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Effects of High Temperatures and Starter Nitrogen on the Growth and Nodulation of Soybean¹

Until recently it was believed that soil temperatures in excess of 30°C are likely to inhibit the establishment of root nodules on soybean [Glycine max (L.) Merr.]; however, using diurnally cycling thermoperiods rather than constant-temperature techniques, we previously found that a root temperature of 45°C is required to suppress soybean nodulation. Considering that the level of soil N also affects growth and nodulation, in this work we examine the effects of daily maxima of 30 to 45°C in combination with different levels of N (KNO₃) applied at planting, using sand-cultured ‘Wilkin’. Plant growth, on combined N or symbioticaily fixed N, was severely inhibited by temperature maxima >41°C, but was improved by 36°C maximum for 9 h daily. With 21-day-old plants, maxima >6° adversely affected nodulation only in the presence of combined N, and after depletion of N excellent nodulation was obtained. Growth and nodulation at 35 days were less adversely affected by temperature stress when a small (e.g., 90 mg N plant^-1) application of N had been made at planting than under N-free conditions. The results show that inhibition of nodulation of soybean by high temperature is exacerbated by: (i) the presence of available N, and (ii) the complete absence of applied N.

Key Words: Glycine max (L.) Merr. • Bradyrhizobium japonicum • Dinitrogen fixation • Temperature stress

² Research associate and associate plant physiologist, respectively, Boyce Thompson Inst. for Plant Res. at Cornell Univ., Ithaca, NY 14853.

Received for publication August 18, 1986.

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