Nitrogen Utilization Efficiency in Canola Cultivars at Grain Harvest

Canola (Brassica napus L.) cultivars with improved nitrogen utilization efficiency (NUE) at grain harvest are of interest to growers to reduce fertilizer inputs. Our objective was to determine whether cultivar-specific responses in NUE (seed yield per N accumulated in the whole plant) could be related to the differences in dry matter and N partitioning among various plant parts. Four spring canola cultivars were grown in a glasshouse under the conditions of low and high N supply. When compared to high-N treatment, deficient N conditions resulted in a similar decrease in dry weight for all cultivars, averaging 46% for shoot, 47% for root, and 45% for dropped leaves. The reductions in N concentrations at low-N compared to high-N treatment were much smaller and averaged 15% for shoot, 16% for root and 10% for dropped leaves. Although significant variations occurred for dry weight, N concentration and N uptake in various plant sections, all cultivars accumulated a similar amount of N in total plant biomass at harvest. However, significant differences in plant biomass, seed yield and consequently, NUE existed because more N-efficient cultivars Eyre and Charlton produced larger seed yields than less N-efficient cultivars Pinnacle and Rainbow. No consistent variations in N concentration in various plant parts could be established among tested cultivars. Thus, cultivar-specific responses in NUE were mainly attributed to the differences in the root-to-shoot ratio and harvest index. N-efficient Eyre produced seed yield similar to the highest yielding Charlton, though it had the smallest plant dry weight of all cultivars. In contrast, N-inefficient Rainbow had the largest plant biomass, but produced the smallest seed yield because of its lowest harvest index and the highest root-to-shoot ratio. The absence of cultivar×N treatment interactions indicated that cultivars performed similarly for plant biomass, N uptake and seed yield across two contrasting N supplies. Canola cultivars significantly differed in NUE despite a similar amount of absorbed N in plant biomass; more N-efficient cultivars outyielded less N-efficient ones primarily because of cultivar-specific variations in the root-to-shoot ratio and harvest index.     

African crop yield reductions due to increasingly unbalanced Nitrogen and Phosphorus consumption

The impact of soil nutrient depletion on crop production has been known for decades, but robust assessments of the impact of increasingly unbalanced nitrogen (N) and phosphorus (P) application rates on crop production are lacking. Here, we use crop response functions based on 741 FAO maize crop trials and EPIC crop modeling across Africa to examine maize yield deficits resulting from unbalanced N : P applications under low, medium, and high input scenarios, for past (1975), current, and future N : P mass ratios of respectively, 1 : 0.29, 1 : 0.15, and 1 : 0.05. At low N inputs (10 kg ha^?1), current yield deficits amount to 10% but will increase up to 27% under the assumed future N : P ratio, while at medium N inputs (50 kg N ha^?1), future yield losses could amount to over 40%. The EPIC crop model was then used to simulate maize yields across Africa. The model results showed relative median future yield reductions at low N inputs of 40%, and 50% at medium and high inputs, albeit with large spatial variability. Dominant low‐quality soils such as Ferralsols, which are strongly adsorbing P, and Arenosols with a low nutrient retention capacity, are associated with a strong yield decline, although Arenosols show very variable crop yield losses at low inputs. Optimal N : P ratios, i.e. those where the lowest amount of applied P produces the highest yield (given N input) where calculated with EPIC to be as low as 1 : 0.5. Finally, we estimated the additional P required given current N inputs, and given N inputs that would allow Africa to close yield gaps (ca. 70%). At current N inputs, P consumption would have to increase 2.3‐fold to be optimal, and to increase 11.7‐fold to close yield gaps. The P demand to overcome these yield deficits would provide a significant additional pressure on current global extraction of P resources.     

