• Glycine max;
  • phytochrome;
  • plant adaptation;
  • plant bioregulation

The influence of row orientation on spectral distribution of light received by growing soybean [Glycine max (L.) Merr. (cv. Coker 338)] plants was measured under field conditions, and light spectrum effects on photosynthate partitioning were studied under controlled environments. Light received by leaves under field conditions differed among those grown in north-south vs east-west oriented rows. In morning and late afternoon, the far-red/ red ratio received by leaves at the surface of the canopy differed about 3-fold from the east to west sides of north-south rows, but only 1.3-fold from the south to north sides of east-west rows.

In controlled environments, brief exposures to red or far-red light at the end of the photosynthetic period influenced partitioning of photosynthate among leaves, stems and roots. The top/root ratios differed significantly between the red and far-red treated plants. Red treated plants partitioned less photosynthate to stems and more to roots than did those treated with far-red. Also, plants with larger root systems developed more nodules. Phytochrome effects on photosynthate partitioning between tops and roots may influence yield of soybean plants grown in soils with low water-holding capacity.