Differential growth of georeacting maize roots

The growth rate of the two sides of 10-mm apical segments prepared from primary roots and of intact primary roots of maize has been analyzed in both vertical and horizontal positions, using a filming method allowing continuous growth recording. The data showed that the georeaction began by a decrease in the overall elongation rate of the roots. This inhibition is effective on the lower side of the bending zone, where the growth is practically stopped during the period of maximum rate of geocurvature. In contrast, the growth is slightly enhanced on the upper part of the elongating zone.     

Architecture and development of the oil-palm (Elaeis guineensis Jacq.) root system

The growth dynamics and architecture of the oil-palm root system are described. Following a transitional juvenile phase, eight different morphological types of roots have been distinguished according to their development pattern and state of differentiation: primary vertical and horizontal roots, secondary horizontal roots, upward growing secondary vertical roots and downward growing secondary vertical roots, superficial and deep tertiary roots and quaternary roots. The relative position of these types of roots determines a morphological and functional unit of the root system called 'root architectural unit' of the oil palm. This root polymorphism enabled us to define a morphogenetic gradient, which reflected the oil-palm root-system ontogenesis.     

Importance of the cap in maize root growth

A large population of primary roots of Zea mays (cv. LG 11) was selected for uniform length at zero time. Their individual growth rates were measured over an 8-h period in the vertical position (in humid air, darkness). Three groups of these roots with significantly different growth rates were then chosen and their cap length was measured. It was found that slowly growing roots had long caps whereas rapidly growing roots had short caps. The production by the cap cells of basipetally transported growth inhibitors was tested (biologically by the curvature of half-decapped roots) and found to be significantly higher for longer root caps than that for shorter ones.     

Modelling the effect of varying soil water on root growth dynamics of annual crops

We present a simple framework for modelling root growth and distribution with depth under varying soil water conditions. The framework considers the lateral growth of roots (proliferation) and the vertical extension of roots (root front velocity). The root front velocity is assumed to be constant when the roots descend into an initially wet soil profile. The lateral growth of roots is governed by two factors: (1) the current root mass or root length density at a given depth, and (2) soil water availability at that depth.Under non-limiting soil water conditions, the increase in root mass at any depth is governed by a logistic equation so that the root length density (R _v) cannot exceed the maximum value. The maximumR _v, is assumed to be the same for all depths. Additional dry matter partitioned to roots is initially distributed according to the current root mass at each depth. As the root mass approaches the maximum value, less dry matter is partitioned to that depth.When soil water is limiting, a water deficit factor is introduced to further modify the distribution of root dry matter. It is assumed that the plant is an energy minimiser so that more root mass is partitioned to the wetter regions of the soil where least energy will be expended for root growth. Hence, the model allows for enhanced root growth in areas where soil water is more easily available.Simulation results show that a variety of root distribution patterns can be reproduced due to varying soil water conditions. It has been demonstrated that broad patterns of root distribution reported in the literature can also be simulated by the model.     

Computer-based video digitizer analysis of surface extension in maize roots

We used a video digitizer system to measure surface extension and curvature in gravistimulated primary roots of maize (Zea mays L.). Downward curvature began about 25 +/- 7 min after gravistimulation and resulted from a combination of enhanced growth along the upper surface and reduced growth along the lower surface relative to growth in vertically oriented controls. The roots curved at a rate of 1.4 +/- 0.5 degrees min-1 but the pattern of curvature varied somewhat. In about 35% of the samples the roots curved steadily downward and the rate of curvature slowed as the root neared 90 degrees. A final angle of about 90 degrees was reached 110 +/- 35 min after the start of gravistimulation. In about 65% of the samples there was a period of backward curvature (partial reversal of curvature) during the response. In some cases (about 15% of those showing a period of reverse bending) this period of backward curvature occurred before the root reached 90 degrees. Following transient backward curvature, downward curvature resumed and the root approached a final angle of about 90 degrees. In about 65% of the roots showing a period of reverse curvature, the roots curved steadily past the vertical, reaching maximum curvature about 205 +/- 65 min after gravistimulation. The direction of curvature then reversed back toward the vertical. After one or two oscillations about the vertical the roots obtained a vertical orientation and the distribution of growth within the root tip became the same as that prior to gravistimulation. The period of transient backward curvature coincided with and was evidently caused by enhancement of growth along the concave and inhibition of growth along the convex side of the curve, a pattern opposite to that prevailing in the earlier stages of downward curvature. There were periods during the gravitropic response when the normally unimodal growth-rate distribution within the elongation zone became bimodal with two peaks of rapid elongation separated by a region of reduced elongation rate. This occurred at different times on the convex and concave sides of the graviresponding root. During the period of steady downward curvature the elongation zone along the convex side extended farther toward the tip than in the vertical control. During the period of reduced rate of curvature, the zone of elongation extended farther toward the tip along the concave side of the root. The data show that the gravitropic response pattern varies with time and involves changes in localized elongation rates as well as changes in the length and position of the elongation zone. Models of root gravitropic curvature based on simple unimodal inhibition of growth along the lower side cannot account for these complex growth patterns.     