Mining of Candidate Maize Genes for Nitrogen Use Efficiency by Integrating Gene Expression and QTL Data

Breeding maize varieties for high nitrogen (N) use efficiency (NUE) by marker-assisted selection using NUE quantitative trait locus (QTL) or by genetic transfer of NUE-associated genes is a viable approach for increasing grain yield in N-limited production areas. In this investigation, we evaluated a set of introgression line populations under N supply and N deficiency conditions. From 42 QTLs for grain yield and yield components, 23 were identified under N supply conditions and 33 from N limited conditions. Meta-analysis of published maize NUE QTLs revealed 37 “consensus” QTLs, of which, 18 was detected under low N conditions. In addition, 258 unique ESTs associated with low N stress response, N uptake, transport, and assimilation were aligned on the maize genome by in silico mapping. Integrating the EST map with the QTL map has resulted in the identification of candidate NUE-associated genes of the following functional categories: N uptake, transport, and assimilation; carbon (C) metabolism and assimilation; and cascades of stress response and signal transduction genes. Nine candidates that have been introgressed into Ye478 significantly altered grain yield/yield components. It is suggested that the dynamics of interactions between C and N metabolism are important for maize yield. A high NUE variety should have a highly efficient C assimilation per unit N and actively express CO₂ assimilation-related genes under N-limited conditions.     

A Large and Deep Root System Underlies High Nitrogen-Use Efficiency in Maize Production

Excessive N fertilization results in low N-use efficiency (NUE) without any yield benefits and can have profound, long-term environmental consequences including soil acidification, N leaching and increased production of greenhouse gases. Improving NUE in crop production has been a longstanding, worldwide challenge. A crucial strategy to improve NUE is to enhance N uptake by roots. Taking maize as a model crop, we have compared root dry weight (RDW), root/shoot biomass ratio (R/S), and NUE of maize grown in the field in China and in western countries using data from 106 studies published since 1959. Detailed analysis revealed that the differences in the RDW and R/S of maize at silking in China and the western countries were not derived from variations in climate, geography, and stress factors. Instead, NUE was positively correlated with R/S and RDW; R/S and NUE of maize varieties grown in western countries were significantly greater than those grown in China. We then testified this conclusion by conducting field trials with representative maize hybrids in China (ZD958 and XY335) and the US (P32D79). We found that US P32D79 had a better root architecture for increased N uptake and removed more mineral N than Chinese cultivars from the 0-60 cm soil profile. Reported data and our field results demonstrate that a large and deep root, with an appropriate architecture and higher stress tolerance (higher plant density, drought and N deficiency), underlies high NUE in maize production. We recommend breeding for these traits to reduce the N-fertilizer use and thus N-leaching in maize production and paying more attention to increase tolerance to stresses in China.     

Effect of nitrogen-deficiency on midday photoinhibition in flag leaves of different rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.) cultivars

Effects of nitrogen (N)-deficiency on midday photoinhibition in flag leaves were compared between two contrastive Japanese rice cultivars, a traditional japonica cultivar with low yield, cv. Shirobeniya (SRB), and a japonica-indica intermediate type with high yield, cv. Akenohoshi (AKN). Both cultivars were grown under high-N and low-N conditions. At midday, low-N supply resulted in more intensive reductions in net photosynthetic rate, stomatal conductance, maximal quantum yield of photosystem II (PSII) and quantum yield of PSII electron transport in SRB than in AKN, indicating that SRB was more strongly photoinhibited than AKN under low-N condition. At midday, the low-N plants of two cultivars showed higher superoxide dismutase (SOD) activities than the high-N plants. However, ascorbate peroxidase (APX) activity was maintained in AKN but significantly decreased in SRB under low-N condition (N-deficiency). In contrast, hydrogen peroxide (H₂O₂) content in SRB significantly increased under low-N condition, indicating that the susceptibility to midday photoinhibition in the low-N plants of SRB is related to the increased H₂O₂ accumulation. It is suggested that the midday depression in photosynthesis may be a result of oxidative stress occurring in the low-N plants in which antioxidant capacity is not enough to cope with the generation of H₂O₂. Therefore, H₂O₂-scavenging capacity could be an important factor in determining the cultivar difference of midday photoinhibition in flag leaves of rice under low-N condition.     

