Intercropping of Faba Bean with Wheat Under Low Water Availability Promotes Faba Bean Nodulation and Root Growth in Deeper Soil Layers

AbstractThe symbiotically fixed N is advantageous to legumes growing under N-limiting conditions, and is also potentially beneficial for subsequent or associated non-legume crops1, 2. However, under stressful conditions such as water limitation and low nutrient availability, legumes may lose the distinct advantage of an unlimited source of symbiotic N2 3, 4, 5. Belowground niche complementarity in legume-cereal intercrops may improve resource use efficiency and adaptability to environmental constraints, but effects of water limitation on legume rooting and nodulation patterns is poorly understood. To advance our knowledge of mechanisms involved in water stress response, faba bean (Vicia faba L., FB) and wheat (Triticum aestivum L.) were grown as mono- and intercrops in soil-filled plexiglass rhizoboxes under water sufficiency (80% of field capacity; FC) and water stress (30% of FC). Water stress decreased shoot biomass in both monocropped and intercropped FB, as well as root length in monocropped FB. Intercropping increased both shoot dry weight and height of FB irrespective of water treatment, while increased root biomass and length in intercropped FB was observed only under water stress. No significant effects of crop or water treatment were found on wheat growth parameters. Intercropping increased overall nodulation (nodule number and nodule dry weight; NDW) regardless of water availability. However spatial patterns of nodulation differed between water treatments: top-15-cm NDW and number increased (27 and 33%, respectively) in sufficiently-watered intercropped FB, while NDW in the same soil layer significantly decreased (45%) in water-stressed intercropped FB. In contrast, below-15-cm NDW and number significantly increased in intercropped FB under both water levels. This enhanced nodulation in the deeper soil layer and the associated increase in root and shoot growth provides evidence for a shift in niche occupancy that improves FB performance when exposed to water limitation in intercropping