Root distribution and morphology of maize seedlings as affected by tillage and fertilizer placement

Suboptimal soil conditions are known to result in poor early growth of maize (Zea mays L.) in no-tillage (NT) systems in contrast with conventional tillage (CT) systems. However, most studies have generally focused on maize roots at later growth stages and/or do not give details on root morphology. In a 2-year field study at two locations (silt loam and loam soils) in the Swiss midlands, we investigated the impacts of tillage intensity, NT vs. CT, and NP-fertilizer sidebanding on the morphology, vertical and horizontal distribution, and nutrient uptake of maize roots at the V6 growth stage. The length density (RLD) and the length per diameter-class distribution (LDD) of the roots were determined from soil cores taken to a depth of 0.5 m and at distances of 0.05 and 0.15 m from both sides of the maize row. The temperature of the topsoil was lower, and the bulk density and penetration resistance were greater in the topsoil of NT compared with CT. The growth and the development of the shoot were slower in NT. RLD was greater and the mean root diameter smaller in CT than in NT, while the vertical and horizontal distribution of roots did not differ between CT and NT. RLD increased in the zone enriched by the sidebanded fertilizer, independent of the tillage system, but LDD did not change. The poorer growth of the roots and shoots of maize seedlings was presumably caused by the lower topsoil temperature in NT rather than by mechanical impedance. The placement of a starter fertilizer at planting under NT is emphasized.     

Gravitropic response growth angle and vertical distribution of roots of wheat (Triticum aestivum L.)

Recent work on root distribution, growth angles and gravitropic responses in Japanese cultivars of winter wheat are reviewed. Vertical distribution of roots, which influences the environmental stress tolerance of plants, was observed in the 12 Japanese cultivars in the field. The root depth index (RDI: the depth at which 50% of the root length has been reached) differed among the cultivars at the stem elongation stage. Since the RDI was closely related to the growth angle of seminal roots obtained in a pot experiment, it was assumed that growth angle is useful for predicting vertical root distribution among wheat genotypes. Gravitropic responses of the primary seminal root of 133 Japanese wheat cultivars assessed by measuring the growth angle in agar medium, were larger in the northern Japanese cultivars and smaller in the southern ones. It was also found that the geographical variation resulted from the wheat breeding process, i.e. genotypes with limited gravitropic responses of roots had been selected in the southern part of Japan where excessive soil moisture is one of the most serious problems.     

Kinetic analysis of root growth and georeaction

The kinetics of growth and georeaction of horizontal or vertical maize (cv. ORLA 264) apical root segments were analysed. Elongation and curvature data were fitted to a mathematical function and the effect of both light and decapitation reported. Elongation of horizontal segments was found to be more sensitive to light than to decapitation. A light treatment or the decapitation presented several effects on the shape of the growth curves. Growth of vertical segments was affected much more by decapitation than by light treatment. The shape of the curves was clearly different for decapitated and intact segments.
Curvature is affected both by light and decapitation but the shape of the bending curves is modified principally by decapitation.
Under the present conditions used, growth and georeaction of root segments do not seem to be strictly correlated.     

Fractal geometry of bean root systems: correlations between spatial and fractal dimension

An obstacle to the study of root architecture is the difficulty of measuring and quantifying the three-dimensional configuration of roots in soil. The objective of this work was to determine if fractal geometry might be useful in estimating the three-dimensional complexity of root architecture from more accessible measurements. A set of results called projection theorems predict that the fractal dimension (FD) of a projection of a root system should be identical to the FD of roots in three-dimensional space (three-dimensional FD). To test this prediction we employed SimRoot, an explicit geometric simulation model of root growth derived from empirical measurements of common bean (Phaseolus vulgaris L.). We computed the three-dimensional FD, FD of horizontal plane intercepts (planar FD), FD of vertical line intercepts (linear FD), and FD of orthogonal projections onto planes (projected FD). Three-dimensional FD was found to differ from corresponding projected FD, suggesting that the analysis of roots grown in a narrow space or excavated and flattened prior to analysis is problematic. A log-linear relationship was found between FD of roots and spatial dimension. This log-linear relationship suggests that the three-dimensional FD of root systems may be accurately estimated from excavations and tracing of root intersections on exposed planes.     