Root hair phenotypes influence nitrogen acquisition in maize

Background and AimsThe utility of root hairs for nitrogen (N) acquisition is poorly understood.MethodsWe explored the utility of root hairs for N acquisition in the functional–structural model SimRoot and with maize genotypes with variable root hair length (RHL) in greenhouse and field environments.Key ResultsSimulation results indicate that long, dense root hairs can improve N acquisition under varying N availability. In the greenhouse, ammonium availability had no effect on RHL and low nitrate availability increased RHL, while in the field low N reduced RHL. Longer RHL was associated with 216 % increase in biomass and 237 % increase in plant N content under low-N conditions in the greenhouse and a 250 % increase in biomass and 200 % increase in plant N content in the field compared with short-RHL phenotypes. In a low-N field environment, genotypes with long RHL had 267 % greater yield than those with short RHL. We speculate that long root hairs improve N capture by increased root surface area and expanded soil exploration beyond the N depletion zone surrounding the root surface.ConclusionsWe conclude that root hairs play an important role in N acquisition. We suggest that root hairs merit consideration as a breeding target for improved N acquisition in maize and other crops.     

Nitrogen Use Efficiency of Rice Cultivars (Oryza sativa L.) Under Salt Stress and Low Nitrogen Conditions

Improving nitrogen use efficiency (NUE) under salt stress has become crucial for rice as it is increasingly facing two major environmental constraints: excessive nitrogen fertilization and soil salinization. However, the interaction between salinity and N levels is very complex and has not yet been considered from the perspective of reduced nitrogen input. We conducted a hydroponic experiment at the early tillering stage on the Yoshida solution to evaluate the impact of rising NaCl and decreasing N application on NUE of four rice cultivars cultivated under three NaCl (0, 56, and 113 mM) and four N (2.86, 1.43, 0.72, and 0.36 mM) concentrations. After 4 weeks, physiological NUE (pNUE), absorption NUE (aNUE), agronomical NUE (agNUE), N transport efficiency (NTE), and physiological traits were evaluated. Significant interactions between N and NaCl-applied concentrations were found in all measured parameters. In all cultivars, increasing the NaCl-applied concentration markedly decreased aNUE and agNUE. For each NaCl treatment, lowering the N applied sharply increased aNUE and agNUE, and this effect was stronger when the NaCl applied was higher. The effect of N lowering on pNUE depended on the NaCl treatment: it enhanced pNUE in the absence of NaCl but had no influence under the highest NaCl-applied concentration. Cultivars largely differed in response to NaCl. The aNUE—but not pNUE—differed between salt-tolerant and salt-sensitive cultivars: aNUE markedly decreased with NaCl concentration in the most salt-sensitive cultivar, whereas it was the highest at the intermediate NaCl concentration in the most salt-tolerant cultivar, especially under low N levels. This finding suggests that under salt conditions, the use of salt-tolerant rice genotypes combined with reducing N level application is necessary to improve NUE. The study of NUE in rice should be focused on the improvement of aNUE with a strong emphasis on the salt tolerance of cultivars.

     

Effects of Controlled-Release Fertiliser on Nitrogen Use Efficiency in Summer Maize

Nitrogen (N) is a nutrient element necessary for plant growth and development. However, excessive inputs of N will lead to inefficient use and large N losses to the environment, which can adversely affect air and water quality, biodiversity and human health. To examine the effects of controlled-release fertilisers (CRF) on yield, we measured ammonia volatilisation, N use efficiency (NUE) and photosynthetic rate after anthesis in summer maize hybrid cultivar Zhengdan958. Maize was grown using common compound fertiliser (CCF), the same amount of resin-coated controlled release fertiliser (CRFIII), the same amount of sulphur-coated controlled release fertiliser (SCFIII) as CCF, 75% CRF (CRFII) and SCF (SCFII), 50% CRF (CRFI) and SCF (SCFI), and no fertiliser. We found that treatments CRFIII, SCFIII, CRFII and SCFII produced grain yields that were 13.15%, 14.15%, 9.69% and 10.04% higher than CCF. There were no significant differences in grain yield among CRFI, SCFI and CCF. We also found that the ammonia volatilisation rates of CRF were significantly lower than those of CCF. The CRF treatments reduced the emission of ammonia by 51.34% to 91.34% compared to CCF. In addition, after treatment with CRF, maize exhibited a higher net photosynthetic rate than CCF after anthesis. Agronomic NUE and apparent N recovery were higher in the CRF treatment than in the CCF treatment. The N uptake and physiological NUE of the four yield-enhanced CRF treatments were higher than those of CCF. These results suggest that the increase in NUE in the CRF treatments was generally attributable to the higher photosynthetic rate and lower ammonia volatilisation compared to CCF-treated maize.     

