Oxygen uptake by tobacco leaves after heat shock

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Abstract
Small discs punched out from leaves of tobacco (Nicotiana tabacum L.) were exposed for 3 min in the dark to a high temperature of around 48°C. This caused a progressive inhibition of the gas‐exchange (oxygen evolution) type photoacoustic signal, resulting‐ finally in phase inversion (i.e. leading to negative values), which indicates that oxygen uptake replaces the normal oxygen evolution. This effect was also observed in various other plant species. Oxygen uptake was rapidly reversed (within ca. 4–5 min) to a certain low value of oxygen evolution (about 20% of the control) by continuous illumination with relatively strong white light (minimum 55 W m−2). However, a few minutes in darkness following this heat treatment induced reappearence of the uptake signal. This photoacoustically detected oxygen uptake after heat shock may be interpreted as reflecting stimulated oxygen photoreduction (Mehler reaction) caused by (light dependent) inactivation of the Calvin cycle by heat, suggesting that oxygen may act as a major photosynthetic electron acceptor under stress conditions. Leaves suffering from such heat shock effects were completely restored to normal behaviour after a 24‐h incubation at room temperature (25 °C). Analysis of the modulation frequency and wavelength dependence of the photoacoustic signals showed that heat shock‐induced oxygen uptake is a very complex phenomenon, composed of at least two components differing in kinetics and sensitivity to DCMU.