Spectral quality during pod development modulates soybean seed fatty acid desaturation

High-density cropping of soybeans results in considerable mutual shading. Consequently, pods mature under a range of light conditions, with those lower in the canopy exposed to drastically altered spectral quality as well as lower irradiance. The influence of spectral quality on reproductive development and seed quality was investigated in soybeans raised to physiological maturity under either broad spectrum or blue-deficient light sources. The absence of blue light had a large influence on vegetative morphology, but the timing of reproductive events was not affected. Total seed yield per plant, dry matter per seed, per cent protein and per cent oil were similar for all treatments. However, seeds harvested from plants matured under broad spectrum illumination contained high levels of oleic acid (18:1) and low linoleic acid (18:2) compared to seeds from plants grown under blue-deficient conditions. In addition to the spectral quality effect, there was a smaller effect of pod position. Seeds from pods lower in the canopy contained less 18:1 and more 18:2 than seeds that matured closer to the top of the canopy. Considering both spectral quality and pod position, the ratio of 18:1 to 18:2 varied four-fold between 0·35 and 1·43, indicative of a possible photoregulatory step in fatty acid desaturation. The spectral effects are consistent with the participation of a photomorphogenetic photoreceptor in the control of fatty acid metabolism during seed maturation and triglyceride accumulation.