Dynamic responses of nitrous oxide emission and nitrogen use efficiency to nitrogen and biochar amendment in an intensified vegetable field in southeastern China

Intensive vegetable production exhibits contrasting characteristics of high nitrous oxide (N₂O) emissions and low nitrogen use efficiency (NUE). In an effort to mitigate N₂O emissions and improve NUE, a field experiment with nine consecutive vegetable crops was designed to study the combined effects of nitrogen (N) and biochar amendment and their interaction on soil properties, N₂O emission and NUE in an intensified vegetable field in southeastern China. We found that N application significantly increased N₂O emissions, N₂O–N emission factors and yield‐scaled N₂O emissions by 51–159%, 9–125% and 14–131%, respectively. Moreover, high N input significantly decreased N partial factor productivity (PFPN) and even yield during the seventh to ninth vegetable crops along with obvious soil degradation and mineral N accumulation. To the contrary, biochar amendment resulted in significant decreases in cumulative N₂O emissions, N₂O–N emission factor and yield‐scaled N₂O emissions by 5–39%, 16–67% and 14–53%, respectively. In addition, biochar significantly increased yield, PFPN and apparent recovery of N (ARN). Although without obvious influence during the first to fourth vegetable crops, biochar amendment mitigated N₂O emissions during the fifth to ninth vegetable crops. The relative effects of biochar amendments were reduced with increasing N application rate. Hence, while high N input produced adverse consequences such as mineral N accumulation and soil degradation in the vegetable field, biochar amendment can be a beneficial agricultural strategy to mitigate N₂O emissions and improve NUE and soil quality in vegetable field.     

黄土高原南部春玉米地膜栽培的水肥效应与氮肥去向

在黄土高原南部采用田间小区和微区试验,研究了春玉米地膜栽培下氮肥-水分-产量关系与氮肥去向。结果表明,相同施肥条件下地膜栽培(N120C)比平作栽培(N120UC)增产显著(46．7％),施用氮肥显著地发挥了地膜的增产潜力,处理N120(尿素氮120kg·hm^-2)、N180(尿素氮180kg·hm^-2)和N120M(尿素氮120kg·hm^-2+有机肥氮60kg·hm^-2),籽粒产量比对照CK(不施氮)分别增产41．8％、43．9％和34．7％,地膜栽培或施用氮肥都极大地改善了玉米水分生产效率(WUE)和降水利用率(RUE),试验中N120C比N120UC水分生产效率提高57．9％。降水利用效率提高54．5％;处理N120、N180和N120M比CK处理WUE分别提高38．4％、47．4％和32．4％,RUE分别提高42．3％、43．9％和34．7％,由于供试有机肥是半腐解的牛粪,比尿素氮素供给迟缓,所以对玉米产量和WUE提高幅度小,试验水分测定反映出,玉米利用的水分73．0％～83．7％来自降雨,表明决定春玉米产量的关键水分是生育期降水,玉米地膜栽培对氮肥去向有微弱影响,相对于平作玉米,氮肥总的回收率差异不大,但氮肥利用率下降7．3个百分点,土壤残留率上升6．4个百分点,土壤当季残留氮主要集中在0～20cm,不会发生向深层大量的淋溶和累积。     

