Crop and weed competition studies rarely determine how plant-to-plant interactions alter the structure and physiology of crop roots. Soybean has the ability to detect neighboring weeds and to alter growth patterns including the allocation of resources to root growth. In this study, we hypothesized that low red : far red light ratio (R : FR) reflected from aboveground vegetative tissue of neighboring weeds would alter soybean root morphology and reduce root biomass and nodule number. All experiments were conducted under controlled conditions in which resources of light, water, and nutrients were nonlimiting. Low R : FR reflected from aboveground neighboring weeds reduced soybean seedling root length, surface area, and volume, including the number of nodules per plant. An accumulation of H2O2, an increase in malondialdehyde (MDA) content, a reduction in flavonoid content, and a decrease in 1,1-diphenyl-2-picrylhydrazyl (DPPH)–radicle scavenging activity were observed. The reduction in flavonoid content was accompanied by a decrease in the transcription of GmIFS and GmN93 and an increase in transcript levels of several antioxidant genes. These molecular and physiological changes may have a physiological cost to the soybean plant, which may limit the plant's ability to respond to subsequent abiotic and biotic stresses that will occur under field conditions.

Flavonoid content was quantified by high performance liquid chromatography (HPLC) and seed-coat colour was recorded from different legume seeds. Soybean seeds generally contained significantly higher amounts of daidzein (315–354 μg/g), genistein (438–458 μg/g), kaempferol (38–68 μg/g) and total measured flavonoids (892–917 μg/g), while cowpea and peanut seeds contained a significantly higher amount of quercetin (214–280 μg/g and 133–289 μg/g, respectively) than the other legumes tested. Significant variation for flavonoid content existed among and within legume species. Daidzein was significantly correlated with genistein and kaempferol (r = 0.92, P < 0.0001; r = 0.68, P < 0.0001), respectively. Genistein was also significantly correlated with kaempferol (r = 0.84, P < 0.0001). Due to differences in genetic background, no consistent relationship was observed between seed-coat colour and flavonoid content. Variation observed in flavonoid content and seed-coat colour would be useful for legume breeding programmes and consumer use.

Legume flavonoids have received considerable attention due to their beneficial effects on human health. Flavonoid content in the seeds of 36 germplasm accessions of guar [Cyamopsis tetragonoloba (L.) Taub.] was quantified by high-performance liquid chromatography (HPLC). On a per 100 g basis, the seed contained, on average, 1.114 mg (0–2.355) daidzein, 0.700 mg (0–1.685) genistein, 0.553 mg (0–1.250) quercetin and 14.460 mg (10.70–19.82) kaempferol. A significant positive correlation was observed between the content of daidzein and genistein; however, significant negative correlations existed between the content of kaempferol and daidzein and of kaempferol and genistein. Compared to soybean seeds, guar seeds contained very low amounts of daidzein and genistein, but a high amount of kaempferol. The information about the levels of flavonoids in guar seeds will be useful to breeding programmes for improving guar seed quality. Furthermore, the high kaempferol content of guar seeds may expand its nutraceutical and pharmaceutical utilization.