夏闲期耕作对旱地小麦土壤水分及植株氮素吸收、运转特性的影响

采用大田试验,研究了夏闲期耕作对旱地小麦播种前和各生育期0～300 cm土壤水分、植株氮素吸收和运转特性的影响.结果表明: 夏闲期耕作可提高播种前和各生育期0～300 cm土壤蓄水量,且枯水年效果较好.夏闲期耕作可显著提高各生育期植株氮素积累量、开花期叶片和茎秆+茎鞘氮素积累量、成熟期籽粒氮素积累量,显著提高茎秆+茎鞘氮素运转量及其对籽粒的贡献率、叶片氮素运转量、花前氮素运转量、花后氮素积累量,最终提高氮素吸收效率,以前茬小麦收获后45 d深翻效果较好.夏闲期耕作条件下,土壤水分与花前氮素运转量及籽粒氮素积累量显著相关,且枯水年关系更密切;播种至开花期土壤水分与花后氮素积累量在丰水年显著相关,而枯水年无显著相关关系.夏闲期耕作,尤其是雨后深翻有利于蓄水保墒及植株氮素吸收和转运.     

Nitrogen redistribution from the roots in post-anthesis plants of spring wheat

The aim of this study was to investigate post-anthesis N redistribution from the roots to other organs in the wheat plant. Two cultivars of spring wheat (Triticum aestivum L.) Sport and WL were grown to maturity in culture solution with stepwise-decreasing N supply and ¹⁵N before sampling. The N concentration of dry weight in the roots decreased from anthesis to full maturity. At maturity the root N concentration was 5.5 mg g⁻¹ dry weight for Sport and 4.9 mg g⁻¹ dry weight for WL, while the main shoot N concentration was 2.1 mg g⁻¹ dry weight for Sport and 2.2 mg g⁻¹ dry weight for WL. The roots were a major sink for N taken up post-anthesis and contained the main proportion of the ¹⁵N during the whole post-anthesis period. The N redistributed from the roots contributed 24.3% of the total N amount in the grains at maturity in cv Sport and 8.7% in cv WL. For cv Sport, 46.3% of the root N amount was redistributed to other parts of the plant and for WL 25.4%. The tiller grains were supplied with N from the ¹⁵N applied post-anthesis to a higher degree than the main shoot grains. The transport capability was maintained until 12 days after complete yellowness. We concluded that wheat has a great ability to redistribute nitrogen from the root system to the grains.     

推迟拔节水对小麦氮素积累与分配和硝态氮运移的影响

摘要：2007—2008年度以高产冬小麦品种济麦22为材料,设置2个拔节水灌溉时期,为拔节期和拔节后10 d;3个目标相对含水量,灌水后0~140 cm土层土壤相对含水量分别达到65%、75%、80%,以W1、W2、W3表示拔节期灌水处理,DW1、DW2、DW3表示拔节后10 d灌水处理;开花期均灌水至0~140 cm土层土壤相对含水量为70%,研究推迟拔节水对小麦氮素积累与分配和硝态氮运移的影响。结果表明：（1）W2和DW2处理有利于提高0~60 cm土层土壤硝态氮含量,促进籽粒氮素积累;营养器官贮藏氮素向籽粒的转运量、籽粒产量和氮肥偏生产力分别高于W1和DW1,与W3和DW3处理无显著差异;开花后植株氮素积累量、籽粒蛋白质含量和水分利用效率分别高于W3和DW3,是拔节期和拔节后10 d灌水的最优处理。（2）W2和DW2处理比较,DW2成熟期100~140 cm土层硝态氮残留量低于W2,籽粒产量、籽粒蛋白质含量、氮素吸收效率、氮肥偏生产力和水分利用效率均显著高于W2,是本试验条件下的最佳灌水方案。2008—2009生长季试验各处理变化趋势同2007—2008年度。     

The utilization of pre-anthesis reserves in grain filling of wheat. Assessment by steady-state 13CO2/12CO2 labelling

δ, C isotope composition relative to Pee Dee Belemnite
WSC, water-soluble carbohydrates
N, nitrogen
C, carbon
cv, cultivar
ME, efficiency of mobilized pre-anthesis C utilization in grain filling (g C g^–1C)

