicia Faba growing in a field, both plants freely rooted and plants in pots buried in the soil.

The experiments resulted in the following observations and conclusions:

1. When leaves are exposed to strong light, the osmotic value at incipient plasmolysis changes not only in the guard cells, but also in the epidermal cells. If the epidermal cells' osmotic value rises, water is sucked from the guard cells and their uptake of water by suction is decreased, which promotes closure and counteracts opening, respectively. If the value falls, the effect is the reverse. The guard cells react passively to these epidermal changes. The passive stomatal movement eliciteed in this way has therefore been denoted as “osmopassive”, in contrast to the long known passive movement caused by a change in turgor of the epidermal cells, and which has therefore been denoted as “turgorpasslve”.

The osmopassive component of stomatal closure has an earlier and more rapid onset than the hydroactive closing reaction, which consists of a decrease in the guard cells' osmotic value. Stomatat closure often starts with the osmopassive rapid process, and is completed and stabilized by the hydroactive process.

It has not been possible to determine whether the osmopassive closing reaction is identical with the rapid reaction previously described, and interpreted as of adenoid nature, and tlius belonging to the active group.

2. The osmotic potential of the guard cells ‐ i.e., the difference between the osmotic value of guard cells and epidermal cells at incipient plasmolysis ‐ is, therefore, formed not only by a cbange in the osmotic value of the former cells, but also by a cbange in that of the latter.

3. Although the pore width runs largely parallel to the osmotic value of the guard cells, there is greater agreement between pore width and osmotic potential. When the water deficit of the leaf exceeds a certain threshold value, potential and stomatal width start to decrease. Closure is completed when the fall in potential approaches the zero value. If the water deficit subsequently continues to increase, the potential becomes negative and the stomata remain closed.

4. The stomatal movements are regulated by physiological processes which form two kinds of equilibrium between increase and decrease of the osmotic potential of the guard cells, i.e. the osmopassive increase ‐ osmopassive decrease and the photoactive increase ‐ hydroactive decrease. These equilibria complement each other in rate and stability. The osmopassive processes start rapidly and as soon as the deficit cbanges; hydroactive closure and sometimes also photoactive opening, are, on the contrary, time‐consuming.

When the water deficit is suboptimal, turgorpassive opening and closing are superadded, but only in those cases in which the osmotic potential of the guard cetls is positive.