Assessment of Root System Dynamics of Species Grown in Mixtures under Field Conditions using Herbicide Injection and 15N Natural Abundance Methods: A Case Study with Pea, Barley and Mustard

Two methods were developed and used to study the root system dynamics of two species grown together or separately under field conditions. The first method, based on herbicide injection at different soil depths, was used to determine the rooting depth penetration rate of each species in pea–barley and pea–mustard mixtures. The roots absorbed the herbicide when they reached the treated zone leading to visible symptoms on the leaves which could be readily monitored. The second method used differences in ¹⁵N natural abundance and N concentration between legume and non-legume species to quantify the contribution of each species to root biomass of a pea–barley mixture. Each contribution was calculated using ¹⁵N abundance and N concentration of root mixtures and of subsamples of roots of individual species within mixtures. Both methods can indeed be used to distinguish roots of species in mixtures and thus to study belowground competition between associated species. The use of these methods demonstrated species differences in root system dynamics between species but also significant effects of interactions between species in mixtures. The rooting depth penetration rate was mainly species specific whereas root biomass was dependant on plant growth, allocation of dry matter between shoot and root components and growth factors such as N fertilization. Root biomass of each species may vary therefore with the level of competition between species.     

Root growth of potato crops on a marine-clay soil

Summary In 1982 and 1983 root samples were taken by auger from potato crops grown on marine clay in the Flevo-Polder. The roots increased their penetration depth throughout the periods of measurement, and ultimately reached depths between 80 cm and 100 cm below the hills. Between 50 and 60 days after emergence, decay of roots commenced, starting in the upper horizons. In the hill mean root length densities varied between 1 and 2 cm cm⁻³. Below the hills root density rarely exceeded 1 cm cm⁻³. The random variation in root density was equivalent to a coefficient of variation of 50%. There were significant effects of the position of sampling (relative to the centre of the plant) on root density; densities were usually lowest beneath the furrow. Depending on season and sampling date, total root length varied between 3.4 and 7.1 km m⁻², and root dry mass varied between 33 and 77 g m⁻². Representative figures for specific root length were 100–120 m g⁻¹ dry weight. About 90% of the root diameters were smaller than 0.44 mm; the most frequent class (35%) were roots with diameters between 0.12 and 0.20 mm.     

Effect of Sowing Date and Cultivar on Root System Development in Pea (Pisum sativum L.)

In many species, root system development depends on cultivar and sowing date, with consequences for aerial growth, and seed yield. Most of the peas (Pisum sativum L.) grown in France are sown in spring or in mid-November. We analyzed the effect of two sowing periods (November and February) and three pea cultivars (a spring cultivar, a winter cultivar, a winter recombinant inbred line) on root development in field conditions. For all treatments, rooting depth at various dates seemed to be strongly correlated with cumulative radiation since sowing. Maximum root depth varied from 0.88 to 1.06 m, with the roots penetrating to greater depths for February sowing than for November sowing in very cold winters. The earlier the crop was sown, the sooner maximum root depth was reached. No difference in root dynamics between cultivars was observed. In contrast, the winter recombinant inbred line presented the highest root density in the ploughed layer. These findings are discussed in terms of their possible implications for yield stability and environmental impact.     

Cotton Root Protection from Plant-Parasitic Nematodes by Abamectin-Treated Seed

Abamectin is nematicidal to Meloidogyne incognita and Rotylenchulus reniformis, but the duration and length of cotton taproot protection from nematode infection by abamectin-treated seed is unknown. Based on the position of initial root-gall formation along the developing taproot from 21 to 35 d after planting, infection by M. incognita was reduced by abamectin seed treatment. Penetration of developing taproots by both nematode species was suppressed at taproot length of 5 cm by abamectin-treated seed, but root penetration increased rapidly with taproot development. Based on an assay of nematode mobility to measure abamectin toxicity, the mortality of M. incognita associated with a 2-d-old emerging cotton radicle was lower than mortality associated with the seed coat, indicating that more abamectin was on the seed coat than on the radicle. Thus, the limited protection of early stage root development suggested that only a small portion of abamectin applied to the seed was transferred to the developing root system.     

