Oscillatory Transpiration and Water Uptake of Avena Plants IV. Transpiratory Response to Sine Shaped Light Cycles

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Abstract
The transpiration rate of individual 6‐day‐old oat plants was forced to oscillate by cyclic sine‐shaped changes in the leaf irradiance (frequency 2 cycles h−1, amplitude and average value 1.4 mW cm−2, red light 620–800 nm). By means of a specially designed cuvette with three chambers the transpiration rate from three different segments of the leaf could be measured simultaneously. The leaf segments were illuminated individually and the illumination on each leaf segment could be modulated independently.The experiments showed that there was a strong correlation between the transpiration rates from the different leaf segments, dependent on a coupling mechanism in the plant. The coupling phenomenon disappeared when the root system was eliminated or when the water potential of the root medium was lowered. It was experimentally shown that CO2 diffusion in the leaf could not be the primary cause for the coupling. Therefore the stomatal dependence on the leaf water potential was considered the most probable reason for the coupling.The frequency of the forcing light cycles could be linearly changed during an experiment and this swept‐frequency technique was used to obtain a frequency response of one single oat plant. The technique made it also possible to study the strength of the coupling between different leaf segments.