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Phenotypic Thermal Stability of Rhizobitoxine-induced Chlorosis and the Nodulation Controlling Gene, rj₁¹

The rj₁ gene in soybeans (Glycine max L Merrill), prevents nodulation with a broad range of strains of Rhizobium japonicum. Strains of R. japonicum that produce rhizobitoxine induced foliar chlorosis can nodulate rj₁ plants in sand or vermiculite culture in the greenhouse. This study was undertaken to determine the effects of temperature on the expression of chlorotic symptoms and nodulation of rj₁ plants, and to clarify the relationship between nodulation and chlorosis. Tests with plants cultured in Leonard jars in environmental control chambers maintained at 21, 27, and 32 C indicated that temperature modified the nodulation response of Clark rj₁ soybeans with the chlorosis inducing strains 61 and 76. Chlorotic symptoms were beast evident at 21 C. Strain 61 produced good nodulation on rj₁ plants at 21 C, but exhibited no induction of chlorosis at that temperature on the cultivar ‘Hawkeye.’ Strain 76 produced strong chlorotic symptoms on both the cultivars Hawkeye and ‘Peking’ at 32 C, but only a low level of nodulation with the rj₁ plants. This differential effect of temperature on these two processes suggested that, while rhizobitoxine is correlated with both processes, separate control mechanisms with different temperature optima govern their expression.

Increasing the temperature from 21 to 32 C mitigated the expression of the ineffective nodulation response of the Rj₁ gene with strain 61 in the cultivar Peking. Despite the low level of nodulation (3.6 nodules per plant) of strain 61 with Peking at 32 C, chlorotic symptoms were clearly apparent.

Key Words: Soybeans • Rhizobium japonicum • N fixation • Symbiosis • Glycine max (L.) Merrill.

² Research geneticist and plant physiologist (support scientist), respectively, Cell Culture and Nitrogen Fixation Laboratory, Beltsville, MD 20705.

Received for publication October 9, 1979.