Significant mobilization of protein and carbohydrates in vegetative plant parts of wheat regularly occurs during grain filling. While this suggests a contribution of reserves to grain filling, the actual efficiency of mobilized assimilate conversion into grain mass (ME) is unknown. In the present study the contribution of pre-anthesis C (C fixed prior to anthesis) to grain filling in main stem ears of two spring wheat (Triticum aestivum L.) cultivars was determined by ¹³C/¹²C steady-state labelling. Mobilization of pre-anthesis C in vegetative plant parts between anthesis and maturity, and the contributions of water-soluble carbohydrates (WSC) and protein to pre-anthesis C mobilization were also assessed. Experiments were performed with two levels of N fertilizer supply in each of 2 years. Pre-anthesis reserves contributed 11–29% to the total mass of C in grains at maturity. Pre-anthesis C accumulation in grains was dependent on both the mass of pre-anthesis C mobilized in above-ground vegetative plant parts (r² = 0·87) and ME (defined as g pre-anthesis C deposited in grains per g pre-anthesis C mobilized in above-ground vegetative plant parts; r² = 0·40). ME varied between 0·48 and 0·75. The effects of years, N fertilizer treatments and cultivars on ME were all related to differences in the fractional contribution of WSC to pre-anthesis C mobilization. Multiple regression analysis indicated that C from mobilized pre-anthesis WSC may be used more efficiently in grain filling than C present in proteins at anthesis and mobilized during grain filling. Possible causes for variability of ME are discussed.     

Effects of timing and supply of nitrogen on nitrogen remobilization from vegetative organs and redistribution to developing seeds of sunflower

A glasshouse study was made of the distribution of ¹⁵N among vegetative organs of sunflower and its later remobilization and redistribution to seeds, as influenced by the developmental stage at which ¹⁵N was provided, and by the N status of the plants. Plants of Hysun 30 sunflower were grown in sand culture and provided with K¹⁵NO₃ for a 3-day period at: (a) 3 days before the end of floret initiation; (b) 3 days before anthesis; (c) the start of anthesis; (d) full anthesis; and (e) 8 days after full anthesis. The plants were grown on a range of N supply rates, from severely deficient to more than adequate for maximum growth. Nitrogen-15 was distributed to all parts of the plant at the end of the ¹⁵N uptake periods. With the exception of the most N-stressed plants, subsequent remobilization of ¹⁵N from roots, stems and leaves occurred irrespective of the time the ¹⁵N was taken up. However, the percentage redistribution to seeds of ¹⁵N taken up at the end of floret initiation was less than for ¹⁵N taken up at anthesis. Remobilization of ¹⁵N from leaves and roots was higher (70%) for ¹⁵N taken up during and after anthesis than for ¹⁵N taken up at the end of floret initiation (45%), except for plants grown on the lowest N supply. By contrast, remobilization of ¹⁵N from the stem was lower for ¹⁵N taken up after full anthesis (40%) than before or during anthesis (>70%). The proportion of ¹⁵N remobilized from the top third of the stem was less than that from the bottom third, and decreased with increasing plant N status. Nitrogen-15 taken up over the 3-day supply periods during anthesis contributed from 2 to 11% of the total seed N at maturity; the contribution to seeds was greatest for plants grown on the highest N supply. Nitrogen taken up just before and during anthesis contributed most of the N accumulated in mature seeds of plants grown on an adequate N supply, but N taken up between the end of floret initiation and just before anthesis, or after full anthesis seemed to make an equally important contribution to mature seeds as N taken up during anthesis for plants grown on a very low N supply. It was concluded that the development of florets and seeds of sunflower is supported by N taken up by the plant between the end of floret initiation and anthesis, and by N redistributed from vegetative organs. Unless soil N is so low as to impair early growth, split applications of N fertilizer would be best made just before the end of floret initiation (‘star stage’) and just before anthesis.     

