Translocation of Photosynthetic Products to Soybean Nodules and Their Role in Nitrogen Fixation"12

Nodules from leguminous plants will continue to fix N2 into acid extractable compounds for several hours after they have been excised from the roots. The capacity of nodules to fix N2 is decreased twoto five-fold by slicing and is abolished by grinding the nodules (2). The decrease in nitrogen fixation which accompanies slicing of the nodules and suspending them in buffer or nutrient solutions has been attributed to the leaching of essential compounds from the slices. Attempts to restore the N2fixing capacity of sliced nodules by the addition of nodule extracts, indoleacetic acid, ascorbic acid, cysteine, glutathionie, citrate, succinate, Versene or vitamins have been unsuiecessful (2, 11). Virtanen et al (20) observed that if pea plants were kept in the dark before their nodutiles were harvested, the capacity of the excised nodules for N2 fixation dropped considerably; upon returning the plants to light, the N2 fixation of their excised nodules increased to a level intermediate between the dark and light conditions. These experiments, and the earlier experiments of Lindstrom et al (10), clearly established the importance of photosynthetic products for N2 fixation. Vernon and Aronoff (19) have employed C14 to study the products translocated in soybeans; they found that sucrose was translocated most rapidly and carried the greatest concentration of C'4. Nelson and Gorham (13, 14) added labeled sugars to soybean plants and followed their movement through the plants; translocation was much more rapid in the (lark than in the light. They also suipplied C1402 and found that products of its fixation were distributed throughout the plant within three hours. Sen and Leopold (17) studied the light and dark fixation of C1402 in Biloxi soybean. They observed a rapid depletion of sugars and a rise in organic acids and amino acids in the leaves in the dark. In the present work, C1402 was furnished to photosynthesizing soybean plants, and the photosynthetic products whicil were translocated to the roots were detected by their radioactivitv. This paper reports the results of these experiments and the effect observed on N2 fixation by nodule slices when rapidly photosynthesized and translocated sugars were added to media in which the slices were bathed.