| Optimal root growth and spatial distribution are beneficial for the efficient uptake of soil resources such as water and nutrients by roots. The accelerated mortality of maize (Zea mays) root system after silking reduces the nutrients uptake capacity. The post-silking nitrogen (N) remobilization and translocation from vegetative tissues to developing kernels affect not only the nutrient use efficiency, but also the carbohydrate production, partitioning, and grain yield. Maize varieties released in different decades in China and one modern hybrid in U.S. were employed in the muti-years of filed experiments in the present studies. The following studies were conducted, (a) the differences in root spatial distribution of different maize varieties and their relationship with soil nutrients uptake, and the shoot-root relationship in determining root growth and nutrient uptake; (b) the characterization of N fluxes after silking, i.e. uptake and remobilization, using 15N isotope, and their relationship with photosynthesis and grain yield; (c) the influences of N fertilizer application, especially after silking, on N uptake and grain yield; (d) the effects of changes of C/N ratio in ear leaf and ear on grain yield by analyzing the different carbohydrates, N compounds, and C/N status in these tissues; (e) the influences of N availability on the anantomic structures and the diurnal changes of key genes involved in sucrose and starch metabolism in ear leaf at silking and 20 days after silking (DAS). The main results were as follows:(1) Maize root dry weight (DW) and length were mainly distributed in the 0-30 cm soil layer. In consistent with the greater shoot biomass and grain yield, new maize varieties had larger root biomass than the old ones. The total root length and its vertical distribution at silking were similar between maize varieties released in different decades. At maturity, however, new maize varieties had longer total root length than old varieties, especially in the 30-60 cm soil layer. The spatial variation of soil Nmin concentration in the 0-60 cm soil profile, especially in 0-20 cm soil layer, was significantly larger than those of soil Olsen-P and available K. The concentration of soil residual Nmin was negatively correlated with root length density in the soil profile.(2) In comparison with the control plants, covering ears at silking significantly enhanced the root growth and root to shoot DW ratio, but reduced the total DW and nutritent uptake of whole plants, which was more pronounced in the ears-removed plants. Since more post-silking DW and nutrients were accumulated in the new maize varieties than the old ones, new varieties were more vulneralble to the treatments of covering or removing ears in regarding to DW and nutrient accumulation. Additionally, larger amounts of N was exported from leaves of new varieties than old ones, especially from the old leaves, while the overall percentage of N remobilization from vegetative tissues was lower in new varieties than in old ones.(3) Maize total DW, N uptake, grain yield, leaves senescence and photosynthesis were less affected by excessive N fertilizer application at silking or three weeks after silking when beyond the optimal N rates. Remobilization of N taken up before silking from vegetative tissues during the entire grain-filling phase was 58%-60%, which contributed to 53%-61% of the total grain N at maturity, and more than 60% of it was remobilized during the 0-30 DAS. Under N starvation, the N allocation to grains originated from post-silking uptake was 45%,56%,70%, and 96% in 0-15,15-30,30-46, and 46-60 DAS, respectively, while this percentage was relatively lower and constant in N-sufficient plants (43%-50%).At 60 or 65 DAS,70%-76% of the N taken up after silking was allocated to grains.(4) Low N supply led to a reduction of 14%-44% and 29%-67% in grain DW and N content at maturity than plants with sufficient N supply, but a significant increase in grain C/N ratio. The carbon and N assimilation in developing grains were positively correlated with each other. Different amounts of N fertilizer application marginally affected the concentration of carbohydrates and soluble protein in both apical and basal kernels of ear. More accumulation of starch, but less soluble protein and amino acids were observed in the N-deficient ear leaf than in the N-sufficent leaf, especially for glutamate, glutamine, alanine, asparingine.(5) Compared to the sufficient N supply, N deficiency significantly limited the photosynthesis, biomass, and grain yield of maize plants. Nevertheless, N-deficient ear leaf had larger and more starch granules in bundle sheath cells, leading to a more starch accumulation. N-deficient ear leaf had open symplastic pathway for sucrose export, while the the relative gene expression levels of transporters involved in sucrose loading to phloem in leaves were down regulated. N deficiency led to starch accumulation in apical cob at both silking and 20 DAS.Overall, compared to the old varieties, the root mortality was delayed in new maize varieties, and more roots was distributed in the 30-60 cm soil layer, both of which were beneficial for more post-silking nutrient uptake. Root growth and nutritent uptake were differently regulated, e.g. by carbon allocation from shoot and shoot growth potential, respectively. Post-silking N application failed to increase N uptake and grain yield when beyond optimal N rates, while reduced N use efficiency. Nitrogen remobilization in vegetative tissues mainly occurred in the fast grain filling phase (0-30 DAS), and the percentage of N allocation to grains originated from the post-silking uptake was greater in N-deficient plants than in N-sufficient plants. Carbon and N assimilation were positively correlated in developing grains. Higher carbohydrates but lower amino acids and protein concentration were observed in N-deficient ear leaf, and the insufficient N supply from source leaves might be one of the key factors limiting maize grain yield under N deficiency. |
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