Leaf Water Balance During Oscillation of Stomatal Aperture

Abstract

Continuous transpiration an,d 3-gauge measurements were made on cotton plants, the stomatal apertures of which had been induced to oscillate, thus allowing a water balance to be made, and leaf potential to be measured as functions of time. Analyses showed phase differences between the water entering and leaving the leaf. Also, from the phase relationship between the flow into the leaf-and the water potential in the leaf it was shown that the water potential in the xylem of the plant also oscillated. This is proposed as a necessary condition for the stomates of all the leaves of a plant 'to oscillate in phase. It is convenient to describe the dynamics of the phenomena using a simple electrical analogue, and the usefulness and limitations of the model are discussed. Cyclic oscillations in stomatal aperture. transpira-tion rate and photosynthetic rate with periods of the order of 1 hr have been observed occasionally during the last 40 years. No one has vet elucidated the mechanism responsible for these oscillations althouigh there is agreenment that all these phenonm,ena involve oscillations in stomatal aperture. For a particular plant. the amplitude of oscillation may vary wAith environimental conditions (2, 7, 14. 17) while the period remains the same. Yet the oscillation's niu-t be dependent on some relatively conservative structural featuire, since many species exhibit more or less the same frequency of oscillation; most commonly the period is 20 to 50 min. There can,, however, he considerable variation within a species. In this laboratory, cotton plants of the same variety grown in glasshouses in the summer mon,ths generally had a period of oscillationl of abouit 40 mmn, but the period in the winter wa's often about an hour. Thus, some conservative property, which is none-the-less influenced by environment, is involved in these oscillations. Fturthermlore, a restricted supply of water to the leaves is a necessary condition for the occurrence of the phenomenon (1, 3. 5, 12). Oscillations in plant water loss, caused by periodic opening and closing of stomates, is associated with cyclic variations ini leaf water status (1, 4). Estimates of leaf water status made with a fl-gauge show that leaf water content is out of phase fronm its normal relationship with stomatal aperture, i.e., stomates are open when leaf water content is low and closed when the leaf is more turgid (4). Similarly , measurements of the water potential of leaves near the one being monitored for water loss indicate that leaf water potential is cycling (1). Changes in leaf water potential give rise to changes in the driv-826 estimates of the superficial density (mg dm 2) of ing force for water-flow into the leaf. Thus it is possible that, wlveni a plant undergoes oscillations in stomatal aperture. there is cycling not only in the rate of water loss from the leaves but also in bothl the rate of water influx through the petiole and the water content of the leaf itself. This work was undertaken to show the relationship between the flow of wate:, into the petiole and the rate of water loss to the atmosphere for a plant which has been induced to cycle, and to try to indicate the mech'anism by which the oscillations are sustained. Materials and Methods Cotton plants (Gossypiumn barbadense L. var. Pima S-2) were grown in Hoagland's solution in a glasshouse anid were about 2 months old when used. Cotton was chosen for this study because it cycles readily and has leaves with large interveinal areas for ,B-gauging. A Plexiglas leaf chamiber (Fig. 1), wlhich had a ,8-gauge built inlto it, was used to enclose a l,eaf. The f8-gauge ('11) was made using a Tc-99 source and a Geiger-Muller tube. and was calibrated with aluminum absorbers. This f8-gauge gave successive the enclosed leaf every 80 sec. Thus the rate of change of leaf water contenit on a unlit area basis, m2 (mg dm-2 min-') could be calculated by taking the derivative with time of the superficial densities. The derivatives at each time, t was obtained from the slope of the least-squares line passing througl the 9 points bracketing 'the time, t. The rate of water loss from the enclosed leaf, Pi7, (mlg dn2 min) was measured with a differential psychometer (1, 13). Flow rates into and out of the leaf chamber, meastured with rotameters and www.plantphysiol.org on June 16, 2020-Published by Downloaded from