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

Abstract

A method was devised to quantify short-term photorespiratory rates in terrestrial plants using 180-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 180-enrichment. Rates of glycolate synthesis were determined from '10-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 02 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% 02 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 C02/02 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 02 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 altemate 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% 10-labeled under an atmosphere of 40% 02. This pool failed to become 100% 180-enriched under lower 02 concentrations.

Utility of basic research in plant/crop physiology in relation to crop improvement: a review and a personal account

“…Most notably, by pioneering: researchers: (1) at Cornell University in field-grown maize where high leaf gas exchange rates were reported for the first time (Musgrave and Moss, 1961;Hesketh and Musgrave, 1962;Baker and Musgrave, 1964;Moss and Musgrave, 1971); (2) at the Hawaiian Sugarcane Plantations where the initial carbon fixation by sugarcane leaves was detected in C 4 -dicarboxylic acids (Kortschak et al, 1965); (3) at the University of Arizona on several tropical grasses such as maize, grain sorghum and bermudagrass, and on the dicot pigweed (A. palmeri) (ElSharkawy and Hesketh, , 1986; (4) at the University of California, Davis, where two more dicot amaranth species, a weedy amaranth (A. retroflexus) and the cultivated, grain amaranth (A. edulis) (syn. A. caudatus edulis ), were discovered to possess C 4 photosynthetic characteristics and have the capacity to reassimilate/ recycle respiratory CO 2 in CO 2 -free air and light (ElSharkawy et al, 1967(ElSharkawy et al, , 1968; (5) in Canada on detached maize leaves where photosynthesis and photorespiration were reported to be insensitive to oxygen level (Forrester et al, 1966); (6) in Australia on sugarcane leaves where the earlier work at Hawaii was confirmed Slack, 1966, 1970); and (7) at the University of North Carolina where maize leaves were reported to absorb oxygen under light proving the existence of photorespiration in C 4 species, but at reduced rates compared to C 3 species Volk, 1969, 1970;Raven, 1972;de Veau and Burris, 1989).…”

Tansley Review No. 112

“…Photorespiration is attenuated by increasing the availability of CO # relative to that of O # , as can be observed in the laboratory and as occurs naturally in many algae and in the bundle sheath cells of many C % leaves. If one can reasonably talk of typical C : O ratios, then these might be 2-3 in well-watered C $ leaves at 20-35mC (Sharkey, 1988 ;De Veau & Burris, 1989 ;Peterson, 1989). Such ratios mean that, for each carbon atom fixed at Rubisco, 0n67-1 atoms pass through the glycollate pool.…”

“…There is controversy over whether catalase has an important role in the hypersensitive response and systemic acquired resistance that can follow pathogen attack (Chen et al, 1993 ;Rao et al, 1997 ;Dat et al, 1998). In maize seedlings, which have higher photorespiratory rates than mature plants ( De Veau & Burris, 1989), enhanced catalase transcripts, protein and activities were associated with decreased chilling-induced oxidative damage (Prasad, 1996). However, this effect was due to induction of the maize-specific mitochondrial isoform : markedly decreased peroxisomal catalase activities did not increase sensitivity to chilling in tobacco (Willekens et al, 1997).…”

Glycolate Metabolism in Low and High CO2-Grown Chlorella pyrenoidosa and Pavlova lutheri as Determined by 18O-Labeling

“…Instead, they have investigated primarily the metabolic flow of carbon during glycolate metabolism (33). The '80-isotope has the added advantage in that nearly 100% of the glycolate pool can becomes 180_ labeled (5,11,12), whereas, in those studies using 14C only a fraction ofthe glycolate pool becomes labeled. This is possibly due to the participation of unlabeled storage carbohydrates in the synthesis of ribulose bisphosphate (30).…”

“…piratory intermediates (glycolate, glycine, and serine) followed a modified procedure of that of Berry et al (5) and has been previously published (11,12). The final yield of glycolate, glycine, and serine after silylation was 54 + 5%, 49 + 4%, and 67 ± 4%, respectively.…”