| Popis: |
THE mechanism of the movement of stomatal guard cells in response to the stimulus of light, although under investigation for nearly a century, is still in doubt. The classical view1 that such movements are due to changes of turgor has received support from many workers, who have found by plasmolytic methods that with stomatal opening the osmotic pressure in the guard cells increases2-6. More recently, the possible collaboration of a 'wall mechanism' in opening the stomata of Cyclamen and Tradescantia has been disproved7., so that a turgor mechanism alone must be concerned ; but how the changes in turgor are produced is still uncertain. Mainly owing to the work of Lloyd8, Hagen9, Loft-field10 and Sayre5 it is very generally accepted that they are due to hydrolysis or condensation of starch in the guard cells, probafily brought about by enzyme action governed by pH. changes in response to light. A careful examination of these and other relevant papers, however, shows that nearly all the evidence for such a mechanism is based on the observation of diurnal changes in starch content and stomatal aperture under natural conditions. These observations leave little doubt that, under such conditions, pore size and starch content are to a large extent, though by no means completely, negatively correlated. A similar inverse relation is shown where change of aperture is experimentally induced by altering the water relations of the leaf2, or by immersing detached pieces of leaf or epidermis in various solutions11-14; but such data are not necessarily relevant to the problem of a light response. Where stomatal movement has been experimentally induced by changes of lighting conditions, at other than the natural times, the evidence for a correlation between starch content and stomatal aperture is not convincing8,15. The correlation of diurnal trends is an unsafe basis for a causal theory, especially in this case, since it has been shown that in cherry laurel16 and Pelargonium17,18 there is a diurnal rhythm of stomatal movement even under conditions of constant illumination. The diurnal rhythm of stomatal starch, which has not been shown to be due to light, may well provide the motive power for such autonomous stomatal movements. |
