656 



N. E. Tolbert 



I 



I 



COp^ concentration : Optimum glycolate accumulation and excre- 

 tion during photosynthesis occurs at low CO2 partial pressures of 

 around 0.17o (5, 26, 28, 29). Below 0.1% CO2 the photosynthetic 

 rate is limited by CO2 availability. Thus the total amount of 

 glycolate synthesis is reduced, but the % of the fixed C^"* in gly- 

 colate products is not reduced (Table 3) (W. Snyder and N. E. 

 Tolbert, unpublished). It seems significant that glycolate se- 

 cretion in large amounts must occur during photosynthesis in 

 nature where the CO2 partial pressure is so limiting. Above 

 about 0.4% CO2 the amount of glycolate which accumulates is re- 

 duced to much lower values (Table 3) (28, 29). At higher CO2 



Table 3. Effect of CO2 concentration on glycolate 

 products in beet leaves. 



Glycine plus serine ^^ 

 % C^4 c/s X 10-3 



25 8 



25 26 



5 6 



3 2 



* Significant amounts of glycolate did not accumulate. 



concentrations much more sucrose is made, but some newly fixed C^^ 

 still appears in glycine and serine. Such experiments suggest 

 that the glycolate pathway was inhibited by higher CO2 concentra- 

 tion. Since glycolate-C^"* is rapidly incorporated into sucrose 

 (1, 7), it is possible that the reservoirs of the glycolate pro- 

 ducts may simply be turning over more rapidly at higher CO2 con- 

 centrations thereby decreasing rather than increasing in size. . 



i 



A theory for the increased percentage of C"'-"* in glycolate ■ 



during photosynthesis at low CO2 pressure has been presented by 

 Calvin's group (37). In changing from high to low CO2 concentra- 

 tion the insufficiency of CO2 resulted in accumulation of RuDP 

 which could be used as a source of glycolate. Although the chem- 

 istry of glycolate synthesis may indeed be consistent with this 

 situation, the hypothesis cannot explain glycolate synthesis in 

 steady state photosynthesis at low CO2 partial pressure nor is it 

 likely to be the determining factor for glycolate synthesis. 



Oxygen concentration ; The requirement of a high (207o or above) 

 oxygen partial pressure during photosynthesis for glycolate pro- 

 duction has been well established (5, 26, 28, 38), but not explained. 

 In 80 or 100% O2 , though photosynthesis is inhibited, the % 0^"^ 

 incorporated into glycolate products is not reduced (N. E. Tolbert, 



