两种不同根系类型湿地植物的根系生长

实验设计了一个水培系统，利用生活污水培养，对4种“须根型”植物美人蕉、风车草、象草和香根草和4种根茎型植物菖蒲、水鬼蕉、芦苇和水烛的根系生长进行比较研究。该系统由用于盛污水的塑料桶（顶部直径36．5cm，底部直径30．Ocm，高34．5cm）和用于固定植物于水面的泡沫板构成。每桶种植1株植物，每种种5株。水培至10周时，须根型植物的平均根数达到1349条／株，而根茎型植物的平均根数只有549条／株。实验结束（水培第21周）时，须根型植物的平均根生物量为11．3g／株，根茎型植物的平均根生物量为7．4g／株。须根型植物根系中，d〈1mm的细根生物量占根系总生物量的51．9％，而根茎型植物d〈1mm的细根的生物量只占25．1％。根茎型植物的根生物量与地上生物量的比值为0．2，显著高于须根型湿地植物（0．1）。须根型湿地植物的根系表面积（6933cm^2／株）极显著地高于根茎型湿地植物（1897cm^2／株）。根茎型湿地植物根的平均寿命（46．6d）较须根型湿地植物根的平均寿命（34．8d）长。美人蕉的平均根数达1871条／株，根表面积达到22832cm^2／株，远较其他种高。     

Effect of mutual shading on the emergence of nodal roots and the root/shoot ratio of maize

The effect of mutual shading on the root/shoot ratio and on the number of nodal roots of maize was studied. Plants of two varieties (Dea and LG2281) were grown in individual pots of 9 L, at three plant densities: 7.5, 11 and 15 plants m^–2. A control experiment was carried out in order to study if root growth was affected by the small size of the pots. Maize plants (cv Dea) were grown at a low plant density (7.5 plants m^–2) in pots of two different volumes (9 and 25 L respectively). In both experiments plants were watered every two hours with a nutrient solution. Some plants were sampled at five dates in the main experiment and the following data were recorded: foliar stage; root, stem and leaf dry weight; number of root primordia and number of emerged roots per phytomer. The final sampling date occurred at silking.Results of the control experiment showed that the root biomass was lower in small pots but the number of nodal roots per phytomer was not affected.Results of the main experiment showed that the total plant biomass and the root/shoot ratio were lower at high plant density. The number of emerged roots was strongly reduced on the upper phytomer (P₈). This reduction was mainly due to a lower percentage of root primordia which elongated. A proposed interpretation is that the number of roots which emerge on upper phytomers is controlled by carbohydrate availability.     

疏叶骆驼刺根系对土壤异质性和种间竞争的响应

近年来, 植物根系对土壤异质性的响应和植物根系之间的相互作用一直是研究的热点。过去的研究主要是针对一年生短命植物进行的, 而且多是在人工控制的温室条件下进行的。而对于多年生植物根系对养分异质性和竞争的综合作用研究很少。该文对塔里木盆地南缘多年生植物疏叶骆驼刺(Alhagi sparsifolia)根系生长对养分异质性和竞争条件的响应途径与适应策略进行了研究, 结果表明: (1)在无竞争的条件下, 疏叶骆驼刺根系优先向空间大的地方生长, 即使另一侧有养分斑块存在, 其根系也向着空间大的一侧生长; (2)在有竞争的条件下, 疏叶骆驼刺根系生长依然是优先占领空间大的一侧, 但是竞争者的存在抑制了疏叶骆驼刺的生长, 导致其枝叶生物量和根系生物量都明显减少(p < 0.01), 而养分斑块的存在促进了疏叶骆驼刺根系的生长; (3)疏叶骆驼刺根系的生长不仅需要养分, 也需要足够的空间, 空间比养分更重要; (4)有竞争者存在的时候, 两株植物的根系都先长向靠近竞争者一侧的空间, 即先占据“共有空间”。研究结果对理解植物根系觅食行为和植物对环境的适应策略有重要意义。     

