The use of oxygen-18 isotopic labelling to assay photorespiration in terrestrial plants and algae

A new method was devised to quantify photorespiration. The assay utilized {dollar}sp{lcub}18{rcub}{dollar}O{dollar}sb2{dollar} to isotopically label intermediates of the glycolate pathway, specifically glycolate, glycine, and serine, for various time periods. The pathway intermediates were isolated and analyzed on a mass spectrometer to determine molecular percent {dollar}sp{lcub}18{rcub}{dollar}O-enrichment. Rates of glycolate synthesis were determined from: {dollar}sp{lcub}18{rcub}{dollar}O-labeling kinetics of the intermediates, derived rate equations, and non-linear regression techniques. The method was adapted to measure photorespiratory rates in both terrestrial plants and algae.; Glycolate synthesis rates in Triticum aestivum L. (var. Hart), a representative C{dollar}sb3{dollar} plant, and Zea mays L. (hybrid PA8810), a representative C{dollar}sb4{dollar} plant, were stimulated by high O{dollar}sb2{dollar} concentrations and inhibited by high CO{dollar}sb2{dollar} concentrations. The synthesis rates were greatest in wheat leaf tissue followed by maize seedlings and 3-month-old maize leaf tissues. The rates were 7.3, 2.1, and 0.7 {dollar}mu{dollar}moles/dm{dollar}sp2{dollar}/min under a 21% O{dollar}sb2{dollar} and 0.035% CO{dollar}sb2{dollar} atmosphere for wheat, maize seedlings and 3-month-old maize, respectively. Photorespiratory CO{dollar}sb2{dollar} evolutionary rates were 27%, 6%, and 2%, respectively, of net photosynthesis for the three groups of plants under the above atmosphere. The results from maize tissue supports the hypothesis that C{dollar}sb4{dollar} plants photorespire albeit at a reduced rate in comparison to C{dollar}sb3{dollar} plants and also indicate that the CO{dollar}sb2{dollar} pumping characteristic of C{dollar}sb4{dollar} metabolism is more active in 3-month-old tissue than in seedling tissue.; Photorespiratory {dollar}sp{lcub}18{rcub}{dollar}O-labeling kinetics were determined for the algae Pavlova lutheri (Droop.) Green and Chlorella pyrenoidosa Chick. Pavlova excreted newly synthesized glycolate into the surrounding medium. The {dollar}sp{lcub}18{rcub}{dollar}O-label did not appear within the glycine or serine pool of this alga. The glycine and serine pools of Chlorella became {dollar}sp{lcub}18{rcub}{dollar}O-labeled even though under certain circumstances a portion of the newly synthesized glycolate was excreted. Glycolate synthesis rates in Chlorella were stimulated by high O{dollar}sb2{dollar} concentrations, inhibited by high CO{dollar}sb2{dollar} concentrations, and 15 to 20 times higher in 1.5 CO{dollar}sb2{dollar}-grown cells than air-grown cells when both were assayed under low CO{dollar}sb2{dollar} concentration. Indirect evidence indicated that glycolate is excreted as a lactone in Chlorella.