It is supposed that oscillations in stomatal conductance are associated with the dynamic properties of the loop in which rate of evaporation affects, through physiological processes, Self-sustained oscillations, having a period of about 40 min, in stomatal aperture, and plant functioning associated with stomatal aperture, are known to occur in many plant species (1,3). Several models of the mechanism responsible for these oscillations have been described (3, 7-9, 1 1). They have in common one basic assumption: that the oscillations are due to the properties of the loop in which rate of evaporation affects, through physiological processes, the aperture of stomata and stomatal aperture in turn affects rate of evaporation. The models differ in the way they describe the physiological processes. The model described by Cowan (3) suggests itself: can we manipulate the total gain of the loop in an unambiguous way and show that the stability of the stomatal mechanism is affected in the manner expected? Any attempt to alter the characteristics of the physiological component of the loop would fall short of what is required because it would presume a detailed understanding of the processes involved. On the other hand, the environmental component of loop gain, which is the sensitivity of rate of evaporation to change in stomatal aperture, can be readily manipulated in a way which is unequivocal because the physical process involved is well understood. We report here on two experiments in which environmental gain has been altered and the effects on the stability of the stomatal mechanism have been observed.
MATERIALS AND METHODSThe plants used were cotton (Gossypiumn hirsutum L. cv.Deltapine Smooth Leaf), 1 month old, having five or six leaves with a total area of 0.05 to 0.06 M2. The apparatus is described in detail elsewhere (6). In brief, it consists of a small closedcircuit wind tunnel. The working section, which contains the upper parts of the plant, is of Plexiglas and is illuminated from above by a xenon arc lamp. The flux of radiation is 80 w m-2 in the wavelength range 370 to 820 nm. Wind speed is about 1 m s-'. The stem of the plant is sealed in the floor of the working section, the roots below being immersed in aerated Hoagland solution.Ambient temperature is regulated, through the combination of a water-cooled heat exchanger and an electrical heater in the air stream approaching the plant, in such a way as to maintain the temperature of the leaves, rather than that of the air stream, as constant as possible. Thermojunctions (44 s.w.g. copper-constantan) are inserted into the main veins of three of the leaves, with the reference junctions in ice. The signals are continuously recorded on strip-chart; one is amplified and used to provide the error signal for proportional control of the electrical heating of the air stream. The bandwidth varies with thermal load, but is always less than 1 C. The humidity of the air stream is sensed by a dew-point hygrometer and is proportionally controlled by regulating the f...