岷江干旱河谷25种植物一年生植株根系功能性状及相互关系

相同条件下相同生长期的植物根系生长与适应策略及其差异性还不清楚。因此,采集岷江干旱河谷地区25种乡土植物(木本15/草本10种)的种子于2009年3月播种在同一干旱环境中,9月测定了1年生植株的最大根深(RDmax)、根幅(RW)与根生物量(RB),计算了总根长(TRL)、比根长(SRL)及细/粗根生物量比(RBf/c),分析了它们之间的关系,进行了根系功能组划分。结果表明:1)25种植物1年生植株RDmax与RW变异较小,总变异率为14.9%和20.7%;TRL和SRL变异相对较大,分别为28.5%和34.7%,草本植物SRL明显大于木本植物;RB和RBf/c种间变异较大,总变异率分别为50.1%和70.5%;2)25种植物的RDmax、RW、RB和TRL间呈显著正相关关系,表明根系较深的物种RW较大,TRL和RB也较高;SRL与RDmax呈极显著负相关关系,与RBf/c呈极显著正相关关系,表明根系垂直分布较浅的物种细根发达,SRL较大;3)主成分分析显示,25种植物可分为3个功能组:第1组具有较大RDmax、RW和RB,资源利用持续时间较长;第2组具有较大TRL、SRL和RBf/c,资源利用效率较高;第3组根系功能性状没有一致的突出特点,可能通过降低自身生理机能适应生存条件。综合分析表明,岷江干旱河谷区25种植物1年生植株根系的功能性状变异明显,可塑性大,历经长期自然选择压力而形成了不同的环境适应策略,但生长型并不必然表达出1年生植株根系功能性状的差异性。     

Penetration of very strong soils by seedling roots of different plant species

The abilities of seedling roots of twenty-two plant species to penetrate a strong growth medium were compared under controlled conditions. Seedlings were grown for 10 days in compression chambers filled with siliceous sandy soil at 0.2 kg kg^–1 water content and mean penetrometer resistance of 4.2 MPa. Root elongation and thickening were measured after growth. The results show that soil strength reduced the elongation of roots of all plant species by over 90% and caused the diameters of the roots to increase compared with control plants grown in vermiculite (0 MPa resistance).Differences in both root elongation and root diameter were observed among plant species. Generally, the roots of dicotyledons (with large diameters) penetrated the strong medium more than graminaceous monocotyledons (with smaller diameters). There was a significant positive correlation (r=0.78, p<0.05) between root diameter and elongation over all the species in the stressed plants. The species were ranked according to the relative root elongation and relative root thickening. Based on this ranking, lupin (Lupinus angustifolius), medic (Medicago scutelata) and faba bean (Vicia faba) were the species with the greatest thickening and elongation while wheat (Triticum aestivum), rhodesgrass (Chloris gayana) and barley (Hordeum vulgare) had the least. The weight of the seeds did not seem to influence either the thickening or elongation of the roots.     

指数施肥对楸树无性系生物量分配和根系形态的影响

为探求楸树不同无性系生物量分配和根系形态的差异,2011年3-8月在甘肃省天水市小陇山林科所,以2年生楸树无性系1-4、7080和015-1组培苗为试验材料,设置了CK、6、10、14 g尿素/株4个处理,研究指数施肥对楸树无性系生物量分配和根系形态的影响.结果表明:(1)同一无性系中,10 g尿素/株的根、茎、叶生物量及总生物量、根长、根表面积、根体积和根平均直径均高于其它处理.无性系015-1的生物量和根系形态参数整体上高于无性系1-4和7080.无性系015-1在10 g尿素/株的根、茎、叶生物量及总生物量分别为89.44 g、61.30 g、79.97 g、230.71 g,是CK的1.48、1.52、2.09、1.66倍;根长、根表面积和根体积为22667 cm、6260 cm2、578.14 cm3,是CK的1.94、1.54、2.43倍.(2)指数施肥和无性系的遗传差异明显影响楸树不同无性系生物量的分配格局.适量施氮明显促进3个楸树无性系生物量的积累,而氮素缺乏或过量均不利于生物量的积累.8月同一无性系的根冠比均随施氮量的增加而降低;同一处理下无性系7080的根冠比高于无性系1-4和015-l.无性系1-4和7080的生物量主要向叶和茎分配,而无性系015-1主要向叶分配.(3)指数施肥在6月和7月主要促进细根根长和根表面积的增加;指数施肥在8月主要促进细根、中等根和粗根体积的急剧增加,分别比7月高达36.88％、124.96％、154.79％.这利于根系在中后期吸收更多养分,从而引起生物量分配格局的变化.(4)生物量参数和根系形态参数关系密切.根生物量、地上生物量、总生物量分别和根长、根表面积、根体积、根平均直径极显著正相关;根冠比和根长、根表面积、根体积、根平均直径极显著负相关;比根长和地上生物量、总生物量显著正相关,和根冠比极显著负相关.     

