The Temperatures of Leaves in Assimilation Chambers, and in the Open

Abstract

The temperatures of apple leaves in assimilation chambers were up to 17° C above the temperature of the outside air in sunlight. Except when in deep shade, enclosed leaves had temperatures higher than ambient. Leaves in the open in the sun were often 2–3° C above ambient, but the greatest difference measured was 4.3° C. In shade, leaves in the open were as much as 0.8° C below the air temperature. Laboratory experiments with an incandescent lamp showed that the temperature difference between an enclosed leaf and the outside air increased linearly with increasing light intensity above a certain value. Below this value it is believed that changes in leaf permeability were sufficiently large to affect the rate of transpiration and therefore the leaf excess temperature-light intensity relationship. Under field conditions leaves may not be in a steady state; this gives rise to more variable measurements which may indicate a non-linear relation between leaf excess temperature and light intensity. Methods of cooling leaves in chambers were examined. Impractically high rates of flow of air at the ambient temperature are necessary to reduce the temperature of enclosed leaves appreciably. Some reduction of the leaf excess temperature can be obtained by filtering the infra-red from the incident light, or by using a chamber made of material which transmits far infra-red, though condensation reduces the effectiveness of the latter measure. Leaves exhibit rapid changes in temperature, so the heating problem cannot be circumvented by brief enclosure. The most effective of the techniques examined is to use a water-cooled chamber, though the temperatures of the leaf and water differ by several degrees centigrade in bright light. A simple solution to the heating problem for field assimilation measurements has not been found.