Effect of soil mechanical impedance on root growth of two rice varieties under field drought stress

Two upland rice varieties, Azucena and Bala, were screened for root growth under droughted and irrigated treatments in two field sites at the West Africa Rice Development Association (WARDA) experimental farm, Côte d’Ivoire, during the dry season of 1999/2000. The sites were chosen to represent contrasting soil profile penetration resistance (PR) characteristics on upland sites, although both were relatively impeding. The number of nodal root axes per unit area passing through horizontal transects (root density) was counted at 35, 56, 77 and 98 days after sowing (DAS) at 10 cm depth intervals. Azucena consistently maintained a greater root density than Bala and a greater proportion of Azucena roots grew to 30 cm depth (22.7% vs. 8.4% at 77 DAS). There was little detectable effect of water regime on root distribution but evidence of lower root numbers at depths below 20 cm in the higher PR site was revealed. A site by variety by soil depth interaction suggests that Azucena roots are more strongly affected by very high PR than those of Bala. PR between 0–30 cm depth increased greatly with decreasing soil water content during the drought as the soil dried. This increase is likely to have prevented or greatly impaired further nodal root growth within this layer. At 40 cm depth, PR was high (3–4 MPa) but did not increase during the drought. At this depth root growth rate was likely to be greatly reduced despite the availability of water. These results demonstrate that varietal differences in root morphology characterised in the laboratory can be also detected in impeding field soils as differences in the density of roots at depth. Relatively poor root growth in these fields in the absence of drought was probably due to the high mechanical impedance and/or the physiological stress of the plants in the dry season. Our results indicate that high mechanical impedance was a more fundamental constraint on root growth than soil water availability during the drought. Thus, varietal differences in root penetration ability might be very important for drought avoidance in soils of this type.     

Growth and development of root systems of winter cereals grown after different tillage methods including direct drilling

Summary A method is described for rapidly estimating the depth of penetration and density of roots of cereal crops under field conditions. Counts of living roots, traversing horizontal faces of soil cores, were made for winter wheat growing on direct-drilled and ploughed land.The rate of penetration of roots of winter wheat in a clay and a sandy loam soil averaged 5 mm per day throughout winters without extremes of cold or wet. Death of roots near the soil surface occurred wilst others continued downward penetration. The rate of root elongation was slower during prolonged periods when the soil was wet and faster,i.e. to greater depths, during dry conditions.Damage sustained to roots during adverse winter conditions ofter varied between direct drilling and ploughing. More roots at depth were consistently recorded on direct-drilled than on ploughed land when measured in spring after a soil water deficit had developed during the preceding month. After prolonged wet soil conditions during the winter on a soil with a large clay fraction and low hydraulic conductivity, root growth and penetration in spring, before the development of a soil water deficit, was more restricted on direct-drilled than on ploughed land.     

Effects of sand burial depth on seed germination and seedling emergence of Cirsium pitcheri

A greenhouse study was conducted to determine the effects of sand burial on seed germination and seedling emergence of Cirsium pitcheri, a threatened species along Lake Huron sand dunes. In October 1996, seeds of C. pitcheri were sorted into three groups (small, medium and large) and artificially buried at 2, 4, 6, 8, 10 and 12 cm depths in plastic pots filled with unsterilized sand from Lake Huron sand dunes. These pots were placed outside in sand boxes for natural stratification. In early February, the pots were brought into the greenhouse for germination of seeds. Data showed that percent seed germination and emergence of seedlings were not related to seed size. However, both variables were negatively correlated with depth. Seedling emergence occurred from a maximum depth of 6 cm with most seedlings emerging from 2 cm depth. Only one seedling emerged from 8 cm depth. Seedlings from large seeds produced longer roots than those of small seeds. Larger root system would probably enhance seedling establishment.     