种间互作和施氮对蚕豆/玉米间作生态系统地上部和地下部生长的影响

为河西走廊绿洲灌区豆科/禾本科间作体系的养分管理提供科学依据,于2007年在武威绿洲农业试验站应用田间原位根系行分隔技术研究了蚕豆/玉米种间互作和施氮对玉米抽雄期的根系空间分布、根系形态和作物地上部生长的影响。研究结果表明:种间互作和施氮均增加了玉米和蚕豆在纵向和横向两个尺度上的根重密度、根长密度、根表面积、根系体积。根长密度和根表面积与两种作物产量和氮素吸收均呈正相关,而与蚕豆的根瘤重呈负相关;抽雄期的土壤含水量与玉米产量和养分吸收呈显著的负相关。玉米根系可以占据蚕豆地下部空间,但蚕豆的根却较少到间作玉米的地下部空间,也就是间作后增加了玉米根系水平尺度的生态位。蚕豆和玉米根系主要分布分别在0-40 cm浅土层和0-60 cm 土层,且间作玉米根系在60-120 cm比单作和分隔的多。因此,种间互作和施氮扩大了两作物根系纵向和横向的空间生态位,改变了作物根系形态,即扩展了两者水分和养分吸收的生态位,增加了作物吸收养分的有效空间,从而提高了间作生态系统的生产力。     

Distribution of growth and proton efflux in gravireactive roots of maize (Zea mays L.)

Roots of Zea mays were maintained in a vertical orhorizontal position and the local elongation rate and H⁺ fluxes were measured using Sephadex beads containing a pH indicator. When the roots were kept horizontally, the growth of the lower side was strongly inhibited and that of the upper side slightly stimulated as compared with vertical roots. The H⁺ extrusion, which was greatest in the elongation zone, was strongly inhibited on the lower side and slightly stimulated on the upper side as compared with vertical roots.     

A growth chamber for idealized studies of seedling root growth dynamics and structure

A root growth chamber is described which allows seedling root growth dynamics and structure to be monitored continuously under a variety of conditions for several weeks. The chamber consists of two cells with inner dimensions 18×20×0.12 cm. To simulate the soil matrix, each cell was filled with spherical glass beads of 0.1 cm diameter. Given the 0.12 cm width of each cell, the glass bead matrix was approximately one bead layer thick. Roots were therefore grown in a quasi -two-dimensional and transparent environment. This enabled root images of high spatial and temporal resolution to be collected and analysed quantitatively using standard image analysis techniques. The chamber was constructed such that the root environment could be manipulated with regard to nutrient distribution, `soil' matrix structure and other perturbations to the system. Preliminary experiments of the growth dynamics of lentil roots (Lens culinaris L. cv. Verte du Puy) in the chamber were conducted. The majority of the primary and lateral roots followed a similar growth pattern with high growth rates between days 5 and 9 and days 14 and 18 separated by a period of low growth rate between days 10 and 12 after seeding in the chamber. Thus, primary and lateral root growth was to a certain extent synchronized. Lateral roots developed after 3 to 8 days on the outer curve (convex side) of the primary root. The roots shared many of the characteristics of roots developed in three-dimensional systems indicating that the chamber did not induce artificial root behaviour. Thus, the idealized and quantitative studies that can be conducted in the chamber may enable many aspects of the complex interactions between the root system and environment to be studied.     

Abscisic Acid and Ethylene Increase in Heterodera avenae-infected Tolerant or Intolerant Oat Cultivars

The relationship between root stunting caused by the cereal cyst nematode and levels of two root growth inhibiting hormones, abscisic acid and ethylene, was investigated in aseptically cultured root segments and in intact roots of two oat cultivars differing in tolerance to the nematode. Cultured root segments of oat cultivars New Zealand Cape (tolerant) and Sual (intolerant) were inoculated with sterilized Heterodera avenae second-stage juveniles. Suppressed growth of root axes and emerged laterals following nematode penetration corresponded to an increase in abscisic acid and ethylene in roots of both intolerant and tolerant cultivars. When the experiment was repeated on intact root systems, nematodes retarded root growth of Sual more than New Zealand Cape despite an increase in ABA and ethylene in both cultivars. Abscisic acid and (or) ethylene may be involved in growth inhibition of H. avenae-infected roots but appear to play no direct role in determining tolerance.     