AM真菌对紫花苜蓿细根生长及其生物量动态特征的影响

细根对植物功能的发挥和土壤碳库及全球碳循环具有重要意义。采用容器法和微根管法于2013年6~10月整个生长季内对紫花苜蓿的细根生物量、生产以及周转规律进行研究。结果表明:(1)紫花苜蓿活细根现存生物量平均值以接种摩西球囊霉(Gm)处理最高(12.46g·m-2),未接种对照最低(7.31g·m-2),并且活细根现存量在9月中旬达到峰值;死细根现存生物量呈先增加后降低再增加的变化趋势,在整个生长过程中未接种处理高于接种处理,接种根内球囊霉(Gi)处理死细根现存平均生物量(3.11g·m-2)又较接种组其他处理低。(2)苜蓿植株细根生长量以接种幼套球囊霉(Ge)处理最大(0.045 mm·cm-2·d-1),接种Gm处理和未接种对照最低(均为0.027mm·cm-2·d-1);而未接菌植株细根死亡量(0.044mm·cm-2·d-1)显著高于接种植株,接种组又以Gi处理最低(0.021mm·cm-2·d-1)。(3)紫花苜蓿在生长季节内细根生产和死亡的高峰分别出现在8月底和10月份,低谷出现在9月底到10月中旬和6月底到8月;接种地表球囊霉(Gv)后细根现存量和年生长量显著高于对照和接种其他菌种处理,细根的周转以对照组最大,而接种Gv和Gm处理较低。研究发现,通过接种丛植菌根真菌可以提高苜蓿细根生物量,降低细根的死亡,增加细根寿命。     

Root development and heavy metal phytoextraction efficiency: comparison of different plant species in the field

Heavy metal phytoextraction is a soil remediation technique which implies the optimal use of plants to remove contamination from soil. Plants must thus be tolerant to heavy metals, adapted to soil and climate characteristics and able to take up large amounts of heavy metals. Their roots must also fit the spatial distribution of pollution. Their different root systems allow plants to adapt to their environment and be more or less efficient in element uptake. To assess the impact of the root system on phytoextraction efficiency in the field, we have studied the uptake and root systems (root length and root size) of various high biomass plants (Brassica juncea, Nicotiana tabacum, Zea mays and Salix viminalis) and one hyperaccumulator (Thlaspi caerulescens) grown in a Zn, Cu and Cd contaminated soil and compared them with total heavy metal distribution in the soil. Changes from year to year have been studied for an annual (Zea mays) and a perennial plant (Salix viminalis) to assess the impact of the climate on root systems and the evolution of efficiency with time and growth. In spite of a small biomass, T. caerulescens was the most efficient plant for Cd and Zn removal because of very high concentrations in the shoots. The second most efficient were plants combining high metal concentrations and high biomass (willows for Cd and Zn and tobacco for Cu and Cd). A large cumulative root density/aboveground biomass ratio (L_A/B), together with a relative larger proportion of fine roots compared to other plants seemed to be additional favourable characteristics for increased heavy metal uptake by T. caerulescens. In general, for all plants correlations were found between L _A/B and heavy metal concentrations in shoots (r=0.758^***, r=0.594^***, r=0.798^*** (P<0.001) for Cd, Cu and Zn concentrations resp.). Differences between years were significant because of variations in climatic conditions for annual plants or because of growth for perennial plants. The plants exhibited also different root distributions along the soil profile: T. caerulescens had a shallow root system and was thus best suited for shallow contamination (0.2 m) whereas maize and willows were the most efficient in colonising the soil at depth and thus more applicable for deep contamination (0.7 m). In the field situation, no plant was able to fit the contamination properly due to heterogeneity in soil contamination. This points out to the importance and the difficulty of choosing plant species according to depth and heterogeneity of localisation of the pollution.     

