Effect of Root Temperature on the Infection Processes and Nodulation inLotusandStylosanthes

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Abstract
Infection threads were observed abundantly in the root hairs of Lotus corniculatus L., but very rarely in L. hispidus, Desf., in response to infection by Rhizobium strains 3001 and 3002. Numbers of infections differed between species and strains and were also affected by temperature. In L. corniculatus all the nodules originated from infection threads, but in L. hispidus most nodules appeared to originate by direct bacterial penetration through the epidermis, and infected root hairs were very rarely seen. Both species of Lotus were tolerant to cold temperatures, the minimum temperature for nodulation being 10 °C. The optimum temperature for nodulation of L. corniculatus was 20 °C with 3001 and between 27 and 30 °C with 3002, a few nodules being formed with both strains at 35 °C. L. hispidus formed more nodules than L. corniculatus and the optimum temperature for both the strains was between 25 and 27 °C. No infection threads were seen in root hairs or nodules of Stylosanthes guyanensis (Aubl.) S. W. and S. humilis H.B.K. infected with Rhizobium strain CB1552, and all the nodules were formed in the axils of lateral roots. Optimum temperature for nodulation in S. guyanensis and S. humilis was around 27 °C; nodulation was completely inhibited at 15 °C and very few nodules were formed at 35 °C. Both in Lotus and Stylosanthes the transfer of plants from suboptimal to optimal and supraoptimal temperatures increased nodulation. Delayed inoculation and excision of root tips increased nodulation.

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