In situ observations of stomatal movements in different light-dark regimes: The influence of endogenous rhythmicity and long-term adjustments

The technical device for continuous microscopic observations of stomatal movements in a gas exchange chamber using digital image analysis, earlier described by Kappen et al, (1994), was technically improved. By electronic remote control, it is now possible repeatedly to record over a period of several days identical stomata on a 25 x 50 mm leaf area, The responses of individual stomata to various light-dark sequences in the light and in the dark phase were investigated on attached leaves of Vicia faba (L.). The amplitude of stomatal oscillations caused by alternating light was highest on the first day of an experimental series and decreased with repeated application, indicating a longterm adjustment to changing light conditions by decreasing sensitivity to intermittent darkening. In the usual dark phase light-dark sequences had little effect on the stomatal aperture, which remained small, These results were widely convergent with the stomatal conductance calculated from the gas exchange measurements, Very prominent is the role of an endogenous rhythmicity of the stomatal response. It could be demonstrated (1) by autonomous stomatal opening before the light phase started; (2) by a suppression of dark response in the early morning hours; (3) by a decreasing light stimulation in the afternoon; (4) by further increase of aperture for several hours if no light was provided in the light phase, Closely adjacent stomata could show divergent dark-opening movements pointing to an autonomous control mechanism located in the guard cells. The endogenously controlled morning opening provides full assimilation capacity in the usually humid morning hours when transpiratory water loss associated with CO2-uptake is comparatively small.