A comparison of minirhizotron techniques for estimating root length density in soils of different bulk densities

Flexible- and rigid-walled minirhizotron techniques were compared for estimating root length density of 14- to 28-day-old Pinto bean (Phaseolus vulgaris L.) and spring whet (Triticum aestivum L.) plants in soil boxes under controlled environment conditions at three soil bulk densities (1.3, 1.5 and 1.7 g cm^–3). The flexible-tube system consisted of bicycle inner tubes inflated inside augered access holes and removed only when measurements were taken. Rigid tubes were constructed of extruded polybutyrate plastic. In both cases tubes were oriented horizontally. Despite similar root densities for wheat and beans based on measurements obtained from soil cores, root densities estimated from both types of minirhizotron were higher in bean than in wheat in uncompacted soil. Estimates of root density by the flexible tube minirhizotron were more closely correlated with soil core image analysis estimates than were those by the rigid minirhizotron system. At high soil bulk density, rigid tube measurements consistently overestimated actual rooting density of both wheat and bean. The relationship between estimated and actual rooting densities in the case of flexible tube measurements was not significantly influenced by soil bulk density. These findings were consistent with the theory that preferential root growth is induced by gaps at the soil-observation tube interface, inherent in the rigid tube technique, and was accentuated under conditions of high soil strength.     

The Wageningen Rhizolab — a facility to study soil-root-shoot-atmosphere interactions in crops

Roots in the Wageningen Rhizolab are observed using two methods: (i) non-destructively, using horizontal, glass minirhizotrons at intervals of 14 days between observations; (ii) with destructive sampling using augers on three dates in the season. This paper reports changes with depth and time in root numbers per unit interface area of the minirhizotron tube (number of intersections) of four crop species (wheat, Brussels sprouts, leek and potato). The number of root intersections of Brussels sprouts, wheat and potato declined with depth at any time, whereas leek showed a different pattern because maximum root growth was observed at a depth of 10–20 cm. Root density generally decreased in the following order: Brussels sprouts, wheat, potato and leek. Plots of root length densities, L_rv(cm. cm^-3), obtained by auger sampling, versus the number of intersections showed considerable variation in slope with species, time in the season and year, implying that a single, universal equation to convert minirhizotron observations into volumetric root densities does not exist. Causes of variation in the slopes are discussed. It is concluded that limited auger sampling combined with minirhizotron observations yield adequate quantitative estimates of relevant root properties.     

A comparison of minirhizotron,core and monolith methods for quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution

Root research has been hampered by a lack of good methods and by the amount of time involved in making measurements. The use of the minirhizotron as a quantitative tool requires comparison with conventional destructive methods. This study was conducted in the greenhouse to compare the minirhizotron technique with core and monolith methods in quantifying barley (Hordeum vulgare L.) and fababean (Vicia faba L.) root distribution. Plants were grown in boxes (80 cm long × 80 cm wide × 75 cm deep) in a hexagonal arrangement to minimize the effects of rooting anistrophy. Minirhizotron observations and destructive sampling to a depth of 70 cm using core and monolith methods were performed at the ripening growth stage. Total root length for the entire depth interval was generally higher in barley (159–309 m) than fababean (110–226 m). Significant correlation coefficients between monolith and core methods for root length density (RLD, cm cm^–3) was observed in both crops (p 0.01). A method and depth interaction showed no significant differences in fababean RLD distribution measured by core and monolith methods. However, the RLD was different for the uppermost 40 cm depth in barley. The relationship for RLD between minirhizotron and core methods was significant only in barley (r=0.77^*). For both crops, estimates of RLD in the top 10-cm layer by the minirhizotron technique were lower than those by core and monolith techniques. In contrast, estimates of RLD were higher in fababean at a depth >30 cm. Destructive sampling still remains the method to quantify root growth in the 0–10 cm soil layer. ei]B E Clothier     

Root characteristics of selected field crops: Data from the Wageningen Rhizolab (1990–2002)

