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

Abstract

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.

The Use and Characteristics of the Photoacoustic Method in the Study of Photosynthesis

“…Another example is the use of the spatial and temporal refractive index changes, caused by the heat modulation. These may be probed by a weak laser beam, using the resulting deviations in the beam direction [photothermal beam deflection (PBD) or "mirage" method] (61). These meth ods have been applied in photosynthesis infrequently, but they are useful for determining the photothermal contribution to the total PA signal (65).…”

“…The absolute value of the photochemical volume change can be calibrated by comparing its value, obtained at 4°C, to the photothermal signal of an external reference measurement (at a temperature far greater than 4°C), using the values of the thermoelastic properties (thermal expansion coefficient, specific heat, and mass density) and the absorbed flash energy (2, 15). However, even in the presence of the photobaric signal, it is possible to observe the same effect by using other photothermal methods (PTR or PBD) (61,65,75). Thermal deactivation spectra were obtained for Anacystis as an indication of photochemical activity (29), but thermal deactivation spectra were used most often in studies on model and viable systems oriented in polymer films (e.g.…”

“…This may be provided by (a) auxillary photothermal measurements [e.g. PTR (65) or PBD(61)]; (b) PA photothermal measurements of water infiltrated samples (72); or (c) PA photothermal measurements at high modulation frequencies, taking into account that PL varies little in the relevant frequency range (about 10-500 Hz) (61,65,72). A rough zero-order approximation, sufficient for indicative purposes, uses PL = 0 (i.e.…”

Bill Arnold and calorimetric measurements of the quantum requirement of photosynthesis?once again ahead of his time

“…For this, a microphone or piezoelectric device is used and the method is so sensitive that a difference of temperature of the order of a millidegree Celsius can be detected easily. There are other ways to sense temperature changes induced by the modulated light: 'photothermal radiometry' (PTR), by which one detects the modulated part of the infra-red radiation emitted from the sample in a strength related to the amplitude of the temperature changes (Kanstad et al 1983), and 'probe beam deflection' (PBD -also known as the 'mirage' effect method), where refractive index changes are induced and alter the trajectory of a probe laser beam (Havaux et al 1990). These last two techniques have been used only infrequently in photosynthesis.…”

Photosynthetic energy storage in aquatic leaves measured by photothermal deflection

“…A detailed theory of PD spectroscopy has been developed and given experimental justification by Jackson et al (1981). The initial PD studies on photosynthetic tissues investigated the effects of DCMU and thermal stress on energy storage in leaves of terrestrial plants (Havaux et al 1989(Havaux et al , 1990.…”