氮肥运筹对晚播冬小麦氮素和干物质积累与转运的影响

氮素平衡对干物质积累与分配的影响是农业生态系统研究的重要内容,在保障产量前提下减少氮肥施用量可减少环境污染与温室气体排放。以晚播冬小麦为研究对象,设置4个施氮量水平:0 kg/hm2(N0)、168.75 kg/hm2(N1)、225 kg/hm2(N2)、281.25 kg/hm2(N3),每个施氮量水平下设置2个追氮时期处理:拔节期(S1)、拔节期+开花期(S2),研究了氮肥运筹对晚播冬小麦氮素和干物质积累与转运及氮肥利用率的影响。结果表明:拔节期追施氮肥(S1)条件下,在225 kg/hm2(N2)基础上增施25%氮肥(N3)对开花期氮素积累总量和营养器官氮素转运量无显著影响;拔节期+开花期追施氮肥(S2)条件下,随施氮量增加,开花期氮素积累总量和花后营养器官氮素转运量升高;S2较S1显著提高成熟期籽粒及营养器官氮素积累量、花后籽粒氮素积累量及其对籽粒氮素积累的贡献率。同一施氮量条件下,S2较S1提高了成熟期的干物质积累量、开花至成熟阶段干物质积累强度和花后籽粒干物质积累量。同一追氮时期条件下,籽粒产量N2与N3无显著差异,氮肥偏生产力随施氮量增加而降低;同一施氮量条件下,S2较S1提高了晚播冬小麦的籽粒产量和氮肥吸收利用率。拔节期+开花期追施氮肥,总施氮量225kg/hm2为有利于实现晚播冬小麦高产和高效的最优氮肥运筹模式。     

Effects of variations in soil water potential,depth of N placement,and cultivar on postanthesis N uptake by wheat

This study was conducted to investigate the influence of soil water potential, depth of N placement, timing, and cultivar on uptake of a small dose of labeled N applied after anthesis by wheat (Triticum aestivum L.) Understanding postanthesis N accumulation should allow better control of grain protein concentration through proper manipulation of inputs. Two hard, red spring-wheat cultivars were planted in early and late fall each yr of a 2-yr field experiment. Less than 1 kg N ha^–1 as K ¹⁵NO₃ was injected into the soil at two depths: shallow (0.05 to 0.08 m) and deep (0.15 to 0.18 m). In both years an irrigation was applied at anthesis, and injections of labeled N were timed 4, 12, and 20 days after anthesis (DAA). Soil water potential was estimated at the time of injection. Mean recovery of ¹⁵N in grain and straw was 57% of the ¹⁵N applied. Recovery did not differ between the high-protein (Yecora Rojo) and the low-protein (Anza or Yolo) cultivars. Mean recovery from deep placement was 60% versus only 54% from shallow placement (p < 0.01). Delaying the time of injection decreased mean recovery significantly from 58% at 4 DAA to 54% at 20 DAA. This decrease was most pronounced in the shallow placement, where soil drying was most severe. Regressions of recovery on soil water potential of individual cultivar x yr x planting x depth treatments were significant only under the driest conditions. Stepwise regression of ¹⁵N recovery on soil water potential and yield parameters using data from all treatments of both years resulted in an equation including soil water potential and N yield, with a multiple correlation coefficient of 0.64. The translocation of ¹⁵N to grain was higher (0.89) than the nitrogen harvest index (0.69), and showed a highly significant increase with increase in DAA. This experiment indicates that the N uptake capacity of wheat remains reasonably constant between 4 and 20 DAA unless soil drying is severe.     

Seasonal patterns of growth and nitrogen fixation in field-grown pea

The seasonal patterns of growth and symbiotic N₂ fixation under field conditions were studied by growth analysis and use of¹⁵N-labelled fertilizer in a determinate pea cultivar (Pisum sativum L.) grown for harvest at the dry seed stage. The patterns of fertilizer N-uptake were almost identical in pea and barley (the non-fixing reference crop), but more fertilizer-N was recovered in barley than in pea. The estimated rate of N₂ fixation in pea gradually increased during the pre-flowering and flowering growth stages and reached a maximum of 10 kg N fixed per ha per day nine to ten weeks after seedling emergence. This was the time of early pod-development (flat pod growth stage) and also the time for maximum crop growth rate and maximum green leaf area index. A steep drop in N₂ fixation rate occurred during the following week. This drop was simultaneous with lodging of the crop, pod-filling (round pod growth stage) and the initiation of mobilization of nitrogen from vegetative organs. The application of fertilizer-N inhibited the rate of N₂ fixation only during that period of growth, when the main part of fertilizer-N was taken up and shortly after. Total accumulation of fixed nitrogen was estimated to be 244, 238 and 213 kg N ha⁻¹ in pea supplied with nil, 25 or 50 kg NO ₃ ⁻ −N ha⁻¹, respectively. About one-fourth of total N₂ fixation was carried out during preflowering, one fourth during the two weeks of flowering and the remainder during post-flowering. About 55% of the amount of N present in pods at maturity was estimated to be derived from mobilization of N from vegetative organs. “Starter” N (25 or 50 kg NO ₃ ⁻ −N ha⁻¹) did not significantly influence either dry matter and nitrogen accumulation or the development of leaf area. Neither root length and root biomass determined 8 weeks after seedling emergence nor the yield of seed dry matter and nitrogen at maturity were influenced by fertilizer application.     