Since being built in 1990, the rhizotron facility in Wageningen, the Wageningen Rhizolab, has been used for experiments on crops (e.g. Alfalfa, Brussels sprouts, common velvet grass, field bean, fodder radish, leeks, lupins, maize, potato, beetroot, ryegrass, spinach, spring wheat, winter rye and winter wheat). In the experiments, horizontal glass minirhizotron tubes combined with auger sampling were used to assess rooting characteristics. For this paper we took the root data from these experiments and looked for a general relationship between thermal time/time after planting and rooting depth, the velocity of the root front and root proliferation. For certain depths (fixed by the depth at which the horizontal minirhizotrons were installed) a simple linear regression was established between the average root number per cm² minirhizotron surface area and thermal time after planting. The compartments selected for each crop were those in which there had been a control treatment and/or in which conditions for rooting were considered to be optimal. We performed regression analyses per compartment and per depth, but only for the period after planting in which a linear increase of root numbers vs. thermal time was observed. After averaging the results, the regression procedure yielded two parameters of rooting for each crop: (a) the actual or thermal time at which the first root appeared at a certain depth and (b) the root proliferation rate after the first root had appeared. In this way, inherent crop differences in rooting behaviour (rooting depth and root proliferation) became apparent. For each crop, the velocity of the root front after planting could be established (calculated in cm(°C day)^–1). This parameter differed greatly between crops. Some crops (such as leeks and common velvet grass) explored the soil profile slowly: the root front moved at a velocity of only 0.07cm(°C day)^–1. Among the crops whose roots grew down much faster (0.18–0.26cm (°C day)^–1) were cereals and fodder radish. For a day with an average temperature of 15°C these rates would have corresponded with the root front travelling approximately 1–4cm per day. In the crops studied the root front velocity did not correlate with the root proliferation rate.     

Soil temperature effects from minirhizotron lighting systems

Observing root dynamics or soil fauna with minirhizotrons requires the use of incandescent or ultraviolet (UV) lighting systems. These light sources can generate heat which would be transferred to the surrounding soil adjacent to the minirhizotron observation tubes and thus may influence root growth and development or fauna activity. The objective of this study was to determine the effect of incandescent and UV light from a minirhizotron camera system on soil temperatures next to minirhizotron tubes. Temperature probes were attached next to and at 0.5 cm from the tube surface and the tubes were then placed in boxes with either a fine sand or a loamy clay soil. Incandescent light was operated stationary for 5 min or moved at 1 cm increments every 10 s down the tube for both dry and wet soils. The UV light was used in a stationary position for 10 minutes in both dry soils. Maximum temperature increases were 3.41–3.52 °C and 1.69–2.14 °C next to the tube for the dry and wet soils, respectively with 5 min of stationary incandescent light. Ultraviolet lights increased soil temperatures to a maximum of approximately 2.5 °C in the dry soil. Probes placed 0.5 cm from the tube surface also showed temperature increases up to 2.15 °C. Moving the light source every 10 s, however, resulted in lower temperature increases (<0.8 °C). Therefore short durations of light resulted in small temperature increases suggesting minimal impact on root development. Increased soil temperatures from longer durations of light, however, may alter root growth and development as well as soil fauna activity and warrants further study.     

Tomato root distribution, yield and fruit quality under subsurface drip irrigation

