Thermoimaging as a tool for studying light-induced heating of leaves Correlation of heat dissipation with the efficiency of photosystem II photochemistry and non-photochemical quenching

Thermoimaging - a highly sensitive and non-invasive method of temperature measurement - was applied to explore the role of changing photosynthetic efficiency in light-induced heating of tobacco (Nicotiana tabacum cv. Samsun) leaves. in the absence of evaporative cooling through the stomata, which was achieved by covering leaves with Vaseline, illumination with 50-1400 mu M photons m(-2) s(-1) intensity of photosynthetically active radiation resulted in approximate to 1-5 degrees C leaf temperature increase in about 2min. The heating effect showed a non-linear correlation with the extent of nonphotochemical quenching (NPQ) resulting in higher leaf temperatures at higher NPQ values. When leaves were adapted to excessive irradiance (1300 mu M photons m(-2) s(-1) for 6h), which resulted in reduction of photosynthetic efficiency and amplification of NPQ the light-induced heating effect was enhanced. The experimental results have been explained on the basis of a simple theoretical model characterizing the balance of energy fluxes in leaves in relation to the efficiency of photosystem II photochemistry and non-photochemical quenching. The role of alternative energy dissipation pathways outside of PSII in the phenomenon of light-induced leaf heating is also discussed. (c) 2008 Elsevier B.V. All rights reserved.