The Variety Differences And Physiological Mechanisms Of Nitrogen Uptake And Translocation For Maize Canopy-Roots-Soil System

Nitrogen use efficiency (NUE) of maize production in China is around 30-35%, which is mainly induced by overuse of nitrogen fertilizer, has not only led to higher costs and fertilizer waste, but also causes a series of resources and environmental problems. Exploring crops potential for efficient nitrogen use is one important way to improve NUE. It is of great importance to identify the differences of nitrogen uptake and translocation among maize variety with different nitrogen efficiency, and claritying its physiological mechanisms is equally important for maize breeding and agronomic regulation for enhancing NUE and sustainable maize production. Based on this purpose, this study compared high NUE hybrids (ZD958 and JS27) and low NUE hybrids (MN2133, ND314 and SD19) at different nitrogen rates in field experiment and at different applying depth of 15N fertilizer in soil pool experiments. In order to systematically understand the nitrogen use diffrences among different varietys, a canopy-roots-soil system perspective was used to analyze the relationships of root traits, nitrogen uptake and nitrate distribution in root layers, and the relationships of canopy nitrogen distribution, transfer dynamics, leaf photosynthesis and nitrogen utilization. Furthermore, the coordination of nitrogen efficiency, nitrogen uptake efficiency and nitrogen utilization efficiency was also investigated. The main research results were as follows.1. The optimum nitrate concentration suitable for root growth was 4 mmol/L in seeding stage. Maize absorbed more nitrogen mainly through increasing fine roots proportion and root surface under low nitrogen stress conditions. Under excessive nitrogen supply conditions, maize uptaked nitrogen by increasing root diameter and by forming high-density branches in roots system. Root length and root volume had greater direct impact on nitrogen uptake efficiency.2. The temporal and spatial distribution of roots of maize variety with different nitrogen efficiency influnced nitrogen uptake and responsed to nitrogen rates. Root length at different soil strata of high NUE variety was remarkably greater than that of low NUE variety. From silking to milking stage, the root senescence period, root decreasing rate of high NUE variety was clearly lower than that of low NUE variety at 0-20cm and below 40cm strata under zero nitrogen treatment. The situation was quite different when N was applied. There was no difference on root decreasing rate at 0-40cm strata, but high NUE varietys showed significant lower root decreasing rate below 0-40cm strata compared to low NUE varietys. The result indicated that root senescence below 40cm strata of high NUE varietys was slower than that of low NUE varietys.3. Nitrogen uptake was significantly correlated to root length. Path analysis indicated that pre-anthesis nitrogen uptake at low and moderate nitrogen treatment was primary controlled by unit root length uptake rate, but when excessive nitrogen was applied, it was mainly controlled by root length. The situation at post-anthesis stage was controlled by root length when low and moderate nitrogen treatment,but controlled by unit root length uptake rate when excessive nitrogen was applied.4. Planting high NUE hybrid could reduce field nitrogen leaching losses. Without nitrogen applying, the maximum soil nitrate content moved 20cm down and the external environment had little effect on it. But with nitrogen applying, soil nitrate content moved 40cm-60cm down, and the interannual difference was mainly influnced by precipitation. Nitrate accumulation in 0-100cm soil layer of high NUE variety was significantly lower than that of low NUE variety. At 300N treatment, the apparent loss of nitrogen of low NUE variety was 1.52-2.89 times of high NUE variety, and 1.45-1.56 times at 450N treatment. Nitrogen uptake was significantly linear negative correlated with soil nitrate accumulation.5. Reducing nitrogen rate and deep fertilization could improve maize nitrogen uptake. At low nitrogen rate, root morphology parameters of 45cm applying depth (F45) was significantly higher than that of 15cm applying depth (F15), nitrogen uptake of F45 was also higher than that of F15, which indicated that deep fertilization promoted the maize root growth, then accelerating nitrogen uptake. At high nitrogen rates, F45 significantly increased root growth in 0-30cm and 60-90cm soil layer, while inhibited root growth in 30-60cm soil layer, and its nitrogen uptake was significantly lower than that of F15.6. Bigger root system and higher nitrogen uptake efficiency of high NUE variety was closely related to physiological traits of middle and bottom leaves of maize canopy. I5N tracing results showed that 30-50% of grain nitrogen was directly derived from nitrogen uptake after silking, and 50-70% was from stover nitrogen translocation after silking, however, difference among varietys was not significant. At different nitrogen rates, nitrogen uptake efficiency(NUpE) and nitrogen utilization efficiency(NUtE) made different contributions to nitrogen use efficiency(NUE). Under low nitrogen conditions, NUtE had a greater effect on NUE, but under high nitrogen conditions, NUpE had a greater effect on NUE. Photosynthesis of middle and bottom leaves and nitrogen assimilation indexes (leaf dry matter, leaf nitrogen accumulation, photosynthetic rate, SPAD value) were significantly correlated with NUpE.