Photorespiratory Rates in Wheat and Maize as Determined by 18O-Labeling

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Abstract
A method was devised to quantify short-term photorespiratory rates in terrestrial plants using 18O-intermediates of the glycolate pathway, specifically glycolate, glycine, and serine. The pathway intermediates were isolated and analyzed on a GC/MS to determine molecular percent 18O-enrichment. Rates of glycolate synthesis were determined from 18O-labeling kinetics of the intermediates, derived rate equations, and nonlinear regression techniques. Glycolate synthesis in wheat (Triticum aestivum L.), a C3 plant, and maize (Zea mays L.), a C4 plant, was stimulated by high O2 concentrations and inhibited by high CO2 concentrations. The synthesis rates were 7.3, 2.1, and 0.7 micromoles per square decimeter per minute under a 21% O2 and 0.035% CO2 atmosphere for leaf tissue of wheat, maize seedlings, and 3-month-old maize, respectively. Photorespiratory CO2 evolution rates were estimated to be 27, 6, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue support the hypothesis that C4 plants photorespire, albeit at a reduced rate in comparison to C3 plants, and that the CO2/O2 ratio in the bundle sheath of maize is higher in mature tissue than in seedling tissue. The pool size of the three photorespiratory intermediates remained constant and were unaffected by changes in either CO2 or O2 concentrations throughout the 10-minute labeling period. This suggests that photorespiratory metabolism is regulated by other mechanism besides phosphoglycolate synthesis by ribulose-1,5-bisphosphate carboxylase/oxygenase, at least under short-term conditions. Other mechanisms could be alternate modes of synthesis of the intermediates, regulation of some of the enzymes of the photorespiratory pathway, or regulation of carbon flow between organelles involved in photorespiration. The glycolate pool became nearly 100% 18O-labeled under an atmosphere of 40% O2. This pool failed to become 100% 18O-enriched under lower O2 concentrations.