EFFECTS ON PHOTOSYNTHESIS 163 



beet and soybean leaves (Table 1-27) were necessary because the inhibitor 

 solutions were allowed to enter the leaf through the petiole, and it is likely 

 that even in 15 min, the longest period used, the iodoacetate was not com- 

 pletely distributed in the leaves. It should be noted that the figures in 

 Tables 1-27 to 1-29 refer to the per cent of the total C^* taken up and not the 

 actual amount of incorporation into the various fractions; to obtain these 

 latter values, it is necessary to take into account the change in the total 

 incorporation, the data for which are to be found in the last row of each 

 table. For example, it might be concluded that sucrose formation in Chlo- 

 rella elipsoidea is not significantly changed by iodoacetate up to 0.01 m.M 

 (Table 1-29), but yet there is 65% inhibition at 0.003 mikf and 91% at 

 0.01 mM iodoacetate; the figures show that the total incorporation of COg 

 is depressed but of that incorporated the same fraction goes to sucrose. 



Summarizing the major changes to be seen in these tables, we can draw 

 the following general conclusions. 



(A) COo incorporation. This is quite potently inhibited by even very low 

 concentrations of the inhibitors. 



(B) Sucrose or polysaccharide synthesis. Total synthesis is always strong- 

 ly depressed, but the pattern of the initial incorporation is unchanged; 

 however, after longer periods (1 hr in Table 1-28) relatively less of the C^* 

 taken up goes to synthesis. 



(C) Hexose phosphates. Incorporation into this fraction is decreased, 

 whether measured by C^^ or P^- (Table 1-30), although in long experiments 

 they may accumulate somewhat as they do in the dark. 



(D) 3-Phosphoglycerate. The results here are more variable; in general 

 there is relatively more of the C^* or P^^ utilized found in this compound. 

 At the lowest concentrations of iodoacetate there is a tendency for absolute 

 incorporation to rise, indicating some accumulation, but at higher concen- 

 trations this is depressed. There is no evidence for marked accumulation. 



(E) Malate. The fraction of the C^^ appearing in malate increases and 

 the total incorporation is usually unchanged or somewhat depressed. 



(F) Amino acids. Relatively more of the label is found in amino acids, 

 showing some diversion of the pathway toward amino acids, but the total 

 synthesis is well depressed, except at the lowest concentrations of iodoace- 

 tate in Chlorclla. 



If one assumes the reversal of the E^I pathway and the photosynthetic 

 carbon cycle to operate as originally postulated by Calvin, the inhibition 

 by iodoacetate can be attributed to a block of 3-PGDH, depressing both 

 the carbon cycle and the formation of sucrose or polysaccharides. Scheme 3 

 represents the carbon cycle and its relation to the EM pathway in a sim- 



