Effect of High Root Temperature and Rhizobium Strain on Nodulation, Nitrogen Fixation, and Growth of Soybeans

The effects of high root temperatures on nodulation, nitrogen (N) fixation, and dry matter production of ‘Lee’ soybean [Glycine max (L.) Merr.] plants inoculated with 13 different strains of Rhizobium japonicum were studied in the greenhouse. Plants were grown in vermiculite, and their root systems were submerged in thermostatically controlled water baths. A treatment without inoculation in which 12 mM of KNO₃ was supplied with the nutrient solution was included. Increasing root temperatures from 28 to 40°C had a detrimental effect on the number of nodules, the specific nitrogenase activity, the N content, and the dry weight of tops and roots of inoculated plants. The magnitude of this effect was highly dependent on the rhizobial strain. For some strains, higher nodule weights and higher total nitrogenase (C₂H₂ reduction) activity were found at 33 than at 28°C, whereas for other strains, these variables declined with increasing temperatures. It appeared that, with increasing temperatures, the processes determining nodule initiation became susceptible at lower temperatures than those associated with nodule growth. When plants formed nodules under nonlimiting temperatures and then were transferred to stressing root temperatures, appearance of additional nodules, N fixation, and plant growth were restricted by high root temperature. These results indicate that the detrimental effect of increasing root temperatures on inoculated plants is due in part to the reduction in number of nodules formed and also to limitations imposed on other processes of the soybean-Rhizobium association. The growth of plants supplied with combined N was also adversely affected by continuous high root temperatures. However, while a temperature of 34°C restricted the growth of plants depending on N fixation, it was stimulatory for the growth of plants supplied with combined N. Therefore, the effect of temperatures near 34°C on the growth of nodulated plants without combined N was due mainly to the effect on the symbiotic system and not on other physiological processes of the plant. The response of different Rhizobium strains in symbiosis with soybean plants to increasing temperature was related to their response to temperature in pure culture and modified the adverse effect of temperature on plant development. Therefore, pure culture screening of R. japonicum may be a useful tool to aid in the selection of strains better suited for soil environments where high temperature is a limiting factor for the symbiosis.