Tomato rooting patterns were evaluated in a 2-year field trial where surface drip irrigation (R0) was compared with subsurface drip irrigation at 20 cm (RI) and 40 cm (RII) depths. Pot-transplanted plants of two processing tomato, `Brigade' (C1) and `H3044' (C2), were used. The behaviour of the root system in response to different irrigation treatments was evaluated through minirhizotrons installed between two plants, in proximity of the plant row. Root length intensity (L _a), length of root per unit of minirhizotron surface area (cm cm^–2) was measured at blooming stage and at harvest. For all sampling dates the depth of the drip irrigation tube, the cultivar and the interaction between treatments did not significantly influence L _a. However differences between irrigation treatments were observed as root distribution along the soil profile and a large concentration of roots at the depth of the irrigation tubes was found. For both surface and subsurface drip irrigation and for both cultivars most of the root system was concentrated in the top 40 cm of the soil profile, where root length density ranged between 0.5 and 1.5 cm cm^–3. Commercial yields (t ha^–1) were 87.6 and 114.2 (R0), 107.5 and 128.1 (RI), 105.0 and 124.8 (RII), for 1997 and 1998, respectively. Differences between the 2 years may be attributed to different climatic conditions. In the second year, although no significant differences were found among treatments, slightly higher values were observed with irrigation tubes at 20 cm depth. Fruit quality was not significantly affected by treatments or by the interaction between irrigation tube depth and cultivar.     

用Minirhizotrons观测柠条根系生长动态

 Minirhizotrons是一种非破坏性、定点直接观察和研究植物根系的新方法。该文介绍了用Minirhizotrons测定植物根系的方法,并同根钻取原状土样法进行了比较;探讨了根系生长动态同土壤含水量间的关系。试验于2004年植物生长季在沙坡头沙漠试验研究站的水分平衡观测场的人工柠条(Caragana korshinskii)林进行,结果表明：Minirhizotrons 管埋入土壤后需要10个月时间允许柠条根系在其周围定居,其观测图片中的根系代表了管子周围2.6 mm土层的根系。柠条根系生长动态和土壤水分变化相关,含水量的升高导致根系的大量繁殖,而根系吸水及蒸发散又导致含水量的减少;在2004年植物生长季,土壤水分和根系的这种相互作用出现了两次,但根系生长高峰比土壤含水量高峰滞后20 d左右。     

Advancing fine root research with minirhizotrons

Minirhizotrons provide a nondestructive, in situ method for directly viewing and studying fine roots. Although many insights into fine roots have been gained using minirhizotrons, a review of the literature indicates a wide variation in how minirhizotrons and minirhizotron data are used. Tube installation is critical, and steps must be taken to insure good soil/tube contact without compacting the soil. Ideally, soil adjacent to minirhizotrons will mimic bulk soil. Tube installation causes some degree of soil disturbance and has the potential to create artifacts in subsequent root data and analysis. We therefore recommend a waiting period between tube installation and image collection of 6-12 months to allow roots to recolonize the space around the tubes and to permit nutrients to return to pre-disturbance levels. To make repeated observations of individual roots for the purposes of quantifying their dynamic properties (e.g. root production, turnover or lifespan), tubes should be secured to prevent movement. The frequency of image collection depends upon the root parameters being measured or calculated and the time and resources available for collecting images and extracting data. However, long sampling intervals of 8 weeks or more can result in large underestimates of root dynamic properties because more fine roots will be born and die unobserved between sampling events. A sampling interval of 2 weeks or less reduces these underestimates to acceptable levels. While short sample intervals are desirable, they can lead to a potential trade-off between the number of minirhizotron tubes used and the number of frames analyzed per tube. Analyzing fewer frames per minirhizotron tube is one way to reduce costs with only minor effects on data variation. The quality of minirhizotron data should be assessed and reported; procedures for quantifying the quality of minirhizotron data are presented here. Root length is a more sensitive metric for dynamic root properties than the root number. To make minirhizotron data from separate experiments more easily comparable, idiosyncratic units should be avoided. Volumetric units compatible with aboveground plant measures make minirhizotron-based estimates of root standing crop, production and turnover more useful. Methods for calculating the volumetric root data are discussed and an example presented. Procedures for estimating fine root lifespan are discussed.     