Large-scale cultivation of adventitious roots of Echinacea purpurea in airlift bioreactors for the production of chichoric acid, chlorogenic acid and caftaric acid

Adventitious roots of Echinacea purpurea were cultured in airlift bioreactors (20 l, 500 l balloon-type, bubble bioreactors and 1,000 l drum-type bubble bioreactor) using Murashige and Skoog (MS) medium with 2 mg indole butyric acid l⁻¹ and 50 g sucrose l⁻¹ for the production of chichoric acid, chlorogenic acid and caftaric acid. In the 20 l bioreactor (containing 14 l MS medium) a maximum yield of 11 g dry biomass l⁻¹ was achieved after 60 days. However, the amount of total phenolics (57 mg g⁻¹ DW), flavonoids (34 mg g⁻¹ DW) and caffeic acid derivatives (38 mg g⁻¹ DW) were highest after 50 days. Based on these studies, pilot-scale cultures were established and 3.6 kg and 5.1 kg dry biomass were achieved in the 500 l and 1,000 l bioreactors, respectively. The accumulation of 5 mg chlorogenic acid g⁻¹ DW, 22 mg chichoric acid g⁻¹ DW and 4 mg caftaric acids g⁻¹ DW were achieved with adventitious roots grown in 1,000 l bioreactors.     

Yields and nitrogen nutrition of intercropped maize and ricebean (Vigna umbellata [Thumb.] Ohwi and Ohashi)

Maize (Zea mays L.) and ricebean (Vigna umbellata [Thumb.] Ohwi and Ohashi) were grown in intercrop and monoculture on Tropaqualf soils under rainfed conditions in Northern Thailand yearly from 1983 to 1986. De Wit's replacement design was used to compare intercrops and monocultures with a constant plant density equivalent to 80 000 maize or 160 000 ricebean plants ha⁻¹. Combined nitrogen was applied at varying levels to 200 kg N ha⁻¹. In the final two seasons the intercrop ratio of maize: ricebean was also varied. At the time of maize maturity intercrops yielded upt 49 kg ha⁻¹ more N in the above ground plant parts than the best monoculture. Dry matter, grain and nitrogen yield of maize and ricebean in intercrop relative to their monoculture yields (RY, relative yield) were significantly greater than their respective share of the plant population. Relative yield totals (RYT) for grain, dry matter and nitrogen were always greater than 1. Nitrogen uptake per maize plant increased with progressive replacement of maize by ricebean plants. This increase was similar to that obtained by applying combined N. Available soil nitrogen tended to decrease with increasing maize:ricebean ratio. Increasing the maize:ricebean ratio increased the % of nitrogen derived from fixation in ricebean, the increase being equivalent to that obtained by decreasing combined nitrogen application. Approximately the same amount of fertilizer and soil nitrogen was taken up by maize plus ricebean in intercrop as the maize monoculture. The results suggest that the improved nitrogen economy of the intercrop resulted from the strong competitiveness of maize in the use of mineral nitrogen and the enhancement of nitrogen fixation in intercropped ricebean which made it less dependent on the depleted pool of soil nitrogen.     

Effects of macro elements and nitrogen source on adventitious root growth and ginsenoside production in ginseng (Panax ginseng C. A. Meyer)

We investigated the effects on ginseng adventitious root growth and ginsenoside production when macro-element concentrations and nitrogen source were manipulated in the culture media. Biomass growth was greatest in the medium supplemented with 0.5-strength NH₄PO₃, whereas ginsenoside accumulation was highest (9.90 mg g^-1 DW) in the absence of NH₄PO₃. At levels of 1.0-strength KNO₃, root growth was maximum, but a 2.0 strength of KNO₃ led to the greatest ginsenoside content (9.85 mg g^-l). High concentrations of MgSO₄ were most favorable for both root growth and ginsenoside accumulation (up to 8.89 mg g^-1 DW). Root growth and ginsenoside content also increased in proportion to the concentration of CaCI₂ in the medium, with the greatest accumulation of ginsenoside (8.91 mg g^-1 DW) occurring at a 2.0 strength. The NH₄/NO₃ ^-- ratio also influenced adventitious root growth and ginsenoside production; both parameters were greater when the NO₃ ^- concentration was higher than that of NH₄ ⁺. Maximum root growth was achieved at an NH₄ ⁺/NO₃ ^- ratio of 7.19/18.50, while ginsenoside production was greatest (83.37 mg L^-1) when NO₃ ^- was used as the sole N source.     

