IODOACETATE INHIBITION OF PHOTOSYNTHETIC CARBON DIOXIDE ASSIMILATION IN SUGAR BEET AND SOYBEAN LEAVES

Abstract

When the petioles of freshly excised sugar beet or soybean leaves were immersed in dilute iodoacetate solutions, the rates of photosynthetic carbon dioxide assimilation were reduced and characteristic changes in the distribution of assimilated C¹⁴were observed. Inhibited sugar beet leaves accumulated C¹⁴in phosphoglyceric acid (PGA) and aspartic acid, but did not incorporate C¹⁴into glyceric acid. Inhibited soybean leaves continued to form glyceric acid, but did not incorporate C¹⁴into PGA and only trace amounts into aspartic acid. Incorporation of C¹⁴into sucrose by the sugar beet leaf ceased when the assimilation of carbon dioxide was reduced below 1/10th of normal, while soybean leaves still formed sucrose at that level of inhibition. It was concluded from these and other data that the mechanisms for photosynthetic carbon dioxide assimilation in the two species were different. The iodoacetate-sensitive, PGA-glyceraldehyde-3-phosphate reduction, which could account for only part of the assimilation by sugar beet leaves, did not appear to be utilized by soybean leaves. An alternate pathway for the reduction of carbon dioxide to the carbohydrate level is postulated to account for the assimilation patterns observed in the two species.