Root dynamics in a salt marsh over three consecutive years

The minirhizotron technique has been used to study root development in a salt marsh in the western part of the Nationalpark Niedersächsisches Wattenmeer during a three-year period. The objective of our study was to evaluate root depth distribution and seasonal changes in growth activities of natural plant root systems. Root number was counted at monthly intervals in the top soil layer (0–0.2 m) for every 2 cm soil depth. The number of roots was regarded as an easily detectable parameter reflecting root growth and decay.In general, highest rooting intensity was found in the soil's subsurface layer (0–0.08 m). The number of roots significantly decreased in deeper horizons of the soil. There was also a significant increase and decrease in the number of roots in the course of a year. The highest rooting intensity was found in late winter to early spring, which substantially decreased towards mid summer when the plants were in their reproductive phase. The data indicate that there is a clear seasonal pattern of root growth of salt marsh species.     

植物根系研究新技术Minirhizotron的起源、发展和应用

根系是土壤和植物的动态界面,对植物和土壤均具有重要意义。但由于根系深处地下,观测研究十分不便,导致根系研究在广度、深度上均落后于地上部分。随着对根系在生态系统以及全球碳平衡中重要作用的认识,根系渐渐成为国际相关领域的研究热点之一。Minirhizotron(微根区管或小观察窗)技术的诞生和应用,使根系研究手段得到了进一步发展,成为根系研究技术发展的重要里程碑。Minirhizotron技术主要由透明观察管、观测设备和记录设备组成,观测设备曾先后使用了普通镜子、观察镜和相机(或摄像机),记录设备也相应地经历了手工绘制、传统黑白、彩色相片或录像带以及高清晰数字图像。同时,还开发了多种图像自动分析系统,使该项技术日臻完善。Minirhizotron技术可以以非破坏方式,定期对同一根系的出现、生长、衰老、死亡和消失进行连续观察,对根系伸长、根系密度、扎根深度、侧根伸展、分枝特性、菌根特性以及细根动态、根系生命周期和分解等进行观测研究,同时,也可开展根系对不同处理响应的研究。因此,Minirhizotron技术必将在农业、林业和环境等科学领域得到越来越广泛的应用。     

Improved scaling of minirhizotron data using an empirically-derived depth of field and correcting for the underestimation of root diameters

Background and aims

Accurate data on the standing crop, production, and turnover of fine roots is essential to our understanding of major terrestrial ecological processes. Minirhizotrons offer a unique opportunity to study the dynamic processes of root systems, but are susceptible to several measurement biases.

Methods

We use roots extracted from minirhizotron tube surfaces to calculate the depth of field of a minirhizotron image and present a model to correct for the underestimation of root diameters obscured by soil in minirhizotron images.

Results

Non-linear regression analysis resulted in an estimated depth of field of 0.78 mm for minirhizotron images. Unadjusted minirhizotron data underestimated root net primary production and fine root standing crop by 61 % when compared to adjusted data using our depth of field and root diameter corrections. Changes in depth of field accounted for >99 % of standing crop adjustments with root diameter corrections accounting for <1 %.

Conclusions

Our results represent the first effort to empirically derive depth of field for minirhizotron images. This work may explain the commonly reported underestimation of fine roots using minirhizotrons, and stands to improve the ability of researchers to accurately scale minirhizotron data to large soil volumes.     

A color composite technique for detecting root dynamics of barley (Hordeum vulgare L.) from minirhizotron images

Quantification of root dynamics by destructive methods is confounded by high coefficients of variation and loss of fine roots. The minirhizotron technique is non-destructive and allows for sequential root observations to be made at the same depth in situ. Observations can be stored on video tape which facilitates data handling and computer-aided image processing. A color composite technique using digital image analyses was adapted in this study to detect barley root dynamics from sequential minirhizotron images. Plants were grown in the greenhouse in boxes (80 × 80 × 75 cm) containing soil from a surface horizon of a Typic Cryoboroll. A minirhizotron was installed at a 45°C angle in each box. Roots intersecting the minirhizotron were observed and video-recorded at tillering, stem extension, heading, dough and ripening growth stages. The images from a particular depth were digitized from the analog video then registered to each other. Discrimination of roots from the soil matrix gave quantitative estimates of root appearance and disappearance. Changes in root appearance and disappearance were detected by assigning a separate primary color (red, green, blue) to selected growth stages, then overlaying the images to create red-green and red-green-blue color composites. The resulting composites allowed for a visual interpretation and quantification of barley root dynamics in situ.     

