Response of cotton root growth and yield to root restriction under various water and nitrogen regimes

Water and nitrogen (N) are two major factors limiting cotton growth and yield. The ability of plants to absorb water and nutrients is closely related to the size of the root system and the rooting space. Better understanding of the physiological mechanisms by which cotton (Gossypium hirsutum L.) adapts to water and N supply when rooting volume is restricted would be useful for improving cotton yield. In this study, cotton was grown in soil columns to control rooting depth to either 60 cm (root-restriction treatment) or 120 cm (no-root-restriction treatment). Four water–N combinations were applied to the plants: (1) deficit irrigation and no N fertilizer (W0N0), (2) deficit irrigation and moderate N fertilizer rate (W0N1), (3) moderate irrigation and no N fertilizer (W1N0), and (4) moderate irrigation and moderate N fertilizer rate (W1N1). Results revealed that root restriction reduced root length density (RLD), root volume density (RVD), root mass density (RMD), superoxide dismutase (SOD) activity, nitrate reductase (NR) activity, total plant biomass, and root : shoot ratio. In contrast, root restriction increased aboveground biomass and yield. The RLD, RVD, RMD, and root : shoot ratio decreased in the order W0N0 > W1N0 > W0N1 > W1N1 in both the root-restriction and no-root-restriction treatments. However, the opposite order (i.e., W1N1 > W0N1 > W1N0 > W0N0) was observed for SOD activity, NR activity, aboveground biomass, and seed yield. Our results suggest that, when N and water supplies are adequate, root restriction increases both root activity and the availability of photosynthates to aboveground plant parts. This increases shoot growth, the shoot : root ratio, and yield.