Optimal Levels of Meloidogyne incognita Inoculum for Infection of Tomato and Peach in Vitro

Penetration of second-stage juveniles (J2) of Meloidogyne incognita into tomato root explants and in vitro propagated peach plantlet roots were compared. Five inoculum levels were used: 25, 50, 75, 100, and 200 J2 for tomato; and 50, 100, 200, 500, and 1,000J2 for peach. The greatest root penetration into tomato was 30% at the 75 J2 level, but the maximum penetration into peach roots was only 8% at the 200 J2 level. The difference (P = 0.05) in penetration of M. incognita at all inoculum levels into these two hosts indicates that penetration versus inoculum density for in vitro studies need to be determined for different plant species.     

Use of Nematodes as Biomonitors of Nonfumigant Nematicide Movement through Field Soil

Three field experiments were established in a loamy sand soil in the Coastal Plain of North Carolina to determine downward movement of aldicarb and fenamiphos with a nematode bioassay. Penetration of bioassay plant roots by Meloidogyne incognita was measured at 1, 3, 7, 14, 21, and 28 days after treatment in the greenhouse as a means of determining nematicide effectiveness. Chemical movement was similar in planted and fallow soil. Nematicidal activity was greater in soil collected from the 0 to 10 cm depth than from the 10 to 20 cm depth. Fenamiphos suppressed host penetration by the nematode more than aldicarb under the high rainfall (19 cm) and low soil temperatures that occurred soon after application in the spring. During the summer, which had 13 cm precipitation and warmer soil temperatures, both chemicals performed equally well at the 0 to 10 cm depth. At the lower soil level (10 to 20 cm), aldicarb limited nematode penetration of host roots more quickly than fenamiphos. Both of these chemicals moved readily in the sandy soil in concentrations sufficient to control M. incognita. Although some variability was encountered in similar experiments, nematodes such as M. incognita have considerable potential as biomonitors of nematicide movement in soil.     

Life histories of two desert species of the bulbous genus Bellevalia

Summary Annual biomass increment and biomass partitioning to leaves, roots and reproduction, and biomass storage in the below-ground bulb was measured in plants of two species of the geophytic genus Bellevalia grown outdoors at three levels of soil moisture. The differences between the species were in accordance with the hypothesis that plants of more arid environments should rely more on internal reserves than plants of more productive environments. In Bellevalia desertorum, a shallow rooted species of the most arid habitats in the Central Negev, leaf and root development during outgrowth at the beginning of winter was rather variable, and followed soil moisture availability to a certain degree. A small portion of its biomass budget was committed to seed production, which varied little among the irrigation regimes. The rest of the biomass was stored in the bulb. The amount of biomass devoted annually to reproduction was mainly determined by the amount of reserves already present in the bulb. In contrast, in B. eigii, which grows in the more productive wadis with its bulb at a depth of 15 to 30 cm, leaf and root growth was not only determined by water availability, but also by initial bulb mass. This resulted in a greater potential relative growth rate than in B. desertorum, but also in a greater risk of accumulating less biomass than it spent in root and leaf construction under poor soil moisture conditions. In this species, reproductive biomass and seed yield were proportional to current biomass gain and, in contrast to B. desertorum, independent of initial bulb mass, provided that the bulb was large enough to initiate flowering.     

不同土壤深度落叶松细根分解及N动态变化

细根分解受根序和土壤深度的潜在影响。使用根序法分根,将落叶松Larix gmelini根系分为两类:一级根、二级根为一类(1—2级根),即低级根;三级根和四级跟为另一类(3—4级根),即高级根。采用埋袋法对落叶松低级根和高级根在不同土壤深度(0—10、10—20 cm和20—30 cm)进行了为期862 d的分解实验,探讨不同根序细根分解规律,养分释放及其影响。结果表明:1—2级根的分解速率比3—4级分解速率慢,这种规律同时存在于不同深度的土壤中。在空间上,低级根和高级根的分解速率均随土壤深度的增加而降低,高级根的降低趋势更明显。随着分解时间的进行,各个土层之间的分解率在低级根之间差异更大。细根分解过程中,落叶松不同根序养分的释放特征不同。N释放速率总体上随细根根序增加而增大,随土壤深度的增加而降低。     