天山林区六种灌木生物量的建模及其器官分配的适应性

灌木全株生物量估算模型的构建仍存在一定困难,对灌木生物量在器官分配上所体现的适应性研究也不够充分。以天山林区6种常见灌木为研究对象,在天山的东段、中段、西段林区分别设置样地进行群落调查,由此以全株收获法取得6种常见灌木若干标准株的全株、根、枝、叶及各径级根的生物量,将D~2H(地径平方与高度的乘积)与V(冠幅面积与高度的乘积)分别选为估测模型的自变量,通过回归分析法建立了各种灌木全株生物量的最优估算模型,然后比较了此6种灌木全株生物量在营养器官上分配差异以及根系生物量在径级上的分配差异。结果表明:(1)天山林区6种常见灌木中,小檗(Berberis heteropoda Schrenk)、忍冬(Lonicera hispida Pall.ex Roem.et Schuet.)、栒子(Cotoneaster melanocarpus Lodd.)的全株生物量约为8.48—9.01 kg,蔷薇(Rosa spinosissima L.)、绣线菊(Spiraea hypericifolia L.)、方枝柏(Juniperus pseudosabina Fisch.et Mey.)的全株生物量约为2.71—3.20 kg;(2)蔷薇、绣线菊、栒子的全株生物量最优估测模型是以V为自变量的函数,小檗、忍冬、方枝柏的全株生物量最优估测模型是以D~2H为自变量的函数,各模型R~2值均在0.850以上,且在P0.05水平上达到显著,模型模拟结果达到了较高的准确度;(3)6种灌木全株生物量在根、枝上的分配比重差异不显著,仅在叶上的分配比重有差异(P0.05);根系生物量在径级上的分配均呈现随根系径级下降而减少的规律,6种灌木在径级大于2 mm根上的分配比重存在差异(P0.05,径级大于20 mm根为P0.01水平);(4)6种灌木全株生物量在营养器官上的分配差异以及根系生物量在径级上的分配差异均体现了各物种对其生境选择的适应策略。     

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

Fibrous carrot root responses to irrigation and compaction of sandy and organic soils

Field experiments were performed in Southern Finland on fine sand and organic soil in 1990 and 1991 to study carrot roots. Fall ploughed land was loosened by rotary harrowing to a depth of 20 cm or compacted under moist conditions to a depth of 25–30 cm by three passes of adjacent wheel tracks with a tractor weighing 3 Mg, in April were contiguously applied across the plot before seed bed preparation. Sprinkler irrigation (30 mm) was applied to fine sand when moisture in the 0–15 cm range of soil depth was 50% of plant-available water capacity. For root sampling, polyvinyl chloride (PVC) cylinders (30 × 60 cm) were installed in the rows of experimental plots after sowing, and removed at harvest. Six carrot plants were grown in each of in these soil colums in situ in the field.Fine root length and width were quantified by image analysis. Root length density (RLD) per plant was 0.2–1.0 cm cm^-3 in the 0–30 cm range. The fibrous root system of one carrot had total root lengths of 130–150 m in loose fine sand and 180–200 m in compacted fine sand. More roots were observed in irrigated than non-irrigated soils. In the 0–50 cm range of organic soil, 230–250 m of root length were removed from loosened organic soils and 240–300 m from compacted soils. Specific root surface area (surface area divided by dry root weight) of a carrot fibrous root system averaged 1500–2000 cm² g^-1. Root length to weight ratios of 250–350 m g^-1 effectively compare with the ratios of other species.Fibrous root growth was stimulated by soil compaction or irrigation to a depth of 30 cm, in both the fine sand and organic soils, suggesting better soil water supply in compacted than in loosened soils. Soil compaction increased root diameters more in fine sand than it did in organic soil. Most of the root length in loosened soils (fine sand 90%, organic soil 80%) and compacted soils (fine sand 80%, organic soil 75%) was composed of roots with diameters of approximately 0.15 mm. With respect to dry weight, length, surface area and volume of the fibrous root system, all the measurements gave significant resposes to irrigation and soil compaction. Total root volumes in the 0–50 cm of soil were 4.3 cm³ and 9.8 cm³ in loosened fine sand and organic soils, respectively, and 6.7 cm³ and 13.4 cm³ in compacted sand and organic soils, respectively. In fine sand, irrigation increased the volume from 4.8 to 6.3 cm³.     

Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California

This research adds to the limited data on coarse and fine root biomass for blue oak (Quercus douglasii Hook and Arn.), a California deciduous oak species found extensively throughout the interior foothills surrounding the Central Valley. Root systems of six blue oak trees were analyzed using three methods — backhoe excavation, quantitative pits, and soil cores. Coarse root biomass ranged from 7 to 177 kg per tree. Rooting depth for the main root system ranged from 0.5 to 1.5 m, with an average of 70% of excavated root biomass located above 0.5 m. Of the total biomass in excavated central root systems, primary roots (including burls) accounted for 56% and large lateral roots (> 20 mm diameter) accounted for 36%. Data from cores indicated that most biomass outside of the root crown was located in fine roots and that fine root biomass decreased with depth. At surface depths (0–20 cm), small-fine (< 0.5 mm diameter) roots accounted for 71%, large-fine (0.5–2.0 mm) for 25%, and coarse (> 2 mm) for 4% of total root biomass collected with cores. Mean fine root biomass density in the top 50 cm was 0.43 kg m⁻³. Fine root biomass did not change with increasing distance from the trees (up to approximately 5 m). Thus, fine roots were not concentrated under the tree canopies. Our results emphasize the importance of the smallest size class of roots (<0.5 mm), which had both higher N concentration and, in the area outside the central root system, greater biomass than large fine (0.5–2.0 mm) or coarse (> 2.0 mm) roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Alley cropping with Senna siamea in South-western Nigeria: II. Dry matter,total N,and urea-derived N dynamics of the Senna and maize roots

Crop and tree roots are crucial in the nutrient recycling hypotheses related to alley cropping systems. At the same time, they are the least understood components of these systems. The biomass, total N content and urea-derived N content of the Senna and maize roots in a Senna-maize alley cropping system were followed for a period of 1.5 years (1 maize-cowpea rotation followed by 1 maize season) to a depth of 90 cm, after the application of ¹⁵N labeled urea. The highest maize root biomass was found in the 0–10 cm layer and this biomass peaked at 38 and 67 days after planting the 1994 maize (DAP) between the maize rows (112 kg ha^–1, on average) and at 38, 67 and 107 DAP under the maize plants (4101 kg ha^–1, on average). Almost no maize roots were found below 60 cm at any sampling date. Senna root biomass decreased with time in all soil layers (from 512 to 68 kg ha^–1 for the 0–10 cm layer between 0 and 480 DAP). Below 10 cm, at least 62% of the total root biomass consisted of Senna roots and this value increased to 87% between 60 and 90 cm. Although these observations support the existence of a Senna root `safety net' between the alleys which could reduce nutrient leaching losses, the depth of such a net may be limited as the root biomass of the Senna trees in the 60–90 cm layer was below 100 kg ha^–1, equivalent to a root length density of only < 0.05 cm cm^–3. The proportion of maize root N derived from the applied urea (%Ndfu) decreased significantly with time (from 21% at 21 DAP to 8% at 107 DAP), while %Ndfu of the maize roots at the second harvest (480 DAP) was only 0.6%. The %Ndfu of the Senna roots never exceeded 4% at any depth or sampling time, but decreased less rapidly compared to the %Ndfu of the maize roots. The higher %Ndfu of the maize roots indicates that maize is more efficient in retrieving urea-derived N. The differences in dynamics of the %Ndfu also indicate that the turnover of N through the maize roots is much faster than the turnover of N through the Senna roots. The recovery of applied urea-N by the maize roots was highest in the top 0–10 cm of soil and never exceeded 0.4% (at 38 DAP) between the rows and 7.1% (at 67 DAP) under the rows. Total urea N recovery by the maize roots increased from 1.8 to 3.2% during the 1994 maize season, while the Senna roots never recovered more than 0.8% of the applied urea-N at any time during the experimental period. These values are low and signify that the roots of both plants will only marginally affect the total recovery of the applied urea-N. Measurement of the dynamics of the biomass and N content of the maize and Senna roots helps to explain the observed recovery of applied urea-N in the aboveground compartments of the alley cropping system.     

