Quantitation of the O2-Dependent, CO2-Reversible Component of the Postillumination CO2 Exchange Transient in Tobacco and Maize Leaves

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Abstract
The postillumination transient of CO2 exchange and its relation to photorespiration has been examined in leaf discs from tobacco (Nicotiana tabacum) and maize (Zea mays). Studies of the transients observed by infrared gas analysis at 1, 21, and 43% O2 in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO2 exchange equivalents (i.e. nanomoles CO2) versus time were derived from the analyzer responses of individual transients. In tobacco (C3), subtraction of the time course of cumulative CO2 exchange under photorespirator conditions (21 or 43% O2) from that obtained under nonphotorespiratory conditions (1% O2) revealed the presence of an O2-dependent and CO2-reversible component within the first 60 seconds following darkening. This component was absent in maize (C4) and at low external O2:CO2 ratios (i.e. < 100) in tobacco. The size of the component in tobacco increased with net photosynthesis as irradiance was increased and was positively associated with inhibition of net photosynthesis by O2. This relatively simple and rapid method of analysis of the transient is introduced to eliminate some uncertainties associated with estimation of photorespiration based on the maximal rate of postillumination CO2 evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.