Increasing nitrogen use efficiency of corn in midwestern cropping systems

Nitrogen (N) loss from agricultural systems raises concerns about the potential impact of farming practices on environmental quality. N is a critical input to agricultural production. However, there is little understanding of the interactions among crop water use, N application rates, and soil types. This study was designed to quantify these interactions in corn ( Zea mays L.) grown in production-size fields in central Iowa on the Clarion-Nicollet-Webster soil association. Seasonal water use varied by soil type and N application rate. Yield varied with N application rate, with the highest average yield obtained at 100 kg ha(-1). N use efficiency (NUE) decreased with increasing N application rates, having values around 50%. Water use efficiency (WUE) decreased as N fertilizer rates increased. Analysis of plant growth patterns showed that in the low organic matter soils (lower water-holding capacities), potential yield was not achieved because of water deficits during the grain-filling period. Using precipitation data coupled with daily water use throughout the season, lower organic matter soils showed these soils began to drain earlier in the spring and continued to drain more water throughout the season. The low NUE in these soils together with increased drainage lead to greater N loss from these soils. Improved management decisions have shown that it is possible to couple water use patterns with N application to increase both WUE and NUE.     

Ideotype root architecture for efficient nitrogen acquisition by maize in intensive cropping systems

The use of nitrogen (N) fertilizers has contributed to the production of a food supply sufficient for both animals and humans despite some negative environmental impact. Sustaining food production by increasing N use efficiency in intensive cropping systems has become a major concern for scientists, environmental groups, and agricultural policymakers worldwide. In high-yielding maize systems the major method of N loss is nitrate leaching. In this review paper, the characteristic of nitrate movement in the soil, N uptake by maize as well as the regulation of root growth by soil N availability are discussed. We suggest that an ideotype root architecture for efficient N acquisition in maize should include (i) deeper roots with high activity that are able to uptake nitrate before it moves downward into deep soil; (ii) vigorous lateral root growth under high N input conditions so as to increase spatial N availability in the soil; and (iii) strong response of lateral root growth to localized nitrogen supply so as to utilize unevenly distributed nitrate especially under limited N conditions.     

施氮时期对高产夏玉米氮代谢关键酶活性及抗氧化特性的影响

选用玉米品种登海661和郑单958为材料,研究了高产条件下施氮时期对夏玉米产量、氮素利用率、氮代谢相关酶及抗氧化酶活性的影响.结果表明:拔节期一次性施氮不利于夏玉米产量提高和氮素积累,分次施氮且增施花粒肥显著提高了植株和籽粒的吸氮量,并提高了籽粒产量.拔节期、10叶期、花后10d按2∶4∶4施氮,登海661产量最高可达14123.0kg· hm-2;基肥、拔节期、10叶期、花后10 d按1∶2∶5∶2施氮,郑单958产量最高可达14517.1 kg· hm-2,这2种施氮方式较拔节期一次性施氮分别增产14.5％和17.5％.花前分次施氮可以显著提高开花期硝酸还原酶活性;登海661和郑单958在花后0～42 d中,施氮处理的谷氨酰胺合成酶、谷氨酸合成酶、谷氨酸脱氢酶活性分别平均提高了32.6％、47.1％、50.4％和14.5％、61.8％、25.6％,减缓了其下降趋势;超氧化物歧化酶、过氧化氢酶活性提高了22.0％、36.6％和13.4％、62.0％,丙二醛含量显著降低.在高产条件下,分次施氮且适当增加花粒肥施入比例可以提高氮代谢相关酶活性,延缓植株衰老,促进氮素吸收利用,进而提高籽粒产量.     

不同硝化抑制剂对红壤氮素硝化作用及玉米产量和氮素利用率的影响

本研究分析添加不同种硝化抑制剂及其组合的高效稳定性氯化铵氮肥对红壤硝化作用、玉米产量和氮肥利用率的影响,旨在筛选出适合酸性红壤的高效稳定性氯化铵态氮肥。在氯化铵中分别添加硝化抑制剂2-氯-6-三甲基吡啶(CP)、3,4-二甲基吡唑磷酸盐(DMPP)和双氰胺(DCD)及其组合,制成6种高效稳定性氯化铵态氮肥,以不施氮肥(CK)和施氯化铵(N)为对照,进行等氮量玉米盆栽试验。结果表明: 与N处理相比,CP+DMPP和DMPP+DCD处理红壤中铵态氮含量提高56%～62%,显著高于CP、DMPP和DCD处理;土壤表观硝化率显著降低33%～34%。添加硝化抑制剂及其组合的6个处理均显著提高了玉米生物量和氮肥吸收利用率。与N处理相比,单独添加硝化抑制剂处理生物量均显著高于硝化抑制剂组合处理,平均提高1.3倍;添加DCD处理效果最显著,玉米籽粒产量、吸氮量和氮肥吸收利用率分别显著提高4.1、6.3和4.4倍。为了达到既能低成本又能提高产量和氮肥利用率的效果,在红壤上添加硝化抑制剂DCD是最佳选择。     

