The objective of this study was to understand the effects of plant spacing on grain yield and root competition in summer maize (Zea mays L.). Maize cultivar Denghai 661 was planted in rectangular tanks (0.54m×0.27m×1.00m) under 27cm (normal) and 6cm (narrow) plant spacing and treated with zero and 7.5g nitrogen (N) per plant. Compared to normal plant spacing, narrow plant spacing generated less root biomass in the 0–20cm zone under both N rates, slight reductions of dry root weight in the 20–40cm and 40–70cm zones at the mid-grain filling stage, and slight variation of dry root weights in the 70–100cm zone during the whole growth period. Narrow plant spacing decreased root reductive activity in all root zones, especially at the grain-filling stage. Grain yield and above-ground biomass were 5.0% and 8.4% lower in the narrow plant spacing than with normal plant spacing, although narrow plant spacing significantly increased N harvest index and N use efficiency in both grain yield and biomass, and higher N translocation rates from vegetative organs. These results indicate that the reductive activity of maize roots in all soil layers and dry weights of shallow roots were significantly decreased under narrow plant spacing conditions, resulting in lower root biomass and yield reduction at maturity. Therefore, a moderately dense sowing is a basis for high yield in summer maize.

Sweet sorghum(Sorghum biocolor L.Moench) is one of the key bio-energy crops.To improve its variety and cultivation method for high yield are of important significance in safeguarding energy supply and securing energy safety.Dry matter per plant and soluble sugar concentration of 30 conventional sweet sorghum varieties were measured under a lower density(15 000 plant/hm2) condition.The results showed that the ranking of direct path coefficient contributed to the SY at maturity were: dry matter and sugar content before heading(DMBH,SCBH),ratio of dry matter after heading(RDMAH) and sugar content after heading(SCAH).The direct path coefficient was similar to their correlation coefficient between DMBH and SY,indicating DMBH was crucial for affecting SY.Since SCBH had a high correlation with DMBH,it then presented a significantly positive effect on SY.SCAH was strongly and negatively correlated with DMBH that its positive effect was smaller compared to the greater negatively indirect effect to SY.RDMAH showed a highly negative interrelationship with SY,mainly because its indirect affects though both SC and DM at heading were larger than its own direct value.Therefore,on the basis of increasing stem sugar contents before and after heading,to obtain high dry matter accumulation before heading is an important breeding target for selecting and breeding sweet sorghum varieties with high sugar yield.

To test the importance of the lower leaves of sweet sorghum to yield of stem sugar.Sweet sorghum Rio was treated with defoliation including cutting half of all leaves,cutting upper 5 leaves and cutting upper 6-10 leaves.All treatments reduced the stem weight,yield of stem sugar,but had no effect on panicle weight and sugar content in stem.Treatment of cutting lower leaves reduced more stem weight and yield of stem sugar than the treatment of upper leaves removal.Thus,our experiment revealed that the lower leaves had more important contribution to sugar yield of sweet sorghum and the panicle had higher competitive advantage to photosynthetic product than stem.To improve stem sugar yield of sweet sorghum,therefore,new cultivars with little panicle and erect upper leaves are suggested in future breeding.

大田条件下采用人工去除分孽的方法,研究了两种种植密度下甜高粱高产品种分蘖特性对糖产量及其干物质生产和氮素吸收利用特性的影响.结果表明,常规种植密度(每公顷75 000株)下,保留分蘖较单茎秆植株对主茎和分蘖茎含糖量影响较小;在较低种植密度(每公顷37 500株)下,保留分蘖植株主茎含糖量明显下降,分蘖茎含糖量与单茎秆植株接近.在不同种植密度条件下,保留分蘖植株较单茎秆植株开花前和开花后茎秆、叶片和穗各器官干物质积累量均显著增加,保留分蘖显著提高了总生物量,达21.9%和81.6%,总糖产量分别增加了17.1%和63.8%.保留分孽植株开花后氮素向茎秆分配比高,同时,保留分蘖植株叶片氮积累及分配比例明显提高,叶片干重占植株总干重比例显著增加,单位氮素糖生产效率下降,但未达显著水平.常规密度和较低密度下分蘖茎糖产量的补偿效应分别可达20%和62%以上,说明常规种植密度下保留分蘖利于增加甜高粱糖产量,并能够达到简化栽培管理和降低经济人力投人的目的.

We conducted an experiment with two maize hybrids (Zea mays L.) to examine the effect of interplant root competition on root growth and to evaluate the impact to total plant performance. Two maize hybrids (Jinhai 5 and Denghai 3719) were grown either with no root competition in their own plot (owners) or as individuals sharing twice the space and nutrients (sharers). Plants were sampled every other week after pollination to track changes in root and shoot biomass. The carbohydrate allocation was smaller in the roots of sharers compared with owners at the pro-phase of grain filling and shoot accumulation was slightly accelerated during this period. However, at the lag phase, the accumulation rate in the shoots of individual plants was distinctly lower than in owners, as a result of earlier root senescence. Overall, shoot mass was reduced by 8% in sharers of both hybrids, while they showed a similar root to shoot ratio compared with the owners. Although the “sharing” treatment was confounded by larger soil spaces, the effects of larger soil volume and interplant root competition were different, and demonstrate that interplant root competition has an inhibitory effect on roots. Maize plants displayed an overcrowding effect (or an escape strategy) by allocating more carbohydrate to the shoots at the expense of the roots when faced with interplant root competition.Key words: Overcrowding effect, interplant root competition, maize (zea mays L.), root discrimination

Sweet sorghum(Sorghum biocolor L.Moench) is one of the leading candidates for cleaner energy crops,characterized by abundant juice and high sugar content in its stalk.As a renewable bioenergy crop,the goal of planting sweet sorghum is to obtain high sugar yield.Therefore,it is necessary for improving sweet sorghum cultivars and cultivation practices to understand sugar yield formation and regulation.In the present paper,we summarized the progress made in research on the sugar components,distribution,accumulation and regulation including sowing time,mineral nutrition,cultivation and planting density,source sink alteration in stem of sweet sorghum,analyzed the existing problems and viewd the research focuses in the future..