Automatic discrimination of fine roots in minirhizotron images

Minirhizotrons provide detailed information on the production, life history and mortality of fine roots. However, manual processing of minirhizotron images is time-consuming, limiting the number and size of experiments that can reasonably be analysed. Previously, an algorithm was developed to automatically detect and measure individual roots in minirhizotron images. Here, species-specific root classifiers were developed to discriminate detected roots from bright background artifacts. Classifiers were developed from training images of peach (Prunus persica), freeman maple (Acer x freemanii) and sweetbay magnolia (Magnolia virginiana) using the Adaboost algorithm. True- and false-positive rates for classifiers were estimated using receiver operating characteristic curves. Classifiers gave true positive rates of 89-94% and false positive rates of 3-7% when applied to nontraining images of the species for which they were developed. The application of a classifier trained on one species to images from another species resulted in little or no reduction in accuracy. These results suggest that a single root classifier can be used to distinguish roots from background objects across multiple minirhizotron experiments. By incorporating root detection and discrimination algorithms into an open-source minirhizotron image analysis application, many analysis tasks that are currently performed by hand can be automated.     

DREB1A promotes root development in deep soil layers and increases water extraction under water stress in groundnut

Water deficit is a major yield‐limiting factor for many crops, and improving the root system has been proposed as a promising breeding strategy, although not in groundnut (Arachis hypogaea L.). The present work was carried out mainly to assess how root traits are influenced under water stress in groundnut, whether transgenics can alter root traits, and whether putative changes lead to water extraction differences. Several transgenic events, transformed with DREB1A driven by the rd29 promoter, along with wild‐type JL24, were tested in a lysimeter system that mimics field conditions under both water stress (WS) and well‐watered (WW) conditions. The WS treatment increased the maximum rooting depth, although the increase was limited to about 20% in JL24, compared to 50% in RD11. The root dry weight followed a similar trend. Consequently, the root dry weight and length density of transgenics was higher in layers below 100‐cm depth (Exp. 1) and below 30 cm (Exp. 2). The root diameter was unchanged under WS treatment, except a slight increase in the 60–90‐cm layer. The root diameter increased below 60 cm in both treatments. In the WW treatment, total water extraction of RD33 was higher than in JL24 and other transgenic events, and somewhat lower in RD11 than in JL24. In the WS treatment, water extraction of RD2, RD11 and RD33 was higher than in JL24. These water extraction differences were mostly apparent in the initial 21 days after treatment imposition and were well related to root length density in the 30–60‐cm layer (R² = 0.68), but not to average root length density. In conclusion, water stress promotes rooting growth more strongly in transgenic events than in the wild type, especially in deep soil layers, and this leads to increased water extraction. This opens an avenue for tapping these characteristics toward the improvement of drought adaptation in deep soil conditions, and toward a better understanding of genes involved in rooting in groundnut.     

A comparison of soil core sampling and minirhizotrons to quantify root development of field-grown potatoes

Root growth of potato (Solanum tuberosum L.) is sensitive to soil conditions. A reduced root system size can result in reduced uptake of water and/or nutrients, leading to impaired crop growth. To understand the mechanisms by which soil conditions affect crop growth, study of temporal and spatial development of roots is required.In field experiments, effects of soil temperature, soil compaction and potato cyst nematodes (Globodera pallida) on root growth of potato cultivars were studied using two methods: core sampling and vertically oriented minirhizotrons.Minirhizotrons showed relatively more roots in deeper soil layers than core sampling, probably because of preferential root growth along the tube. Spatial distribution of roots should therefore be analysed by core sampling.To eliminate differences in spatial distribution, total root systems as measured by both methods were compared. Nematodes, cultivars and time did not affect the relationship between both methods. Soil compaction, however, affected it because of a strong response of root length in bulk soil and small differences in root number against the minirhizotron, suggesting that soil coring has to be used to study effects of different bulk densities.With both methods, sequential measurements of roots give the net effect of root growth and decay. Data on root turnover can only be obtained with minirhizotrons by comparing video recordings of different dates. Other information obtained with minirhizotrons is the average orientation of roots. Moreover, the minirhizotron method has the advantage of demanding less labour.     

