Effect of Photosynthate Availability on N₂- Fixation in Soybean Nodules

Response of photosynthate availability on the nitrogen fixation rate in soybean nodules was studied in situ either by restricting carbohydrate flow to roots or by raising the temperature around root or shoot. For arresting photosynthate supply to nodules, cold water (4°C) girdle was applied around the stem. Raising ambient temperature (27°C around shoot was more than twice effective in enhancing the acetylene reduction rate compared to raising root temperature. Girdling drastically reduced the sugar content of the lower organs and also reduced acetylene reduction and hydrogen evolution significantly. The data show that the current photosynthesis serves as major source of carbohydrate to support nodular N₂-fixation.