青海省森林细根生物量及其影响因子

细根是植物吸收水分和养分的主要器官。全球变暖背景下,研究森林细根生物量及其环境因子的变化对生态系统碳平衡、碳收支及其贡献率具有重要意义。采用土钻法和室内分析法对青海省森林6个海拔梯度上5种林分类型的细根生物量和土壤理化性质进行测定,并分析了与环境因子之间的相互关系。结果表明:(1)青海省森林0—40 cm土层总细根生物量平均为8.50 t/hm~2,随着海拔梯度的增加先降低后升高,不同海拔梯度细根生物量差异显著(P0.05),最大值出现在2100—2400 m处。(2)5种林分0—40 cm土层总细根生物量为:白桦白杨云杉圆柏山杨,不同林分间细根生物量差异不显著。(3)细根垂直分布随土层深度增加而减少,且70%的细根集中在表层(0—20 cm)。(4)土壤容重深层(20—40 cm)显著大于表层(P0.05),并随海拔梯度逐步增加,且林分间差异较大。(5)全碳(Total carbon, TC)、全氮(Total nitrogen, TN)、全磷(Total phosphorus, TP)含量表层显著高于深层。TC、TN随海拔升高先增后降低,TP则随海拔逐步降低。不同林分间土壤养分差异较明显。(6)结构方程模型分析得到海拔、土层、容重直接影响细根生物量,细根生物量直接影响土壤养分。林分类型通过土壤容重间接影响细根生物量。因此,林分和海拔通过影响土壤微环境而影响到细根生物量及其空间分布格局。     

Above-ground competition does not alter biomass allocated to roots in Abutilon theophrasti

We tested whether plants allocate proportionately less biomass to roots in response to above-ground competition as predicted by optimal partitioning theory. Two population densities of Abutilon theophrasti were achieved by planting one individual per pot and varying spacing among pots so that plants in the two densities experienced the same soil volume but different degrees of canopy overlap. Density did not affect root:shoot ratio, the partitioning of biomass between fine roots and storage roots, fine root length, or root specific length. Plants growing in high density exhibited typical above-ground responses to neighbours, having higher ratios of stem to leaf biomass and greater leaf specific area than those growing in low density. Total root biomass and shoot biomass were highly correlated. However, storage root biomass was more strongly correlated with shoot biomass than was fine-root biomass. Fine-root length was correlated with above-ground biomass only for the small subcanopy plants in crowded populations. Because leaf surface area increased with biomass, the ratio between absorptive root surface area and transpirational leaf surface area declined with plant size, a relationship that could make larger plants more susceptible to drought. We conclude that A. theophrasti does not reallocate biomass from roots to shoots in response to above-ground competition even though much root biomass is apparently involved in storage and not in resource acquisition.     

Carbon (13C) uptake and allocation in pasture plants following field pulse-labelling

