Changes in light quality alter physiological responses of soybean to thiamethoxam

The interaction between neighboring weed-induced far-red enriched light and thiamethoxam can significantly alter soybean seedling morphology, nodulation, isoflavone levels, UV-absorbing phenolics, and carbon and nitrogen content. Neonicotinoid insecticides that are widely used on major crop plants can enhance plant growth and yield. Although the underlying mechanism of this enhanced growth and yield is not clear, recent studies suggest that neonicotinoids such as thiamethoxam (TMX) may exert their effects at least in part via signals that involve salicylic acid (SA) and jasmonic acid (JA). In the current research, effects of TMX on morphological and physiological responses of soybean have been compared under far-red-depleted (FR-D) and far-red-enriched (FR-E) light reflected by neighboring weeds. TMX significantly enhanced shoot and root growth but did not prevent stem elongation under FR-E light. Also, TMX did not prevent reductions in shoot carbon content and shoot carbon to nitrogen ratio under FR-E light. Despite similarities between these TMX effects in soybean and those known for SA and JA in other plant species, TMX significantly enhanced root-nodule numbers per plant and levels of root isoflavones malonyl-daidzin and malonyl-genistin under FR-E light only. These results suggest that the combined effect of FR-E light and TMX triggers a mechanism that operates concomitantly to enhance root isoflavones and nodulation in soybean.