高温下干旱和渍水对冬小麦花后旗叶光合特性和物质转运的影响

以蛋白质含量不同的两个冬小麦品种扬麦9号和豫麦34为材料,研究了不同温度和水分条件下小麦花后旗叶光合特性的变化、营养器官花前贮藏干物质和氮素转运特征及其与籽粒产量和品质形成的关系.结果表明,高温及干旱和渍水均明显降低了旗叶光合速率和叶绿素含量（SPAD值）,但高温下干旱和渍水对光合作用的影响加重.小麦营养器官花前贮藏干物质、氮素转运量和转运率在适温下表现为干旱>对照>渍水,高温下则表现为对照>干旱>渍水.适温下花后同化物积累量表现为对照>渍水>干旱,高温下则表现为对照>干旱>渍水.花后氮素积累量在适温和高温下均表现为对照>渍水>干旱.籽粒淀粉含量以适温适宜水分处理最高,高温渍水下最低;蛋白质含量以高温干旱下最高,适温渍水下最低.温度和水分逆境下小麦粒质量和淀粉含量的降低与花后较低的光合能力及干物质积累有关,而蛋白质含量则与花前贮藏氮素的转运量和转运率有关.     

Role of cytokinin in enhanced productivity of maize supplied with NH4 + and NO3 −

Supplying both N forms (NH₄ ⁺+NO₃ ⁻) to the maize (Zea mays L.) plant can optimize productivity by enhancing reproductive development. However, the physiological factors responsible for this enhancement have not been elucidated, and may include the supply of cytokinin, a growth-regulating substance. Therefore, field and gravel hydroponic studies were conducted to examine the effect of N form (NH₄ ⁺+NO₃ ⁻ versus predominantly NO₃ ⁻) and exogenous cytokinin treatment (six foliar applications of 22 μM 6-benzylaminopurine (BAP) during vegetative growth versus untreated) on productivity and yield of maize. For untreated plants, NH₄ ⁺+NO₃ ⁻ nutrition increased grain yield by 11% and whole shoot N content by 6% compared with predominantly NO₃ ⁻. Cytokinin application to NO₃ ⁻-grown field plants increased grain yield to that of NH₄ ⁺+NO₃ ⁻-grown plants, which was the result of enhanced dry matter partitioning to the grain and decreased kernel abortion. Likewise, hydroponically grown maize supplied with NH₄ ⁺+NO₃ ⁻ doubled anthesis earshoot weight, and enhanced the partitioning of dry matter to the shoot. NH₄ ⁺+NO₃ ⁻ nutrition also increased earshoot N content by 200%, and whole shoot N accumulation by 25%. During vegetative growth, NH₄ ⁺+NO₃ ⁻ plants had higher concentrations of endogenous cytokinins zeatin and zeatin riboside in root tips than NO₃ ⁻-grown plants. Based on these data, we suggest that the enhanced earshoot and grain production of plants supplied with NH₄ ⁺+NO₃ ⁻ may be partly associated with an increased endogenous cytokinin supply.     

基于植株碳流的水稻籽粒淀粉积累模拟模型

通过解析水稻(Oryza sativa)植株碳素积累和转运的动态规律及其与环境因子和基因型之间的定量关系, 构建基于植株碳流动态的水稻籽粒淀粉积累模拟模型。水稻籽粒中的淀粉积累速率取决于库限制下的淀粉积累速率和源限制下的可获取碳源。库限制下的淀粉积累速率是潜在淀粉积累速率及温度、水分、氮素、淀粉合成能力等因子综合影响的结果; 源限制下的可获取碳源取决于花后光合器官生产的即时光合产物和营养器官向籽粒转运的储存光合产物。花后植株即时光合产物随花后生长度日呈对数递减。花后营养器官向籽粒转运的储存光合产物又分为叶片和茎中积累碳素的转运。利用不同栽培条件下的独立田间试验资料对籽粒淀粉积累的动态模型进行了检验, 结果显示籽粒淀粉积累量和含量的模拟值和观测值之间的根均方差均值分别为3.61%和4.51%, 决定系数分别为0.994和0.959, 表明该模型对不同栽培条件下的水稻单籽粒淀粉积累量和含量具有较好的预测性, 为水稻生产中籽粒淀粉指标的动态预测和管理调控提供了量化工具。     

