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Nodule Development in a Split-Root System in Reponse to Red and Far-Red Light Treatment of Soybean Shoots¹

A split-root (sides A and B) study with soybean [Glycine max (L.) Merr. (‘Lee’)l and USDA strain 3I1b110 Bradyrhizobium japonicum was conducted to assess the effect of red and far-red light treatments of shoots on the timing of B. japonicum-induced suppression of root nodulation. Plants were grown at 25°C in a growth-chamber with 12-h daily photosynthetic periods that ended with 5-min exposures to red or far-red light. Day 0 for inoculation was 7 days after planting at which time roots had extended into both sides of the split-root system. When side B was inoculated 2 to 4 days or longer after side A, nodulation and nodule mass on side B were significantly less than on side A. The number of nodules formed on roots of either side A or B were lower when shoots were treated with far-red than with red light. Therefore, total suppression of nodulation on side B occurred more rapidly with far-red than red light treatment. When far-red treatment was followed immediately by red treatment, plants responded similarly to the red treatment for nodule weight as well as nodule number. These data show that the timing of suppression of secondary nodulation on the roots can be affected by spectral balance of light received by the shoots, and the reversible nature of the effects of far-red light with red light strongly indicates phytochrome involvement. This interaction could have significant effects on nodulation patterns of soybean grown in different populations, row widths, and row orientations because these treatments alter the far-red/red ratio of light received in soybean canopies. It may also be important in the interpretation of greenhouse and growth-chamber studies of phenomena such as soybean nodulation and B. japonicum strain preference by different soybean cultivars.

Key Words: Glycine (L.) Merr. • Bradyrhizobium japonicum • Nodulation suppression • Canopy light • Spectral composition • Phytochrome

² Soil scientist, research plant physiologist, and oil scientist, respectively, USDA-ARS, Coastal Plains Soil and Water Conservation Research Center, Florence, SC 29502-3039.

Received for publication October 20, 1986.