Root architecture and nitrogen metabolism in roots of apple rootstock respond to exogenous glucose supply in low carbon soil

To investigate the response of root architecture and nitrogen metabolism of apple rootstock to glucose supply in low-carbon (C) soil, Malus baccata (L.) Borkh. in gravel soil was treated with glucose C equal to the soil microbial biomass carbon (MBC)-C value (G1), five times the soil MBC value (G2), or with no glucose (CK). The roots samples were harvested after treatments for 7, 15 and 30 days. The roots tended to become larger, more dichotomous and showed a larger link branching angle in G1 and G2 than in CK, especially in the G1 treatment for 30 days. Plant height and biomass were increased by G1. Nitrate (NO3--N) and nitrite (NO2--N) contents were increased, but ammonium (NH4+-N) concentration was decreased in the roots treated with G1 and G2 in all treatment periods. Also, the activities and transcript levels of nitrate reductase, glutamine synthetase, glutamate dehydrogenase, glutamate synthase were generally increased in roots treated with glucose, especially under G1. The activities of glutamic oxalacetic transaminases and glutamic-pyruvic transaminase were higher under G1 than under either G2 or CK. Exogenous carbon source that equals to the native MBC effectively regulated the root architecture and supported increasing nitrogen absorption and metabolism in plants growing under carbon-restricted conditions.