Isotopic trapping of the carbon flowing through the glycolate pathway by exogenous glycolate, glycine andL-serine was investigated during14CO2photosynthesis at different CO2concentrations in tomato leaves.L-Serine markedly trapped the carbon flowing from14CO2. The amounts of14C incorporated into serine decreased at a high CO2concentration, but increased with an increase in the CO2concentration in the presence of exogenous serine during 10-min photosynthesis in14CO2. When14CO2was fed for 5 to 40 sec at 1300 ppm CO2to tomato leaves which had been givenL-serine, an increase in the accumulation of14C-serine began after 20 sec, and the14C-serine molecules formed at 20 and 40 sec were labeled uniformly. In the presence of exogenous serine during 10-min photosynthesis in 1300 ppm CO2, isonicotinic acid hydrazide increased the incorporation of14CO2into glycine with a corresponding decrease in the accumulation of14C-serine, but it did not inhibit serine accumulation completely; an evidence for that some serine was formed by a pathway other than the glycolate pathway. The effect of the CO2concentration on the glycolate pathway is discussed in terms of serine synthesis in the presence of exogenous serine.
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