Identification of quantitative trait loci for nitrogen use efficiency in maize

Intensively managed crop systems are normally dependent on nitrogen input to maximize yield potential. Improvements in nitrogen- use efficiency (NUE) in crop plants may support the development of cropping systems that are more economically efficient and environment friendly. The objective of this study was to map and characterize quantitative trait loci (QTL) for NUE in a maize population. In preliminary experiments, inbred lines contrasting for NUE were identified and were used to generate populations of F2:3 families for genetic study. A total of 214 F2:3 families were evaluated in replicated trials under high nitrogen (280 kg/ha) and low nitrogen (30 kg/ha) conditions in 1996 and 1997. Analysis of ear-leaf area, plant height, grain yield, ears per plant, kernels number per ear, and kernel weight indicated significant genetic variation among F2:3 families. The heritability of these traits was found to be high (h2=0.57–0.81). The mapping population were genotyped using a set of 99 restriction fragment length polymorphism (RFLP) markers. A linkage map of these markers was developed and used to identify QTL. Between two and six loci were found to be associated with each trait. The correspondence of several genomic regions with traits measured under nitrogen limited conditions suggests the presence of QTL associated with NUE. QTLs will help breeders to improve their maize ideotype of a low-nitrogen efficiency by identifying those constitutive and adaptive traits involved in the expression of traits significantly correlated with yield, such as ear leaf area and number of ears per plant. This revised version was published online in June 2006 with corrections to the Cover Date.     

Mapping QTLs for root traits under different nitrate levels at the seedling stage in maize (Zea mays L.)

Nitrogen (N) loss is a worldwide problem in crop production. Apart from reasonable N fertilizer application, breeding N efficient cultivars provides an alternative way. Root architecture is an important factor determining N acquisition. However, little is known about the molecular genetic basis for root growth in relation to N supply. In the present study, an F₈ maize (Zea may L.) recombinant inbred (RI) population consisting of 94 lines was used to identify the QTLs for root traits under different nitrate levels. The lateral root length (LRL), axial root length (ARL), maximal axial root length (MARL), axial root number (ARN) and average axial root length (AARL) were evaluated under low N (LN) and high N (HN) conditions in a hydroponics system. A total of 17 QTLs were detected among which 14 loci are located on the same chromosome region as published QTLs for root traits. A major QTL on chromosome 1 (between bnlg1025 and umc2029) for the AARL under LN could explain 43.7% of the phenotypic variation. This QTL co-localizes with previously reported QTLs that associate with root traits, grain yield, and N uptake. Our results indicate that longer axial roots are important for efficient N acquisition and the major QTL for AARL may be used as a marker in breeding N efficient maize genotypes.     

Genetic variability of nitrogen accumulation during vegetative development and remobilization during the forcing process in witloof chicory tuberized root (Cichorium intybus L.)

A better knowledge of genetic variability of traits related to nitrogen use efficiency (NUE) is a potential strategy to optimize N fertilization and to reduce environmental pollution without decreasing marketable yield and quality. To this aim, in this study, 13 cultivars of witloof chicory were compared with three reference cultivars known for their adaptation to low, intermediate and high N availability in the field during the vegetative phase of development. Pertinent criteria used for this study were determined by a thorough comparison of nitrogen reserve accumulation in tuberized roots during vegetative development and mobilization during the forcing process in the three reference cultivars. Cluster analysis allowed us to sort the cultivars into four main groups we named G1, G2, G3 and G4. Cultivars of group G4, better adapted to soils with high nitrogen contents (N-demanding cultivars), showed higher total N, nitrate, total amino acids (AA), glutamine contents and lower total N and AA mobilization for chicon growth than did cultivars of group G1, adapted to soils with low nitrogen content (N-sensitive cultivars). An intermediate behavior was exhibited by cultivars of groups G2 and G3, characterized as N tolerant. It is proposed that either chicory growers or breeders may take advantage of the genetic variability revealed in the present study to gain flexibility in choosing the right cultivar for the type of soil available (N-rich soil vs N-poor soil) or to adapt the level of N fertilization to the type of cultivar (N-demanding vs N-sensitive) in order to target the highest NUE for the best chicon yield and trade quality.     

