THE EFFECTS OF LOW ROOT-ZONE TEMPERATURE STRESS ON 2 SOYBEAN (GLYCINE-MAX) GENOTYPES WHEN COMBINED WITH BRADYRHIZOBIUM STRAINS OF VARYING GEOGRAPHIC ORIGIN

In areas with short growing seasons, poor early vegetative growth of s oybean (Glycine max [L.] Merr.) is often attributed to the restrictive effect of cool soil conditions on nodulation and N2-fixation by this subtropical grain legume. However, there are few studies regarding pot ential genetic variability of soybean and Bradyrhizobium japonicum gen otypes for nodulation at cool root-zone temperatures (RZT). Experiment s were conducted to (1) test for a threshold temperature for low RZT i nhibition of soybean nodulation and (2) ascertain whether this thresho ld temperature response depends mainly on the micro- or macrosymbiont. In experiment 1 soybean seedlings (Glycine max [L.] Merr. cv. Maple A rrow) were inoculated with 1 ml of a log phase culture of B. japonicum strain 532C, H8 or H15 (the latter two strains were isolated from col d soils of Hokkaido, northern Japan) and maintained at either 16, 17.5 , 19 or 25-degrees-C RZT. In experiment 2 seedlings of cv. Maple Arrow and a cold-tolerant Evans isoline were combined with strain 532C and two Hokkaido strains (H5, H30) at both 19 and 25-degrees-C RZT. Result s indicated that N2-fixation at 44 days after inoculation was substant ially reduced (30-40%) by RZT as high as 19-degrees-C, due to developm ent of less nodule mass and to a delay in the onset of N2-fixation and a small decrease in the number of nodules formed. However, the number of nodules formed was sharply reduced and the time required for the f irst appearance of nodules was significantly delayed below an RZT of 1 7.5-degrees-C. Differences between cultivars for nodulation and N accu mulation were apparent at 25-degrees-C, but were abolished by growth a t 19-degrees-C, indicating that, in spite of differences in growth pot ential between the cultivars under optimum RZT. both cultivars were eq ually limited by low RZT. Differences between B. japonicum strains wer e consistent across temperatures and were largely attributable to high er rates of specific nodule activity recorded for strain 532C, which s eemed well adapted to low RZT. These results suggest that the host pla nt mediates the sensitivity of N2-fixation under low RZT and that inoc ulation with B. japonicum strains from cold environments is unlikely t o enhance soybean N2-fixation under cool soil conditions.