Cyclic Changes in Transpiration of Sunflower Leaves in a Steady Environment

Abstract

Concurrent cyclic changes of leaf temperature, transpiration rate, and stomatal aperture were found to occur in plants of sunflower (Helianthus annuus L.) exposed to a steady aerial environment and with their roots in water. These cyclic changes were less regular with plants in drying soils and did not occur at all with plants in soils with a water potential less than −120 J kg⁻¹. A sequence of possible events which lead to the repeated cycling is suggested; these involve relative changes in the turgor of epidermal and guard cells. The possibility that repeated cycling may be caused by a changing concentration of carbon dioxide in the sub-stomatal cavities is also discussed. The irregularity and eventual cessation of these cyclic changes are considered to be due to the reduction in turgor of the epidermal and guard cells as the availability of water to these cells is reduced.