Corn(Zea mays L.) plant density is known as a major factor determining the degree of competition between plants. Modern corn hybrids have several responses to light decline at large densities,such as keeping a erectophile plant type and elongating the maintenance of green leaves. Some researchers have pointed out that modern hybrids showed high levels in root vigor,ATPase activity,and soluble protein contents as well as the deep-vertical but narrow-horizontal distribution of roots in soils. However,the root competitive ability among cultivars that released in different years is little known. Denghai 3719 created a yield record of 21 t ha-1 in 2005. To understand the root spatial distribution and competitive ability of Denghai 3719,an experiment was conducted in rectangle tanks for comparing the roots characteristics and the biomass of aboveground part between Denghai 3719 and Jinhai 5(a hybrid widely cultivated in China as control). The cultivars were planted in two models,i.e.,two plants per tank with the same cultivar and with each plant from both cultivars. Each model treated with two nitrogen levels(0 and 7.5 g N plant-1). Under the nitrogen application condition,the two cultivars had a distinct biomass accumulation,but their root/shoot ratios were similar. Denghai 3719 took on superiority over Jinhai 5 both in the proportion and root vigor of shallow(0–20 cm) roots,as a result of an increase of nitrogen accumulation by 6% and biomass by 8% under competition,respectively. Under nitrogen deficiency,root/shoot ratio increased in both hybrids than that under nitrogen application,with Denghai 3719 and Jinhai 5 increased by 4% and 21%,respectively. Owing to the inferiority of Denghai 3719 over Jinhai 5 in respects of root/shoot ratio,and the proportion and root vigor of shallow(0–20 cm) roots under nitrogen deficiency,Denghai 3719 had a decrease in nitrogen accumulation by 8% and in biomass by 2% under competition,respectively. The results indicate that Denghai 3719 is more adapted to nitrogen-efficient environment in view of root distribution and root vigor.

【Objective】The diference of soluble sugar yield between diferent sweet sorghum varieties was discussed to reveal the relationship between sugar yield(SY) with dry matter production and nitrogen utilization of different sweet sorghum varieties.【Method】Thirty sweet sorghum varieties were randomly cultivated at a low density of 15 000 plant/ha.Clustering analysis,correlation analysis and pass analysis were used to analyse dry matter(including stem,leaf and sheath,ear) production per plant at both heading and maturing stages,as well as nitrogen utilization among different organs at maturing stage.【Result】Through hierarchical cluster analysis of sugar yield per plant,the tested sweet sorghum varieties were sorted into 3 types,i.e.,low-SY with short duration,mediate-SY with short duration,high-SY with mediate duration and high-SY with long duration,with high-SY type accounted for only 23.3% of the tested varieties.Results showed that,nitrogen content was significantly lower in high-SY type,especially in stalk and leaf and sheath,as compared to the low and mediate types,which benefited its high sugar content to some extent because of the reverse correlation between sugar content and nitrogen content(-0.592**).As for nitrogen partition between various organs for high-SY type,largest proportion of nitrogen was distributed to stalk preferentially,followed with leaf and sheath,with panicle took the least participation in nitrogen accumulation,with a similar tendency of the dry matter partition of various organs,that contributed to obtaining high stalk biomass and ample leaf area to providing enough photosynthetic assimilate.Results of both correlation analysis and path analysis indicated that the correlation coefficients between SY with the dry matter accumulation and nitrogen productivity efficiency for SY was significantly positive;further these two items exerted direct effect on SY.However,nitrogen content of whole plant had a negative effect on SY,i.e.,the higher nitrogen content the lower SY obtained.【Conclusion】Increased dry matter productivity especially in stalk with a relatively high sugar concentration,meanwhile,sustaining a relative low plant nitrogen level and high nitrogen productivity efficiency for sugar yield are the primary characteristics of sweet sorghum varieties aimed for producing high sugar yield.

【目的】探讨不同甜高粱品种单株糖产量差异及其与含糖量和干物质生产特性的关系。【方法】在充分发挥个体生产潜力条件下（大田种植密度15000株•ha-1），研究了国内外30个甜高粱品种可溶性糖含量和抽穗前后干物质生产特性。【结果】相关分析和通径分析表明，各时期单株干物重和含糖量均与糖产量呈极显著正相关，且前者对糖产量的直接和间接效应都较高。各指标对成熟期糖产量均起正直接作用，由大到小排序为：抽穗期干物重、抽穗期含糖量、抽穗后干物重所占比例和抽穗后含糖量。其中，抽穗期干物重对糖产量的直接通径系数（1.0036）与其简单相关系数（0.9326**）接近，说明该性状是影响糖产量的重要因素；抽穗期含糖量与此期干物重达极显著正相关，也与最终糖产量密切相关（r=0.6971**）；抽穗后含糖量增加值与糖产量负相关不显著，但直接效应较大，亦是影响糖产量的主要性状指标；抽穗后干物重所占比例与糖产量则极显著负相关（r=-0.5304**），主要是其通过抽穗期含糖量和干物重的负间接效应大于自身正直接效应所致。【结论】奠定抽穗前高糖产量基础，增强抽穗后糖分积累能力，重视提高整个生育时期干物质生产能力，是甜高粱高糖品种选育及高产栽培的重要目标。