Characteristics of the circadian rhythm in diffusive resistance of Abutilon theophrasti leaves in humid and dry environments.

Diffusive resistance of the upper and lower leaf surfaces of Abutilon theophrasti Medic, was measured with an automatic diffusive resistance meter. Plants were maintained in growth chambers under 4 different environments: 1. high humidity, well-watered, 2. high humidity, dry, 3. low humidity, well-watered, 4. low humidity, dry. Diffusive resistance measurements were obtained every 4 h for up to 3 days when the plants were subjected to either daily regimes consisting of 15 h of light followed by 9 h of darkness or to continuous illumination. Leaf movements were also monitored. Diffusive resistance was rhythmic (circadian) under all the environmental conditions examined. The pattern of diffusive resistance was the same regardless of whether the humidity and/or soil moisture were high or low. The rhythms of diffusive resistance of the upper and lower leaf surfaces differed, however, in amplitude and in the phase at which the minimum value was reached. Diffusive resistance of the leaf was highest (peak of the rhythm) when the leaf blade was in a nearly horizontal position.