烯效唑干拌种对小麦氮素积累和运转及籽粒蛋白质品质的影响

通过田间试验,研究了不同烯效唑干拌种剂量对3个不同筋力小麦品种植株氮素积累、运转和籽粒蛋白质品质的影响,结果表明,基因型、环境及烯效唑处理对小麦品质的影响效应依次减小,且均达到了极显著水平,但三者的互作效应较小。烯效唑处理后提高了不同生态点下不同小麦品种籽粒蛋白质含量和产量,处理后的面筋含量和沉淀值增加,面团形成时间和稳定时间延长;干拌种增加了开花期各营养器官中的氮素含量和单株氮素积累量,花后氮素总转移量、总转移率及其对籽粒氮的贡献率极显著提高,且处理后旗叶中可溶性蛋白质含量在花后15 d内均显著高于对照;对籽粒中氮含量而言,烯效唑处理后提高了灌浆初期籽粒中的非蛋白氮含量,花后5—20 d内均高于对照,灌浆期间籽粒蛋白氮含量均高于对照,因而处理后的粗蛋白质含量变化动态特点为谷底高、回升快。研究认为,烯效唑处理如同基因、环境一样独立影响小麦籽粒品质,而烯效唑处理后提高了开花初期旗叶中的可溶性蛋白质含量和花前营养器官中氮素含量及花后氮素转运量,可能是其提高籽粒非蛋白氮含量、促进籽粒蛋白质含量增加和蛋白质质量提高的重要原因之一,烯效唑干拌种对小麦籽粒蛋白质品质的改善具有广适性。     

Fate and effects on yield components of extra applications of nitrogen on spring wheat (Triticum aestivum L.) grown in solution culture

Spring wheat (Triticum aestivum L.) was grown with daily additions of nitrate-N. The relative addition rate of nitrate-N was decreased stepwise, and after 125 days of growth, 58 mg N plant^-1 had been introduced. The fate and effect of an extra addition of nitrate (20 mg N plant^-1) at six different times during the ontogeny (37, 54, 66, 79, 94 and 108 days from sowing) on grain yield and grain protein concentration was investigated. The plants absorbed all or most of the extra nitrate at all stages of development evaluated. Dry matter production of both aerial vegetative parts and grains, but not roots, generally increased as a result of the extra nitrate addition. The increase in grain dry matter was mainly an effect of an increased number of grains per plant. Extra nitrate applications had large effects on grain nitrogen content at all stages, but the effect on main shoot and tiller ears varied depending on the time of application. Early applications, i.e. before anthesis, mainly led to increased yield with unchanged protein concentration whereas late applications also led to increased grain protein concentration. The largest effect on grain nitrogen concentration (25–30% increase) was obtained when the extra nitrate was applied late after sowing, i.e. less than four weeks before final harvest. As the extra dose of nitrate was labelled with ¹⁵N, it was possible to follow the movement of the extra nitrogen addition within the plant. Samples were taken at one and five days after ¹⁵N-addition and at final harvest. There were differences in the movement of ¹⁵N depending on when it was introduced. Generally, net movement of the ¹⁵N-labelled N into the grain increased with age at application until 94 days after sowing when a maximum of 90% of the added ¹⁵N ended up in the grain.Abbreviations RN Relative increase in nitrogen content - RA Relative nitrogen addition rate - RG Relative growth rate - N nitrogen     

Nitrogen uptake and use of two contrasting maize hybrids differing in leaf senescence