The allocation of carbon (C) to plant roots and conversion to soil organic matter is a major determinant of the size of the terrestrial C pool in pastoral ecosystems. The aim was to quantify C allocation to roots in contrasting pastoral ecosystems. Pastures on long-term research sites in Canterbury, New Zealand were pulse-labelled using ¹³CO₂ within portable gas-tight enclosures. Sites included Winchmore (with or without superphosphate fertiliser, and with or without irrigation) and Tara Hills (low, medium or high grazing intensity with continuous or alternating grazing). Separate micro-plots were labelled in late spring, summer and autumn at Winchmore and in spring at Tara Hills. Herbage label ¹³C recoveries were greatest one hour after pulse labelling and declined by 21 days, whereas in roots they were initially lower but generally continued to increase until 21 days. The greatest recoveries of ¹³C in roots, one hour and 21 days after labelling, were in summer and autumn respectively. The proportion of label ¹³C allocated to roots by 21 days was 0.50 in the absence of superphosphate and 0.41 in the superphosphate treatment, and was 0.39, 0.43 and 0.51 respectively in spring, summer and autumn. Irrigation had no significant effect on root allocation. The low stocking rate at Tara Hills, which had the greatest herbage biomass, also had greater total ¹³C, tussock herbage ¹³C and root ¹³C recoveries than the higher stocking rate treatments. Inter-tussock root recovery and allocation of ¹³C to roots increased with increasing stocking rate, whereas tussock root allocation was greatest in the high and least in the medium stocking rate treatment. By 21 days there was a greater inter-tussock and tussock root recovery and lower inter-tussock herbage recovery in the continuous than in the alternating grazing management treatment. The root allocation was generally greater in the continuous than in alternating grazed treatments, except for tussocks one hour after labelling where the reverse was the case. In conclusion the ¹³C pulse labelling showed pasture plants allocate more C to roots with low soil fertility, high grazing intensity, continuous grazing, and in autumn.     

西洋参根残体对自身生长的双重作用

无论在自然生态环境还是在人工农田环境下,植株残体进入土壤后都会对土壤的物理化学性质以及后茬植物的生长产生重要影响。西洋参（Panax quinquefolium L.）为人参属多年生名贵药材,在栽培生产中存在严重的连作障碍问题。为了探明秋后残留在土壤中的须根降解产物对来年植株生长的影响,以及收获后残留在田间的根茬对连作西洋参生长的作用,本实验以3年生西洋参苗为研究对象,采用室内水培试验以及田间盆栽试验,通过添加西洋参根的粉碎物模拟根残体,测定其对西洋参生长的影响。水培试验中全营养液中分别添加0.02 mg/mL、0.1 mg/mL、0.5 mg/mL西洋参根粉碎物,处理后每隔5天测定植株叶片展开情况、株高、冠幅等生长指标。盆栽试验在土壤中添加0.1 mg/g根粉碎物,于栽种后1-2个月测定西洋参叶片展开情况、株高、冠幅等生长指标;水培及盆栽试验均于展叶期、现蕾期、结果期测定地上部及地下部生物量。采用高效液相色谱法（HPLC）测定根围土壤中8种酚酸类化合物的含量。试验结果表明,水培溶液中添加0.02-0.5 mg/mL根残体,可显著抑制西洋参自身地上部分生长,推迟展叶期,结果期生物量降低14.9%-45.0%;对地下部分的影响主要表现为在展叶期显著促进须根生长(p<0.05)。与水培试验相比,盆栽土壤中添加0.1 mg/g根残体同样导致西洋参展叶期推迟;不同的是处理组的地上、地下部及须根的平均生物量均高于对照。另外,添加根残体后盆栽西洋参根围土壤中丁香酸、香草醛、p-香豆酸、阿魏酸等酚酸类化感物质含量下降49.1%-81.4%,但作为逆境信号物质的水杨酸含量升高59.9%。以上结果可以初步确认根残体对西洋参早期生长具有自毒和促进的双重作用,表现为抑制地上部分生长,导致生物量显著下降;同时在生长早期促进须根生长;但在田间环境下,自毒作用可能受根残体降解速度以及土壤对降解产物吸附的影响有所减弱,使促进作用更为明显。     

