Soybean (<i>Glycine max</i>) modulation and N₂‐fixation as affected by exposure to a low root‐zone temperature

Abstract

Low root‐zone temperatures (RZTs) are known to reduce soybean N 2 ‐fixation. However, the relative sensitivity of the various stages of symbiosis establishment and function (N 2 ‐fixation) to suboptimal RZTs is unresolved. We conducted experiments to examine the effect of exposure to a RZT of 15°C on nodulation. The control RZT was 25°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 (approximately 10 −8 cells) of Bradyhizobium japonicum strain 532C. They were then (1) maintained continuously at RZTs of 15 or 25°C, transferred to 15 or 25°C from the alternate temperature 7 days after inoculation (DAI), or transferred to 15 or 25°C at 14 DAI, and (2) maintained at 15 or 25°C, or transferred at either 1, 4 or 7 DAI. When seedlings were maintained at a RZT of 25°C nodule primordia (&lt;1 mm) were visible at 7 DAI and N 2 ‐fixation commenced at 14 DAI. Nodule function (N 2 ‐fixation) appeared to be relatively insensitive to low RZTs since exposure of plants to 15°C following the onset of N 2 ‐fixation (14 DAI) resulted in 68% of the N fixed and 78% of the dry weight of the 25°C RZT, although N partitioning to shoot tissues was reduced. In contrast, exposure to the low RZT shortly after inoculation declayed the onset of N 2 ‐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, respectively.