Root responses to canopy far-red light in cereal-legume crop mixtures

Abstract Background and aims: Above- and belowground signals for and responses to competition in mixed-species stands have been studied mostly separately, leaving it unclear if aboveground signals specific to mixed stands have consequences for how plants respond and compete belowground. This study investigated the effect of an important aboveground signal, the ratio of red to far-red light (R/FR), on plant biomass allocation and root architecture and morphology when plants grow with different species in the vegetative stage. Methods: A greenhouse experiment with a mixture of wheat (Triticum aestivum) and faba bean (Vicia faba) and their respective mono stands was carried out in deep pots (22.6 L, 72cm in depth). LED lights were used to enrich FR levels (i.e., reduce the R/FR) of the light environment. After 52 days of growth, biomass allocation and root traits were measured. Results: Exposure to FR light predominantly affected adventitious roots, with species-specific effects. Especially in species mixtures, increased FR caused wheat to produce more adventitious roots per tiller, while adventitious root mass of faba bean decreased. The influence of FR on rooting depth (D75, i.e. depth of 75% of root biomass) was species specific, with higher FR levels causing a reduction in D75 in wheat, but a greater D75 in faba bean. Conclusions: Our findings demonstrate the remarkable plasticity of adventitious roots, showing them to be highly responsive to low R/FR signals and influenced by the neighboring plant's identity. This highlights the complexity of plants responses to environmental cues and how they modify interspecific interactions.