Comparison of tomato root distributions by minirhizotron and destructive sampling

Calibration of minirhizotron data against root length density (RLD) was carried out in a field trial where three drip irrigation depths: surface (R0) and subsurface, 0.20 m (RI) and 0.40 m depth (RII) and two processing tomato cultivars: `Brigade' (CI) and `H3044' (CII) were imposed. For each treatment three minirhizotron tubes were located at 10, 37.5 and 75 cm of the way from one plant row to the next. Roots intersecting the minirizotrons walls were expressed as root length intensity (L _a) and number of roots per unit of minirhizotron wall area (N _ra). Root length density (RLD) was calculated from core samples taken for each minirhizotron tube at two locations: near the top of the minirhizotron (BI) and 15 cm apart from it, facing the minirhizotron wall opposite the plant row (BII). Minirhizotron data were regressed against RLD obtained at BI and BII and with their respective means. The results show that for all the situations studied, better correlations were obtained when RLD was regressed with L _a than with N _ra. Also was evident that the relationship between L _a and RLD was strongly influenced by the location of soil coring. RLD was correlated with L _a trough linear and cubic equations, having the last ones higher determination coefficients. For instance at 10 cm from the plant row when values from the top layer (0–40 cm) were analysed separately, L _a was significantly regressed with RLD measured at BII and described by the equations: RLD = 0.5448 + 0.0071 L _a (R ² = 0.51) and RLD = 0.4823 + 0.0074L _a + 8×10^–5 L _a ² – 5×10^–7 L _a ³ (R ² = 0.61). Under the 40 cm depth the highest coefficients of determination for the linear and cubic equations were respectively 0.47 and 0.88, found when L _a was regressed with RLD measured at BI. For minirhizotrons located at 75 cm from the plant row and for location BI it was possible to analyse jointly data from all depths with coefficients of determination of 0.45 and 0.59 for the linear and cubic equations respectively.     

Improving propagation methods of Ricinodendron heudelotti Baill. from cuttings

Ricinodendron heudelotii is a valuable multipurpose tree from Africa that is lacking an efficient and inexpensive vegetative propagation method. To improve multiplication, a series of nursery experiments were conducted to assess the effect of propagation media, plant growth regulators (NAA and IBA), accession source harvest timing, cutting type, and pre-treatment with honey. Maximum survival and rooting percentages of 90.2 and 93.7% respectively were achieved in all experiments. The maximum number of leaves and roots were respectively, 7.4 and 8.7; maximum root length was 15.6 cm. Fine sand was superior to sawdust or fine sand:sawdust mixture. IBA, at 100 μg/l was more effective than NAA for rooting and growth. Principal component analysis showed that, independent of the accession source, cuttings harvested from April to May were more suitable for propagation, giving the highest rates of rooting and growth. In contrast, variable results were achieved with apical cuttings (highest rooting), basal single-node leafy cuttings (highest number of leaves and length of root) and basal two-node leafy cuttings (highest number of roots). Cuttings were more successful when they were soaked in honey for 60 min. After acclimatization, maximum survival rate of plants was 67.6%. This improved protocol can be incorporated into agro forestry system in which propagation of R. heudelotii is a component.     