Rooting characteristics of two widely distributed woody plant species growing in different karst habitats of southwest China

A series of studies claimed that deep root development of plant established in karst regions was facilitated by fractured bedrock beneath the shallow soils; however, bedrock is not a uniform medium for root proliferation. We hypothesized plant species that survived in different karst habitats had some other rooting characteristics rather than deep penetration. To test the hypothesis, coarse root systems of two widely distributed woody species (one tree and one shrub) growing in three typical rocky karst habitats (shallow soil, loose rocky soil and exposed rock) were excavated in karst region of southwest China. Root systems were investigated based on four parameters: maximum rooting depth, maximum radial extent, root tapering pattern and root curvature. In all the three habitats, maximum rooting depths were no deeper than 120 and 40 cm for the tree and shrub species, respectively. Maximum radial extents were extremely large compared with maximum rooting depth, indicating that rooting characteristics were dominated by horizontal extension rather than deep penetration. Roots of both species growing in shallow soil habitat tapered gradually and curved slightly, which was consistent with the specific characteristics of this habitat. On the contrary, roots of the tree species growing in the other two habitats tapered rapidly but curved slightly, while roots of the shrub species tapered gradually but curved strongly. It was speculated that limited depths and rapid tapering rates of the tree roots were likely compensated by their utmost radial extensions, while the shrub species might benefit from its root curvature as the associated root tropisms may increase the ability of root to encounter more water and contribute to potentially high resource absorption efficiency. Our results highlight the importance of taking shallow-rooted species into account in understanding the distribution of natural plant communities and predicting future vegetation dynamics in karst regions.     

Relationships between root characteristics and seed size in two contrasting floras

Plants vary greatly in root system characteristics, but the causes of this variation are poorly understood. We hypothesised that root system size is closely linked to the plant’s ecological strategy, and that seed size is correlated with root diameter, as a result of anatomical constraints. We analysed the relationships between root characteristics – root depth, basal root diameter and root type – and other plant attributes in more than 300 plant species from two ecologically and geographically contrasted areas: Britain and NE Spain. We used statistical tests that took into account phylogenetic patterns in the data. Apart from plant life span, only plant height and seed size were related to root size in the adult plants. Plant species with shallow or thin main roots had smaller seeds than species with deep or thick main roots, and species with taproots had bigger seeds than plants with fibrous or especially with adventitious roots. These relationships were consistent in the two floras. Seed size was related to plant height, but this association was weaker than that between seed size and root depth. Root depth explained a significant proportion of the variation in seed weight, independently from life form or dispersal mode and, in some cases, more than either of them. These results suggest that traditional ecological explanations do not adequately explain the relationship between seed and plant adult size, and that there will be other, complementary explanations. In particular, we propose that the relationship between seed size and plant height is secondary. The putative causal sequence is that deep-rooted plants (which are generally taller) have large seeds because of allometric and developmental constraints that mean that only large seeds can produce the thick roots that can grow rapidly to depth.     

Do propagation methods affect the fine root architecture of African plum (Dacryodes edulis)?

Belowground tree growth attributes determine whether associations will be complementary or competitive in an agroforestry context. A study on fine root (d?≤?2?mm) distribution patterns of Dacryodes edulis based on root density (RD), root length density (RLD) and root weight density (RWD) was conducted to evaluate the effect of propagation methods on rooting distribution. Results showed that D. edulis trees of seed origin had greater RD (P?≤?0.001) than trees of vegetative origin (cuttings and marcots) in the upper soil stratum (0–30?cm). Similarly, in the uppermost soil stratum (0–10?cm), RLD and RWDs varied significantly (P?<?0.01). Trees of seed origin had an exponential distribution pattern for fine RD, RLD and RWD with depth to 80?cm. In contrast, the distribution pattern of fine roots of trees of vegetative origin (cuttings and marcots) were quadratic for the same variables which increased in the 20–30?cm soil depth stratum before declining steadily to a depth of 80?cm. The findings of this study suggest that D. edulis trees of vegetative origin (cuttings and marots) are likely to be less competitive than trees of seed origin when intercropped with shallow-rooted annual plants in an agroforestry system for belowground resources.     

