Glycolate Pathway and Serine Synthesis in Relation to CO2 Concentration in Tomato Leaves

Isotopic trapping of the carbon flowing through the glycolatepathway by exogenous glycolate, glycine and L-serine was investigatedduring ¹⁴CO₂ photosynthesis at different CO₂ concentrationsin tomato leaves. L-Serine markedly trapped the carbon flowingfrom ¹⁴CO₂. The amounts of ¹⁴C incorporated into serine decreasedat a high CO₂ concentration, but increased with an increasein the CO₂ concentration in the presence of exogenous serineduring 10-min photosynthesis in ¹⁴CO₂. When ¹⁴CO₂ was fed for5 to 40 sec at 1300 ppm CO₂ to tomato leaves which had beengiven L-serine, an increase in the accumulation of ¹⁴C-serinebegan after 20 sec, and the ¹⁴C-serine molecules formed at 20and 40 sec were labeled uniformly. In the presence of exogenousserine during 10-min photosynthesis in 1300 ppm CO₂, isonicotinicacid hydrazide increased the incorporation of ¹⁴CO₂ into glycinewith a corresponding decrease in the accumulation of ¹⁴C-serine,but it did not inhibit serine accumulation completely; an evidencefor that some serine was formed by a pathway other than theglycolate pathway. The effect of the CO₂ concentration on theglycolate pathway is discussed in terms of serine synthesisin the presence of exogenous serine. (Received June 1, 1981; Accepted September 30, 1981)