植物吸收根的增殖和生长与养分变异的关系——臭椿、翠菊、加拿大一枝黄花分根实验的启示

构件理论认为植物根可以相对独立地吸收养分和对所处环境的养分条件做出响应。根据成本-收益理论, 单个根(构件)的生死、生长发育与其吸收的养分收益和自身建造、维持的消耗有关。基于此, 该文提出两个关于吸收根生死条件的假设: 1)当可利用养分低于低临界值, 根死亡在一段时滞(数天到几周)后发生; 2)当可利用养分高于高临界值并持续一段时间, 新的侧根产生。为了检验这两个假设, 用臭椿(Ailanthus altissima)、翠菊(Callistephus chinensis)、加拿大一枝黄花(Solidago canadensis)作实验物种, 设计了温室分根实验。每株植物选3个一级根, 分别引入3个不同养分水平的斑块: 0、20、200 μg N·g^-1。每4天将根暴露并拍照, 查数新根数并测量细根总长度和一级侧根长。由于高养分处理斑块内根的快速生长, 实验在开始后8天或12天结束。结果显示: 除臭椿在0养分处理外, 三物种在各养分处理下都有侧根产生, 总根长均有增加; 臭椿、翠菊、加拿大一枝黄花在不同观测时间和养分水平处理间的侧根数目和总根长差异显著, 而一级侧根长除臭椿外变异均较小; 整个过程中没有根死亡。研究结果部分支持两个假设。本研究还为进一步探究根模块构件增殖、生死过程机制提出新的建议, 即除需要更长的实验时间外, 还应该考虑: 1)多种资源各自及联合对根生长、生死过程的影响; 2)资源斑块和整个根系生长背景的资源丰度对比; 3)根构建和根维持的相对C消耗。     

Root distribution of a Mediterranean shrubland in Portugal

The distribution of roots of an Erica (Erica scoparia and Erica lusitanica) dominated Mediterranean maquis was studied using three different approaches: root counts on trench walls (down to 120 cm), estimation of the maximum rooting depth using an allometric relationship and estimation of fine root biomass and fine root length using soil cores (down to 100 cm). Roots were classified according to diameter (fine, 1.0 mm; small, 1.1–5.0 mm; medium, 5.1–10.0 mm; coarse, >10.0 mm) and species (Erica sp., Pteridium aquilinum, Rubus ulmifolius and Ulex jussiaei). The depth corresponding to 50% of all roots (D ₅₀) was determined by fitting a new model to the cumulative root distribution. Fine roots represented 96% of root counts. Root counts of Erica represented 59%, Ulex 34%, Rubus 6% and Pteridium 1%. Overall root counts showed a D ₅₀ of 26 cm. D ₅₀ was higher for Ulex (40 cm) and Erica (22 cm), than for Pteridium (9 cm) and Rubus (3 cm). D ₅₀ for fine roots was 27 cm, for small roots 11 cm, for medium roots 6 cm and for coarse roots 4 cm. The estimated average maximum rooting depth of the 28 deepest Erica roots was 222 cm. The deepest Erica root was estimated to reach 329 cm. A total of 82% of roots growing deeper than 125 cm were not reaching more than 175 cm. The overall fine root length density ranged from 4.6 cm/cm³ at 10 cm to 0.8 cm/cm³ at 80 cm. The overall fine root biomass ranged from 7.7 mg/cm³ at 10 cm to 0.6 mg/cm³ at 40 cm. D ₅₀ for root biomass was 12 cm and D ₅₀ for root length was 14 cm. Fine root biomass was estimated as 1.6 kg/m² and the respective root length as 18.7 km/m².     

Phosphorus acquisition from soil by white lupin (Lupinus albus L.) and soybean (Glycine max L.), species with contrasting root development

White lupin and soybean have contrasting root morphologies: white lupin develops proteoid or cluster roots, roots with discreet clusters of short, determinate branch roots (rootlets) while soybean develops a more fibrous root system with evenly distributed, longer branch roots. Growth and P acquisition by white lupin and soybean were compared in a soil high in bound, total P, with or without additional inorganic P applied in solution. Additional P increased biomass by 25% and doubled total P in soybean. In contrast, white lupin did not respond to additional P in biomass or total P. However added P decreased cluster development on proteoid roots indicating that white lupin sensed the added P. The reduction in cluster weight per plant was exactly countered by an increase in dry weight of other roots. Soybean root development responded to P application, proliferating branch roots with active meristems in the upper portion of the soil profile where P was applied, and reducing root weight to plant weight by 13%. White lupin did not proliferate roots in response to P application. When P was not added to soil, soybean and lupin acquired similar P per unit root dry weight. However, white lupin accumulated 4.8 times more P per unit root length, suggesting that P acquisition in these plants involved other mechanisms such as the exudation of P solubilizing compounds. Soybean accessed P by developing more root length thus colonising more soil volume than white lupin and, therefore, was better able to take advantage of the added P. Pericycle and root tip meristem activities were critical to the differences in root development between white lupin and soybean, and therefore their responses to plant and soil P.