Quantitation of the O(2)-Dependent, CO(2)-Reversible Component of the Postillumination CO(2) Exchange Transient in Tobacco and Maize Leaves.
The postillumination transient of CO(2) 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% O(2) in an open system were extended using the nonsteady state model described previously (Peterson and Ferrandino 1984 Plant Physiol 76: 976-978). Cumulative CO(2) exchange equivalents (i.e. nanomoles CO(2)) versus time were derived from the analyzer responses of individual transients. In tobacco (C(3)), subtraction of the time course of cumulative CO(2) exchange under photorespiratory conditions (21 or 43% O(2)) from that obtained under nonphotorespiratory conditions (1% O(2)) revealed the presence of an O(2)-dependent and CO(2)-reversible component within the first 60 seconds following darkening. This component was absent in maize (C(4)) and at low external O(2):CO(2) 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 O(2). 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 CO(2) evolution. This method also provides a useful and complementary tool for detecting variation in photorespiration.
Peterson
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Department of Biochemistry and Genetics, The Connecticut Agricultural Experiment Station, P.O. Box 1106, New Haven, Connecticut 06504.