Light-stimulated heat tolerance in leaves of two neotropical tree species, Ficus insipida and Calophyllum longifolium.

Previous heat tolerance tests of higher plants have been mostly performed with darkened leaves. However, under natural conditions, high leaf temperatures usually occur during periods of high solar radiation. In this study, we demonstrate small but significant increases in the heat tolerance of illuminated leaves. Leaf disks of mature sun leaves from two neotropical tree species, Ficus insipida Willd. and Calophyllum longifolium Willd., were subjected to 15min of heat treatment in the light (500µmol photons m-2s-1) and in the dark. Tissue temperatures were controlled by floating the disks on the surface of a water bath. PSII activity was determined 24h and 48h after heating using chlorophyll a fluorescence. Permanent tissue damage was assessed visually during long-term storage of leaf sections under dim light. In comparison to heat treatments in the dark, the critical temperature (T50) causing a 50% decline of the fluorescence ratio Fv/Fm was increased by ~1°C (from ~52.5°C to ~53.5°C) in the light. Moreover, illumination reduced the decline of Fv/Fm as temperatures approached T50. Visible tissue damage was reduced following heat treatment in the light. Experiments with attached leaves of seedlings exposed to increasing temperatures in a gas exchange cuvette also showed a positive effect of light on heat tolerance.