Allocation of nitrogen and sucrose in maize seedling under low nitrogen stress

Objectives The study aimed to explore the characteristics of nitrogen (N) uptake, transport and utilization in seedlings of maize inbred lines, in order to reveal the changing patterns of phenotypic and physiological characteristics under low nitrogen stress.

Methods A hydroponic experiment was conducted using two maize inbred lines XY4 and PH4CV as experimental materials. Nutrient solution of normal N (2 mmol/L, NN) and low nitrogen stress (0.04 mmol/L. LN) was prepared. Since 3 h of culturing, the biomass, photosynthetic characteristics, root traits, nitrogen and sucrose contents of maize seedlings were measured, the measurement was done in 3 days interval and the last one was at the 12th day.

Results Root responses to low nitrogen stress were earlier than those of shoot. The root dry weights of PH4CV and XY4 treated with LN for 3 and 6 days increased significantly compared with NN treatment, increased by 65.15% and 84.63%, respectively. However, after 9 days treatment in low nitrogen level, the shoot dry weights of XY4 and PH4CV decreased significantly compared with NN treatment, resulting in the root to shoot ratio increasing. In LN treatment, except for intercellular CO₂ concentration (Ci) and water use efficiency (WUE), the SPAD values, net photosynthetic rate (Pn), stomatal conductance (Tr) and transpiration rate (Gs) in seedling leaves of both inbred lines were lower than those of NN treatment, and XY4 decreased more than PH4CV. Under LN treatment, the sources of variation of root dry weight in two inbred lines were different, the increase of XY4 in root dry weight was related to the increase of total root length, root surface area, root volume, lateral root number and primary root length, while that of PH4CV was mainly related to the increase of lateral root number. Compared with NN treatment, the nitrogen and sucrose contents in seedling shoots of two inbred lines decreased significantly, and nitrogen contents in the old leaves of XY4 decreased faster than that of PH4CV, while nitrogen contents in roots, physiological nitrogen use efficiency of individual plants and sucrose contents in roots increased significantly under LN treatment, and XY4 increased more than those of PH4CV.

Conclusions Low nitrogen stress promotes maize seedlings to assign less nitrogen and sucrose to the shoot, results in limiting the accumulation of shoot biomass and the photosynthetic capacity of leaves, while more nitrogen and sucrose are allocated to the roots, thus promoting the formation of root morphology and facilitating their absorption of more nitrogen.