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Soybean (Glycine max) modulation and N2-fixation as affected by exposure to a low root-zone temperature
Authors:D H Lynch  D L Smith
Institution:Plant Science Dept. McGill Univ., Macdonald Campus, 21111 Lakeshore Road, Ste. Anne de Bellevue, Quebec, Canada H9X 1C0.
Abstract:Low root-zone temperatures (RZTs) are known to reduce soybean N2-fixation. However, the relative sensitivity of the various stages of symbiosis establishment and function (N2-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 (<1 mm) were visible at 7 DAI and N2-fixation commenced at 14 DAI. Nodule function (N2-fixation) appeared to be relatively insensitive to low RZTs since exposure of plants to 15°C following the onset of N2-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 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, respectively.
Keywords:Glycine max            low root-zone temperature  N2-fixation  nodulation  soybean
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