根区交替控制灌溉条件下玉米根系吸水规律

为了阐明根区交替控制灌溉(CRDAI)条件下玉米根系吸水规律,通过田间试验,在沟灌垄植模式下采用根区交替控制灌溉研究玉米根区不同点位(沟位、坡位和垄位)的根长密度(RLD)及根系吸水动态。研究表明,根区土壤水分的干湿交替引起玉米RLD的空间动态变化,在垄位两侧不对称分布,并存在层间差异;土壤水分和RLD是根区交替控制灌溉下根系吸水速率的主要限制因素。在同一土层,根系吸水贡献率以垄位最大,沟位最低;玉米营养生长阶段,10—30 cm土层的根系吸水速率最大;玉米生殖生长阶段,20—70 cm为根系吸水速率最大的土层,根系吸水贡献率为43.21%—55.48%。研究阐明了交替控制灌溉下根系吸水与土壤水分、RLD间相互作用的动态规律,对控制灌溉下水分调控机理研究具有理论意义。     

A gradual rather than abrupt increase in soil strength gives better root penetration of strong layers

The purpose of this paper is to test the hypothesis that gradual rather than abrupt increases in strength lead to increased root penetration of strong layers. This was tested experimentally in a model system with multi-layered wax discs with a total thickness of up to 6 mm that either increased in strength with depth, or decreased in strength with depth. Strength was varied by altering the proportions of hard paraffin wax and white soft paraffin. Multi-layered wax discs consisted of three layers, each 1.5 or 2 mm thick, to give a disc thickness of either 4.5 or 6 mm. ‘Test’ wax discs had a 40% hard wax layer above a 60% wax layer, which was above an 80% wax layer. This led to a gradual increase in strength as roots encountered the disc from the sand growing medium. In ‘control’ discs, the order was reversed so that 80% wax was encountered immediately below the sand. These treatments were used to challenge roots of the rice cvs. Azucena and Bala. Compared with the control, the test layers increased root penetration of Azucena from a mean of 5.9 root axes to 12.4, while penetration of Bala was increased from 0.5 to 5.6. These results show that there was increased root penetration when roots encountered a gradual increase in strength rather than a sudden increase in strength. This was explored further with numerical simulations of stress distributions at the tips of roots. These simulations indicated that gradual rather than abrupt increases in strength decreased stress concentration at the root tip, consistent with the experimental results.     

Interaction Between “Safe Sites” and “Safe Sides” for Germination of Neurada procumbens (Neuradaceae) in the Middle East

The seed bank of Neurada procumbens, a prostrate annual common in sandy desert habitats, is vulnerable to surface disturbances. Moreover, its seeds are highly heteromorphic, having both spinose and non-spinose sides, which affects both the vertical distribution of seeds and diaspores, and the precise position in which they lie in the soil. Here we explore the ecological implications of two factors, “safe sites” within habitats and seed orientation or “safe sides”, on seedling establishment and the seed bank. In three natural populations, the vertical distribution of buried, germinable diaspores decreased with soil depth. Most diaspores occurred at the surface (0–1 cm below the surface), while none were observed below 10 cm. Seedling emergence decreased with depth and reached zero in diaspores below 5 cm. Seedling emergence also proved highly sensitive to the orientation of diaspores in soil. Diaspores lying on their smooth side with the spinose upper side directed upwards reached the highest seedling emergence of 61 %. Root length ranged from 10 to 16 cm, and the root to shoot ratio varied between 22.8 and 30.3 cm, depending on diaspore size. Diaspores near the surface may constitute a more transient component of the seed bank, as few of these remained viable for more than a year. Persistence of Neurada seed banks seems to depend not only on vertical movement of diaspores among habitat “safe sites” but also on different seed position alternatives.     

