Light Intensity and Transpiration

Burton Edward Livingston
1911 Botanical Gazette  
effects of these conditions and by certain responses to other conditions. One secondary effect of variations in light intensity is seen in the opening of stomata when many plants are transferred from darkness to diffuse or stronger light. These openings close, or tend to close, in many plants when light gives place to darkness or to very dim light. But there seems to be no evidence for thinking that stomata] movement is at all marked as long as the intensity of illumination is above a certain
more » ... s above a certain minimum, about what is known as weak diffuse light. Of course they close with wilting under any light conditions (see LLOYD, Pubi. 82 of the Carnegie Institution). Another secondary effect of high evaporation conditions, whether caused by direct sunlight or by dryness of the air, etc., is the removal of water from the leaves at a rate more rapid than its rate of entrance, so that the cells are plasmolyzed and general wilting occurs. It is probable that this effect is felt long before actual wilting is to be observed; whenever transpiration surpasses the rate of water supply to the transpiring tissues it must be supposed that a gradual lowering of the vapor tension of the water films held in the moist cellulose walls will ensue, just as a semi-dry piece of filter paper will exhibit a much lower vapor tension than a similar piece saturated. Long before plasmolysis occurs we should expect to find that the capillary menisci of the cell membranes abutting upon the internal atmosphere of the leaf would become more and more concave, and would perhaps break and retreat into the pores of the membrane. In the one case, the vapor tension of the water film, in the other their actual superficial extent should be reduced. It may thus come about that an increase in the evaporating power of the air or of solar insolation might produce, by its very accelerating influence, a retardation in the transpiration rate. Such a phenomenon is common in soils, where an increase in the rate of water loss above the rate of diffusion of water to the soil surface causes the water films to retreat into the soil and thus decreases the rate of water loss. It is thus that the "dust mulch" is produced, by which adaptation the soil seeks to reduce water loss in a dry time! A measurable falling off in the relative transpiration rate, occurring in the forenoon, when the evaporating power of the air and the light intensity are both still increasing in their daily march,
doi:10.1086/330684 fatcat:tolice6hr5adpnit6qw4j7zy7e