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

Abstract

Symbiotic N fixation by Trifolium subterraneum L., inoculated with 3 strains of Rhizobium trifolii Dang., was examined over the range of root temperatures 8-28[degree]C. The plants were transferred from an optimal temperature for nitrogen fixation (23[degree]C) to other temperatures at three stages, namely immediately after inoculation, 3 days after germination; 14 days after germination, when N fixation had commenced; and 21 days after germination, when the plants had been fixing N for at least 7 days. N increase and dry weight accumulation were determined for 2 growth periods[long dash]days 14-21 (I) and days 21-28 (II). The total amount of N fixed during any period was determined by 4 factors[long dash]the root temperature during the period, the amount of N previously fixed, the N percentage of the plants at the start of the period, and the bacterial strain that formed the nodules. The plants forming nodules at 8[degree]C fixed little or no N during the course of the experiment, while those at 13[degree]C did not fix any N until day 21. The longer the period at 23[degree]C before transfer to a lower or higher temperature, the greater the amount of N in the plants at the start of any growth period, and the greater the amount of N fixed during that period. However, the root temperature controlled the relative N assimilation rate (RN), and for plants transferred to 8-18[degree]C on day 21, the additional N fixation during period I did not enable them to achieve a higher RN than that of the plants growing at these temperatures from day 14. At all lower temperatures, differences in the rate of nodule establishments, as indicated by the amount of N fixed during the early stages, were found between strains, although the time to first visible nodule and rate of nodule appearance were the same. At 28[degree]C, the RN of plants transferred on day 21 was similar to that of the day 14 plants during period I. For all strains, the Rn of both day 3 and day 14 plants at 28[degree]C declined markedly from period I to period n, and for strain NA30, and a lesser extent strain CC17, the plants appeared to be dying despite the relatively high RN during period I. Under some conditions it was observed that exceptionally high RN values were achieved by both nodulated and N control plants, and this was accompanied by low relative growth rates (Rw). As the percentage N level in the plants increased, the Rn declined, and there was an increase in Rw, such that the ratio RN/RW approximated to unity. A form of regulatory mechanism ensuring that the plant is achieving the most efficient use of resources is proposed.