Far-red light perception by the shoot influences root growth and development in cereal-legume crop mixtures

Background and aims: Above- and below-ground signals for and responses to competition in mixed-species stands have mostly been studied separately. This leaves unclear if aboveground signals specific to mixed stands have consequences for how plants respond and compete belowground. This study investigated the effect of an 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. 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, 72 cm in depth) for 52 days of growth. Light-emitting diode (LED) lights were used to enrich FR levels (i.e., reduce the R/FR) of the light environment. Results: Exposure of the shoots to FR-enriched 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 plasticity of adventitious roots; they are highly responsive to the R/FR level to which the shoot is exposed and to whether neighboring plants are of the same or a different species. This highlights the complexity of plants' responses to environmental cues and how they modify interspecific interactions.