SOYBEAN (GLYCINE-MAX) NODULATION AND N2-FIXATION AS AFFECTED BY EXPOSURE TO A LOW ROOT-ZONE TEMPERATURE

Low root-zone temperatures (RZTs) are known to reduce soybean N2-fixat ion. However, the relative sensitivity of the various stages of symbio sis establishment and function (N2-fixation) to suboptimal RZTs is unr esolved. We conducted experiments to examine the effect of exposure to a RZT of 15-degrees-C on nodulation. The control RZT was 25-degrees-C . Root temperatures were controlled by circulating cooled water around pots on a growth bench. Soybean seedlings [Glycine max (L.) Merr. cv. Maple Arrow] were inoculated with 1 ml of a log-phase culture (approx imately 10(-8) cells) of Bradyrhizobium japonicum strain 532C. They we re then (1) maintained continuously at RZTs of 15 or 25-degrees-C, tra nsferred to 15 or 25-degrees-C from the alternate temperature 7 days a fter inoculation (DAI), or transferred to 15 or 25-degrees-C at 14 DAI , and (2) maintained at 15 or 25-degrees-C, or transferred at either 1 , 4 or 7 DAI. When seedlings were maintained at a RZT of 25-degrees-C nodule primordia (<1 mm) were visible at 7 DAI and N2-fixation commenc ed at 14 DAI. Nodule function (N2-fixation) appeared to be relatively insensitive to low RZTs since exposure of plants to 15-degrees-C follo wing the onset of N2-fixation (14 DAI) resulted in 68% of the N fixed and 78% of the dry weight of the 25-degrees-C RZT, although N partitio ning to shoot tissues was reduced. In contrast, exposure to the low RZ T shortly after inoculation delayed the onset of N2-fixation for 4 to 6 weeks, primarily by inhibiting the early stages of nodulation. This resulted in fixed N and dry weight levels of 9% and 22% of controls, r espectively.