Photothermal beam deflection: a new method for in vivo measurements of thermal energy dissipation and photochemical energy conversion in intact leaves.

A novel photosynthetic technique, photothermal deflection spectroscopy, is presented which is based on the 'mirage effect' and allows the rapid measurement of thermal deactivation of excited pigments in leaf samples placed in an open cell. Modulated heat emission from leaves illuminated with intensity-modulated light was measured via the detection of the periodic deflection of a laser beam parallel to the sample surface. Photothermal deflection signals can be monitored in vivo in leaves placed in various, liquid or gaseous, environments with a satisfactory signal-to-noise ratio close to 60-80 (in distilled water) at low modulation frequencies (below 50 Hz). Using this new and simple photothermal method, it was possible to easily obtain useful information on the leaf photochemical activity and its light-saturation characteristics under normal or stress conditions, suggesting that in vivo deflection signals could be used for assaying the photosynthetic state of health of crop plants. The beam deflection method presented in this paper appears to be a potentially useful photosynthetic tool complementary to the related photoacoustic technique.