Effect of host plant selection and temperature on the structure of root nodules of red clover (Trifolium pratense L.)

Summary

• 1. Nodule structure was examined in the red clover cultivar S123 and in lines (designated H²) separately bred for high yield withRhizobium trifolii strains RCR 0403 and RCR 5. Plants were grown at 16/11, 22/17 and 27/22°C in a 16 h day at 25,000 lux.

• 2. The larger yields of bred lines compared with S123 were directly correlated with larger aggregate nodule size, larger infected zones and with more bacteroid tissue. Over all treatments the correlation of plant dry weight with aggregate areas of bacteroid tissue, assessed in median longitudinal section, accounted for 79% of the variance.

• 3. Yields were unrelated to the proportion of uninfected cells and vacuoles within the bacteroid zone.

• 4. Effects of host type and strain on nodule structure and yield were greatest at the moderate temperature and least and most irregular at high temperature.

• 5. Bacteriod tissue degenerated more rapidly in S123 than in the bred lines, more rapidly with RCR 0403 than RCR 5 and more rapidly at the high temperature than at moderate or low temperatures.

• 6. Aggregate nodule size irrespective of treatment was correlated with the aggregate sizes of nodule meristem and differentiating tissues. Nodule comtex comprised a larger proportion of the nodule section at 17 days than subsequently. Other than as specified above, the size relationships of the different tissues of the nodule were unaffected by host selection, temperature, bacterial strain or plant age.

• 7. At 22/17°C over a 17–25 day period the increment in dry matter per mm² active nodule tissue was similar for H² and S123 plants and was less for RCR 0403 (3.33 mg) than for RCR 5 (6.87 mg). At 27/22°C the effeciency of bacteroid tissue was much less,viz. 0.67 and 1.23 mg per mm² respectively.

• 8. Nodule tissue areas and volumes were closely related so that unit volumes of active bacteriod tissue in general promoted similar dry matter increment in S123 and bred lines,viz. about 50.5 mg mm⁻³ at 22/17° for RCR 0403 and 80.8 mg mm⁻³ for RCR 5. The latter was associated with a fixation rate of 0.157 mg N mm⁻³ active tissue day⁻¹.