In eastern Canada, the use of fertilizer N has been identified as the most energy-consuming component of maize (Zea mays L.) grain production. As the economic and environmental costs of excessive N fertilization rise, there is an increased emphasis on selection of hybrids with greater N use efficiency (NUE; defined as the ratio of the amount of ¹⁵N recovered in grain or stover dry matter to the amount of fertilizer ¹⁵N applied to the soil in this study). Using an ¹⁵N-labelling approach, a field study was conducted on a tile-drained Brandon loam soil (Typic Endoaquoll) on the Central Experimental Farm at Ottawa, Canada (45°22 N, 75°43 W) in 1993 and 1994. Fertilizer N uptake and partitioning within the plant in relation to dry matter changes were monitored during development of a current stay-green maize hybrid and an older early-senescing hybrid grown with three fertilizer N levels (0, 100, 200 kg N ha^-1). Dry matter, N concentration and¹⁵ N atom% enrichment of plant components were determined at five growth stages. The current stay-green hybrid, Pioneer 3902 had greater NUE than the old early-senescing hybrid, Pride 5, which was associated with 24% more dry matter production and 20% more N uptake during grain fill for Pioneer 3902. There was no indication of greater allocation of N to the grain in Pioneer 3902. Our data suggest that prolonged maintenance of green leaf area for photosynthate production during grain fill and the ability to take up available soil N later in grain filling are characteristics of maize hybrids with greater NUE.     

Assessing post-anthesis nitrogen uptake, distribution and utilisation in grain protein synthesis in barley (Hordeum vulgare L.) using 15N fertiliser and 15N proteinogenic and non-proteinogenic amino acids

We investigated the effects of nitrogen (N) availability during the vegetative phase on (a) post‐anthesis N uptake and (b) its translocation into ears in barley plants grown in a greenhouse at two levels of N: low (50 mg N kg^?1 sand) and optimal N supply (150 mg N kg^?1 sand). Plants in the two N treatments were fertilised with the same amount of labelled ¹⁵N [50 mg ¹⁵N kg^?1 sand at 10% ¹⁵N_exc (N_excess, i.e. N_exc, is defined as the abundance of enriched stable isotope minus the natural abundance of the isotope) applied as ¹⁵NH₄¹⁵NO₃] 10 days after anthesis (daa). In a separate experiment, the uptake and transport into ears of proteinogenic and non‐proteinogenic amino acids were studied to determine whether a relationship exists between amino acid transport into ears and their proteinogenic nature. Plants were fed with either ¹⁵N‐α‐alanine, a proteinogenic amino acid, or ¹⁵N‐α‐aminoisobutyric acid, a non‐proteinogenic amino acid. Both these amino acids were labelled at 95.6% ¹⁵N_exc. Results showed that N accumulations in stems, leaves and especially in ears were correlated with their dry matter (dm) weights. The application of 150 mg N kg^?1 sand significantly increased plant dm weight and total N accumulation in plants. During their filling period, ears absorbed N from both external (growth substrate) and internal (stored N in plants) sources. Nitrogen concentration in ears was higher in optimal N‐fed plants than in low N‐fed plants until 10 daa, but from 21 to 35 daa, differences were not detected. Conversely, ¹⁵N_exc in ears, leaves and stems was higher in low N‐fed plants than in optimal N‐fed plants. Ears acted as strong sink organ for the post‐anthesis N taken up from the soil independently of pre‐anthesis N nutrition: on average, 87% of the N taken up from the soil after anthesis was translocated and accumulated in ears. Low N‐fed plants continued to take up N from the post‐anthesis N fertiliser during the later grain‐filling period. The increase of pre‐anthesis N supply rate led to a decrease in the contribution of nitrogen derived from post‐anthesis ¹⁵N‐labelled fertiliser (N_dff) to total N in all aboveground organs, especially in ears where 44% and 22% of total N originated from post‐anthesis N uptake in low N‐fed and optimal N‐fed plants, respectively. The experiment with labelled amino acids showed that there was greater transport of proteinogenic amino acid into the ear (50% of total ¹⁵N) than non‐proteinogenic amino acid (39%). However, this transport of the non‐proteinogenic amino acids into ear suggested that the transport of N compounds from source (leaves) to sink organs (ear) might not be intrinsically regulated by their ability to be incorporated into storage protein of ears.     

Low concentrations of ammonium inhibit specific nodulation (nodule number g-1 root DW) in soybean (Glycine max [L.] Merr.)