Small differences in root distributions allow resource niche partitioning

1. Deep roots have long been thought to allow trees to coexist with shallow‐rooted grasses. However, data demonstrating how root distributions affect water uptake and niche partitioning are uncommon.
2. We describe tree and grass root distributions using a depth‐specific tracer experiment six times over two years in a subtropical savanna, Kruger National Park, South Africa. These point‐in‐time measurements were then used in a soil water flow model to simulate continuous water uptake by depth and plant growth form (trees and grasses) across two growing seasons. This allowed estimates of the total amount of water a root distribution could absorb as well as the amount of water a root distribution could absorb in excess of the other rooting distribution (i.e., unique hydrological niche).
3. Most active tree and grass roots were in shallow soils: The mean depth of water uptake was 22 cm for trees and 17 cm for grasses. Slightly deeper rooting distributions provided trees with 5% more soil water than the grasses in a drier season, but 13% less water in a wetter season. Small differences also provided each rooting distribution (tree or grass) with unique hydrological niches of 4 to 13 mm water.
4. The effect of rooting distributions has long been inferred. By quantifying the depth and timing of water uptake, we demonstrated how even small differences in rooting distributions can provide plants with resource niches that can contribute to species coexistence. Differences in total water uptake and unique hydrological niche sizes were small in this system, but they indicated that tradeoffs in rooting strategies can be expected to contribute to tree and grass coexistence because 1) competitive advantages change over time and 2) plant growth forms always have access to a soil resource pool that is not available to the other plant growth form.

     

Comparison of root distributions of species in North American grasslands using GIS

Abstract. We quantified the spatial distribution of roots of individual plants using detailed drawings from the literature of species of grasses, forbs, and shrubs in the Central Great Plains grasslands of North America. We scanned each two-dimensional drawing electronically and used ARC/INFO, a Geographic Information System, to calculate root length (cm) and density (cm root length/cm soil) with depth in the soil profile. We then selected one of three mathematical models that best fit the data, and classified each species as either shallow-, medium- or deep-rooted. 66 root drawings from 55 species were evaluated. Most plants were shallow-rooted with largest root densities occurring at depths < 20 cm; most maximum rooting depths were > 1m. Grasses had the shallowest maximum depth and shrubs the deepest. Deep-rooted forbs had the smallest root densities by depth. Most plants had two sections to their distribution of root density: an initial increase from the soil surface followed by a decrease in density with increasing depth. Our results suggest that the abundance and importance of different species and growth forms in North American grasslands is related to similarities and differences in the spatial distributions of their root systems. Our approach provides quantitative information on root distributions to be used for comparisons among species, and in simulation modeling analyses that could not be done with conventional methods that are qualitative in nature.     

Cultivar and planting date effects on soybean root growth

To avoid late summer drought, soybean [Gylcine max (L) Merrill] producers in many southern and border states of the USA modify their cropping systems. Options include use of unadapted cultivars and changing planting dates. Because root function is important to plant health and yield, this study was conducted to determine if planting date and soybean cultivar affect root growth and distribution. Seeds of one cultivar from each of four maturity groups (MG III, IV, V, and VI) were sown in mid-April, mid-May, and mid-June in 1992 and 1993 on a Tiptonville silt loam near Portageville, MO. Root observations were performed 30 and 60 days after emergence (DAE) using a minirhizotron system. Cultivars differed for root length density (RLD) only in the 15 to 28 cm depth in 1992 and in the 15 to 28 cm and 29 to 42 cm depths in 1993, but differences were not related to maturity classification of cultivar. Average RLD was 1.02 cm^–3 for MG III and IV cultivars and 1.21 cm cm^–3 for MG V and VI cultivars. Average RLD for the mid-June planting date was 1.65 cm cm^–3 but only 0.73 cm cm^–3 for the mid-April planting date. An increase in RLD between 30 and 60 DAE occurred at all soil depths. For both years, MG V and VI cultivars produced higher yields than the MG III cultivars. Earlier than normal planting dates inhibited early root growth, but did not reduce yield. Cultivars differed only slightly for the rooting characteristics measured in this study. These rooting characteristics may not be important criteria for cultivar selection.Abbreviations MG maturity group - VCR videocassette recorder - DAE days after emergence - RLD root length density - CRLD change in root length density Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153Contribution from the Missouri Agric. Exp. Station Journal Series Number 12, 153