Low nitrogen stress regulates chlorophyll fluorescence in coordination with photosynthesis and Rubisco efficiency of rice

Nitrogen (N) is the basis of plant growth and development and, is considered as one of the priming agents to elevate a range of stresses. Plants use solar radiations through photosynthesis, which amasses the assimilatory components of crop yield to meet the global demand for food. Nitrogen is the main regulator in the allocation of photosynthetic apparatus which changes of the photosynthesis (P_n) and quantum yield (F_v/F_m) of the plant. In the present study, dynamics of the photosynthetic establishment, N-dependent relation with chlorophyll fluorescence attributes and Rubisco efficacy was evaluated in low-N tolerant (cv. CR Dhan 311) and low-N sensitive (cv. Rasi) rice cultivars under low-N and optimum-N conditions. There was a decrease in the stored leaf N under low-N condition, resulting in the decreased P_n and F_v/F_m efficiency of the plants through depletion in the activity and content of Rubisco. The P_n and F_v/F_m followed the parallel trend of leaf N content during low-N condition along with depletion of intercellular CO₂ concentration and overall conductance under low-N condition. Photosynthetic saturation curve cleared abrupt decrease of effective quantum yield in the low-N sensitive rice cultivar than the low-N tolerant rice. Also, the rapid light curve highlighted the unacclimated regulation of photochemical and non-photochemical quenching in the low-N condition. The low-N sensitive rice cultivar triumphed non-photochemical quenching, whereas the low-N tolerant rice cultivar rose gradually during the light curve. Our study suggested that the quantum yield is the key limitation for photosynthesis in low-N condition. Regulation of Rubisco, photochemical and non-photochemical quenching may help plants to grow under low-N level.

     

玉/豆和玉/薯模式下氮肥运筹对玉米氮素利用和土壤硝态氮残留的影响

过量施用氮肥造成的环境问题日益严重,氮肥合理使用已成为人们研究的热点.本文研究了西南玉米两种主要套作模式下氮肥运筹对玉米氮素利用和土壤硝态氮残留的影响.结果表明：连续分带轮作种植玉/豆模式后,玉米收获期植株中的氮素积累较玉/薯模式平均提高了6.1%,氮收获指数增加了5.4%,最终使氮肥利用效率提高4.3%,氮素同化量提高了15.1%,氮肥偏生产力提高了22.6%;玉米收获后硝态氮淋溶损失减少,60～120 cm土层中硝态氮残留玉/豆模式较玉/薯模式降低了10.3%,而0～60 cm土层中平均提高了12.9%,有利于培肥地力,两年产量平均较玉/薯模式高1249 kg·hm^-2,增产22%;增加施氮量提高了植株氮素积累,降低了氮肥利用率,显著提高了表层土壤中硝态氮的累积,60～100 cm土层中硝态氮的累积量在0～270 kg·hm^-2处理间差异不显著,继续增加施氮量会显著增加土壤硝态氮的淋溶;氮肥后移显著提高了土壤0～60 cm土层硝态氮的积累.两种模式下施氮量和底追比对玉米氮素吸收和硝态氮残留的影响结果不一致,玉/豆模式以施氮180～270 kg·hm^-2、按底肥∶拔节肥∶穗肥=3∶2∶5的施肥方式有利于提高玉米植株后期氮素积累、氮收获指数和氮肥利用效率,减少了氮肥损失,两年最高产量平均可达7757 kg·hm^-2;而玉/薯模式在180 kg·hm^-2、按底肥∶穗肥=5∶5的施肥方式下,氮素积累利用及产量均优于其他处理,两年平均产量为6572 kg·hm^-2,可实现两种模式下玉米高产、高效、安全的氮肥管理体系.