Experiments on peas (Gulden and Vessey, 1997) have indicated that NH ₄ ⁺ stimulates both whole plant (nodules plant^-1) and specific nodulation (nodules g^-1 root DW). The effect of low concentrations of NH ₄ ⁺ on the soybean/Bradyrhizobium symbiosis is unknown. The objectives of the current study were to determine the immediate and residual effects of NH ₄ ⁺ on nodulation and N₂ fixation in soybean (Glycine max [L.] Merr.) in sand culture. Soybean (cv. Maple Ridge) were exposed to 0.0, 0.5, 1.0 and 2.0 mM of ¹⁵N-labelled (NH₄)₂SO₄ for 28 days after inoculation (DAI). From 29 to 56 DAI the plants were grown on NH ₄ ⁺ -free nutrient solution. Plants were harvested at 7, 14, 21, 28 and 56 DAI for root, shoot and nodule dry weight (DW), total N content, nodule counts and ¹⁵N enrichment of plant composites. Nitrogenase activity was measured by gas exchange at 28 DAI. The plants in the control (0.0 mM NH ₄ ⁺ ) treatment had consistently lower relative growth rates than the plants in the NH ₄ ⁺ treatments during the first 28 DAI. Plant growth was also less at 2.0 mM NH ₄ ⁺ compared to growth at 0.5 and 1.0 mM NH ₄ ⁺ . At 28 DAI, plants exposed to 0.5 and 1.0 mM NH ₄ ⁺ had significantly more nodules per plant and larger individual nodules than either the NH ₄ ⁺ -free controls or the 2.0 mM NH ₄ ⁺ -supplied plants. However, specific nodulation (nodule number g^-1 root DW) and specific nitrogenase activity (nitrogenase activity g^-1 nodule DW) were on average approximately 286% and 60% higher in the control plants, respectively, than for plants in the NH ₄ ⁺ treatments at 28 DAI. Also at 28 DAI, specific nodule DW (nodule DW g^-1 root DW) were 17, 44 and 53% higher in control plants than plants that had been exposed to 0.5, 1.0 and 2.0 mM NH ₄ ⁺ . At 56 DAI, after an additional 4 weeks of NH ₄ ⁺ -free nutrition, the plants which had previously received 0.5 and 1.0 mM NH ₄ ⁺ still maintained the highest plant DW and N contents, however, specific nodule DW had become similar at 600 mg nodule DW g^-1 root DW among all treatments. It is concluded that NH ₄ ⁺ has a negative effect on the nodulation process in the soybean/Bradyrhizobium symbiosis (as best indicated by the negative effect of NH ₄ ⁺ on specific nodulation). Despite this negative effect on specific nodulation, 0.5 and 1.0 mM NH ₄ ⁺ resulted in higher whole plant nodulation and N₂ fixation due to a compensating, positive effect on overall plant growth (i.e. fewer nodules g^-1 root DW, but much larger roots). Once NH ₄ ⁺ was removed from all treatments, the soybean plants appeared to move toward a consistent level of nodule DW relative to root DW.     

Effects of exogenous ABA and cytokinin on leaf photosynthesis and grain protein accumulation in wheat ears cultured <Emphasis Type="Italic">in vitro</Emphasis>

A detached culture system and steady-state ¹⁵N labeling technique were used to study the effects of exogenous ABA and ZR on photosynthetic characteristics, nitrogen remobilization and the activities of key enzymes for nitrogen metabolism in detached wheat parts during grain protein accumulation. The differences in net photosynthetic rate, chlorophyll content (SPAD value) and soluble protein content in the flag leaves of detached culture system between the treatments of ABA and ZR showed that ABA facilitates the post-anthesis senescence course compared to the ZR treatment. The differences in the changes of ¹⁵N amount in different organs in the detached culture system between the ABA and ZR treatments showed that nitrogen remobilization from vegetative organs to the grain is accelerated by the ABA treatment but is delayed by ZR. The activities of GS and GPT in grains treated with ABA were significantly higher than those with the control treatment at 5 DAC, but reduced significantly compared with control at 11 DAC. The two enzyme activities in grains were reduced significantly by ZR at 5 DAC and increased significantly at 11 DAC, compared with those treated with ABA. The above changes of enzyme activity showed that the ABA treatment hastens amino acid conversion into grains and protein accumulation in grains, whereas the ZR treatment delays these processes. A significant reduction in grain weight with ABA treatment is associated with the reduction of net photosynthesis, chlorophyll content, and soluble protein content in flag leaves. Compared with the control and ZR treatments, a significant increase in grain protein content with the ABA treatment may result from the accelerating effects of ABA on N remobilization, amino acid conversion into grains and protein accumulation in grains.