Regulatory Effect of Light and Rhizobial Inoculation on the Root Architecture and Plant Performance of Pasture Legumes

Abstract

Rhizobial associations with leguminous plants are some of the most important symbioses on Earth, and they have economic relevance in agriculture. Because their interactions are positive and have advantages for both partners, nitrogen-fixing rhizobia also demand significant carbohydrate allocation in exchange for key nutrients, and this demand is reflected in the anatomy of roots. In the current scenario of climate change, rhizobia–legume interactions can be affected, and plants may need to compensate for carbon loss when light availability is not correct. Under such conditions, roots can modify their anatomy to accommodate symbionts’ needs, and the outcome of an interaction can switch from mutualism to parasitism, resulting in changes in root allocation. We experimented with two legume species originating from well-irradiated environments (Coronilla juncea L. and Ornithopus compressus L.) and two species from shaded environments (Trifolium repens L. and Vicia sativa L.). We applied high radiation, intermediate radiation, and low radiation to two treatments of microbial inoculation (inoculation and control). After an incubation period of 105 days, we quantified the root area, size, and complexity, as well as the nodule production and mass, plant relative growth, and below-ground allocation. For plants originating in shaded environments, nodulation, root complexity, and below-ground allocation were enhanced in inoculated plants when they were transferred to conditions of high irradiance. Strikingly, plants from environments exposed to high light radiation were less plastic when exposed to changing light availability, and the symbionts were less beneficial than expected in stress-free environments. Our study proved that the stress imposed on plants due to high irradiance is overcome when plants are inoculated, and the positive effect is more evident in plants that are usually grown in shaded environments (e.g., Trifolium repens and Vicia sativa).