Physical Environment and Symbiotic Nitrogen Fixation. V. Effect of Time of Exposure to Unfavourable Root Temperatures

Symbiotic nitrogen fixation by Trifolium subterraneum L., inoculated with three strains of Rhizobium trifolii Dang., was examined over the range of root temperatures 8-28°0. The plants were transferred from an optimal temperature for nitrogen fixation (23°0) to other temperatures at three stages, namely (1) immediately after inoculation, 3 days after germination, (2) 14 days after germ· ination, when nitrogen fixation had commenced, and (3) 21 days after germination, when the plants had been fixing nitrogen for at least 7 days. Nitrogen increase and dry weight accumulation were determined for two growth periods-days 14-21 (I) and days 21-28 (II)