Oscillations in Apparent Hydraulic Conductance of Cotton Plants

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Abstract
Cotton plants were grown in pots that could be encased in a pressure chamber so that a pneumatic pressure could be applied to the roots while the shoot was enclosed in an unpressurized cuvette. Enough pressure (the 'balancing pressure') was applied to the roots to raise the pressure of the xylem sap in the shoot to that of the atmosphere, so that a cut in the xylem of the shoot was on the verge of bleeding. The balancing pressure was accurately maintained, despite any changes in the transpirational flux, by means of an automatic pressure regulator. When the transpirational flux was suddenly changed by changing the humidity in the cuvette, the balancing pressure started to oscillate even though flux was steady at its new level, i.e. the apparent hydraulic conductance (the ratio of transpirational flux to the pressure drop across the plant) oscillated. In plants that were phosphorus-deficient, the oscillations were large and had a period of 30-50 min; though damped, they persisted for several cycles. Phosphorussufficient plants gave one, critically-damped, oscillation.