Coarse root spatial distribution determined using a ground-penetrating radar technique in a subtropical evergreen broad-leaved forest, China

Coarse roots play a critical role in forest ecosystems and both abiotic and biotic factors affect their spatial distribution. To some extent, coarse root density may reflect the quantity of root biomass and biotic competition in forests. However, using traditional methods (e.g., excavation) to study coarse roots is challenging, because those methods are time-consuming and laborious. Furthermore, these destructive methods cannot be repeated in the same forests. Therefore, the discovery of non-destructive methods for root studies will be very significant. In this study, we used a ground-penetrating radar technique to detect the coarse root density of three habitats (ridge, slope and valley) and the dominant tree species (Castanopsis eyrei and Schima superba) in a subtropical forest. We found that (i) the mean of coarse root density for these three habitats was 88.04 roots m^?2, with roots being mainly distributed at depths of 0–40 cm. Coarse root densities were lower in deeper soils and in areas far from the trunk. (ii) Coarse root densities differed significantly among the three habitats studied here with slope habitat having the lowest coarse root density. Compared with S. superba, C. eyrei had more roots distributed in deeper soils. Furthermore, coarse roots with a diameter >3 cm occurred more frequently in the valleys, compared with root densities in ridge and slope habitats, and most coarse roots occurred at soil depths of 20–40 cm. (iii) The coarse root density correlated negatively with tree species richness at soil depths of 40–60 cm. The abundances of the dominant species, such as C. eyrei, Cyclobalanopsis glauca, Pinus massoniana, had significant impacts on coarse root density. (iv) The soil depth of 0–40 cm was the “basic distribution layer” for coarse roots since the majority of coarse roots were found in this soil layer with an average root density of 84.18 roots m^?2, which had no significant linear relationships with topography, tree species richness, rarefied tree species richness and tree density. Significant relationships between coarse root density and these factors were found at the soil depth of 40–60 cm, which was the “potential distribution layer” for coarse root distribution.     

柠条人工林细根不同分枝根序寿命估计

植物细根在发育结构上表现的形态特征和生理功能异质性影响细根寿命的准确估计,因此了解分枝根序细根寿命差异对于深入认识细根的周转过程和陆地生态系统碳分配具有重要意义。采用微根管(Minirhizotron)技术对晋西北黄土区的五年生柠条(Caragana Korshinskii Kom.)人工林细根的生长过程进行了为期3a(2007—2009年)的追踪观测,分析了不同因素(土层深度、季节变化、空间位置)对一级根和高级根寿命的影响。结果表明:(1)在各土层深度处,一级根的中值寿命均低于高级根中值寿命,其中一级根中值寿命表现随土层深度增加而增加趋势,而高级根除表层0—20 cm中值寿命较短外,各土层间变化趋势不明显,40—60 cm、80—100 cm土层高级根在观测期结束时其累积存活率仍在50%以上;(2)不同季节出生一级根和高级根的中值寿命季节性表现为:秋季夏季春季,并且在各个季节均表现,高级根寿命显著大于一级根寿命(P0.01);一级根仅夏季与秋季差异性不显著(P0.05),而高级根仅春季与秋季存在极显著差异(P0.01);(3)一级根和高级根距树干基部0 cm处细根中值寿命均大于50 cm处一级根和高级根细根的中值寿命。同一位置处高级根寿命要大于一级根寿命。在距树干基部0 cm处和50 cm处,一级根和高级根的寿命均存在极显著差异(P0.01),但高级根却在距树干基部0 cm和50 cm处差异不明显,而一级根却表现极显著差异(P0.01)。     

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。