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

The temperatures of apple leaves in assimilation chambers wereup to 17° C above the temperature of the outside air insunlight. Except when in deep shade, enclosed leaves had temperatureshigher than ambient. Leaves in the open in the sun were often2–3° C above ambient, but the greatest differencemeasured was 4.3° C. In shade, leaves in the open were asmuch as 0.8° C below the air temperature. Laboratory experimentswith an incandescent lamp showed that the temperature differencebetween an enclosed leaf and the outside air increased linearlywith increasing light intensity above a certain value. Belowthis value it is believed that changes in leaf permeabilitywere sufficiently large to affect the rate of transpirationand 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 indicatea non-linear relation between leaf excess temperature and lightintensity. Methods of cooling leaves in chambers were examined.Impractically high rates of flow of air at the ambient temperatureare necessary to reduce the temperature of enclosed leaves appreciably.Some reduction of the leaf excess temperature can be obtainedby filtering the infra-red from the incident light, or by usinga chamber made of material which transmits far infra-red, thoughcondensation reduces the effectiveness of the latter measure.Leaves exhibit rapid changes in temperature, so the heatingproblem cannot be circumvented by brief enclosure. The mosteffective of the techniques examined is to use a water-cooledchamber, though the temperatures of the leaf and water differby several degrees centigrade in bright light. A simple solutionto the heating problem for field assimilation measurements hasnot been found.