The Influence of Root Temperature, Rhizobium Strain and Host Selection on the Structure and Nitrogen-fixing Efficiency of the Root Nodules of Trifolium subterraneum

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Abstract
Low root temperature greatly affected the structure and N2-fixing efficiency of root nodules. More nodule tissue was formed per plant at 11 and 15 °C than at 7 and 19 °C. Low root temperatures either prevented or slowed bacteroid differentiation; the differentiation zone was 19 per cent of the total nodule tissue at 7 °C but only 5 per cent at 19 °C. The amount of bacteroid tissue formed at the different root temperatures by the two fully effective strains TAi and SU297 reflected the environment from which they originated. Both formed the same amount at 15 and 19 °C but only TAI, which originated from a cold environment formed bacteroids at 7 °C. At 7 °C a bacteroid-filled cell did not degenerate until after 20 days, cf. less than 10 days at 19 °C. At 7 and 11 °C all strains formed more bacteroids in the abundantly nodulating than in the sparse host independently of nodule number. Strain 0403 was most sensitive to both temperature and host; it formed bacteroids in nodules on the sparse host at 19 °C only, but formed bacteroids in the abundant host between 7–19 °C. The amount of bacteroid tissue formed by TAI and SU297 depended directly on nodule number and was approximately constant between 20–40 days only at 19 °C when nodule formation had almost stopped. The optimum temperature for maximum fixation of nitrogen was not necessarily that for maximum efficiency of fixation, which for these experiments was 51 ug N mm-3 bacteroid tissue per day.

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