Heliotropic Leaf Movements in Common Beans Controlled by Air Temperature

Abstract

Heliotropic leaf movements were examined in common beans (Phaseolus vulgaris cv Blue Lake Bush) under outdoor and labo-ratory conditions. Heliotropic leaf movements in well-watered plants were partly controlled by temperature, and appeared to be independent of atmospheric humidity and CO2 concentration. When environmental conditions were held constant in the labo-ratory, increased air temperature caused bean leaves to orient more obliquely to a light source. Ambient C02, intercellular C02, and net photosynthesis were not correlated with the temperature-induced changes in heliotropic movements, nor did they signifi-cantly affect these movements directly. The effect of air temper-ature on leaf movements need not be mediated through a change in leaf water potential, transpiration, or leaf conductance. Air temperature modified laminar orientation in light through its effect on tissue temperature in the pulvinal region, not that of the lamina or petiole. However, under darkness the temperature effects on leaf movements were not expressed. Active heliotropic move-ments in response to air temperature allowed lamina temperature to remain close to the thermal optimum of photosynthesis. This temperature effect underlies a commonly observed pattem of leaf movements under well-watered conditions: a tendency for