Interspecific root interactions and water-use efficiency of intercropped proso millet and mung bean

Proso millet (Panicum miliaceum L.)/mung bean (Vigna radiata L.) intercropping could serve as a new cereal/legume planting pattern to increase crop production. However, how these two crop species compete for resources by stimulating root development and water uptake is unknown. An experiment was conducted at the Northwest A&F University experimental site in northwestern China from 2017 to 2019 to investigate the mechanisms governing the yield responses of both crops and compare interspecific interactions by analyzing the root distribution and water-use efficiency (WUE) of the crops. Intercropped proso millet dominated, as indicated by the highly positive aggressivity (Apm; averaging 1.66) and relative crowding coefficient (Kpm; averaging 2.66). The root growth and distribution of both crops increased under intercropping, with lateral expansion to the middle interrow spaces during the cogrowth period, resulting in greater root length density, surface area density, and volume density in the upper layer (0-50 cm) and close to the plants. This allowed the absorption of more soil water, which helped increase the WUE of intercropped proso millet. These improvements occurred due to significant correlations between soil water content and root structure. However, the relatively short mung bean was shaded by the taller proso millet, creating a disadvantage reflected by the decreased plant biomass and WUE of the former. Compared with grain yield under monocultures, the grain yield of the intercropped proso millet and mung bean increased by 13.9-50.0 % and decreased by 31.9-47.0 %, respectively. Two rows of proso millet intercropped with four rows of mung bean (2P4M) is recommended, as this system, although reducing mung bean yields, increases proso millet yields and enables the greatest land-use efficiency in arid regions of Northwest China.