NaCl induced changes in ionically bound peroxidase activity in roots of rice seedlings

The changes in ionically bound peroxidase activity in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mM progressively decreases root growth. The reduction of root growth by NaCl is closely correlated with the increase in ionically bound peroxidase activity. Since proline and ammonium accumulations are associated with root growth inhibition caused by NaCl, we determined the effects of proline or NH4Cl on root growth and ionically bound peroxidase activity in roots. External application of proline or NH4Cl markedly inhibited root growth and increased ionically bound peroxidase activity in roots of rice seedlings in the absence of NaCl. An increase in ionically bound peroxidase activity in roots preceded inhibition of root growth caused by NaCl, NH4Cl or proline. Mannitol inhibited root growth, but decreased rather than increased ionically bound peroxidase activity at the concentration iso-osmotic with NaCl. The inhibition of root growth and the increase in ionically bound peroxidase activity in roots by NaClis reversible and is associated with ionic rather than osmotic component. This revised version was published online in June 2006 with corrections to the Cover Date.     

Comparison of radial increment and volume growth in stems and roots of Quercus petraea

Dendrochronological studies dealing with roots, stems and branches are very rare or often take the form of short notes. The difficulties of detecting rings and of quantifying the radial growth in roots have already been described for various species. In oak the anatomical root structure differs from stemwood. The roots are radial-porous or diffuse-porous, and there is often no clear distinction between individual rings. In our study visual and radiographic techniques were used to examine radial increment in roots of sessile oak (Quercus petraea L.) which was compared with radial growth in branches and along the stem. Coarse roots were cut from four 30- to 34-year-old trees that had been uprooted mechanically and disks were taken at different distances from the stem-root base. Ring widths were measured in the stem at height of 0.3 m, at breast height (1.3 m), beneath the crown, in branches of the crown, and in roots every 20 cm. The ring widths were cross-dated, and the heterogeneity of growth within a root and within the root system were analysed. Asymmetric growth frequently occurred in roots so that ovals, I-beam and T-beam shapes were developed. With the method used in our study the annual growth layers close to the central cylinder could be distinguished as well as beneath the bark. Pointer years were detected in all sections of the tree and permitted correction of ring widths in roots. Root system, stem and branch showed a basic similarity in their radial sequence of ring width. The annual biomass increment was weaker and more variable with several consecutive changes in the roots than in the stems. The root/shoot ratio reached a minimum rather early, beginning at the cambial age of 20 years. This revised version was published online in June 2006 with corrections to the Cover Date.     

Adaptive longitudinal growth of first-order lateral roots of a woody species (Spartium junceum) to slope and different soil conditions—upward growth of surface roots

First-order lateral roots originating in the upper part of the taproot of a woody species, usually termed surface roots, grow close beneath the soil surface, even on irregular or sloping ground. In slope condition, in fact, the surface roots can assume upward as well as downward growth. Existing knowledge on the controls over root direction does not fully explain these field observations.

Two different soil types and sloping conditions were selected in field condition to explore the behaviour of the surface roots in the woody species Spartium junceum L. The root system 3D architecture was measured with a 3D digitizer and the angle of growth (0° = vertically downwards) and the radial direction (0° = horizontally downslope or northwards) of all root segments measured.

Surface roots were more numerous in clay soil than in loam soil, independently from the slope inclination. They had initial angles larger than 90°, i.e. they grew upwards only in clay soil. The subsequent angles of growth maintained this value only in steep-slope condition, showing a clear soil type x slope inclination interaction. The initial angle of all first-order lateral roots decreased linearly with depth of origin on the taproot always in relation to the soil type, with this relationship being stronger in clay soil.

These findings showed that the liminal angle (the preferred angle of growth) of surface roots was mainly affected by the soil type rather than the soil surface inclination. Thus, upward growth must stand in the plasticity of the plagiotropic response of these secondary laterals rather than in a strong internal control.     

Relations between root length densities and root intersections with horizontal and vertical planes using root growth modelling in 3-dimensions

The root growth simulation model of Diggle (ROOTMAP; 1988) was modified to allow the numerical output of data on root intersections with horizontal and vertical planes. ROOTMAP was used to generate two three-dimensional model structures of fibrous root systems. The lateral roots were oriented randomly (geotropism index=0) but the main axes were positively gravitropic (geotropism index=0.6). The mean density of root intersections (n, cm^-2) with the sides of a series of 5×5×5 cm cubic volumes was related approximately linearly to the root length density (L_t cm^-2) within each volume by the equation L_t=2.3n (correlation coefficient, r=0.981). This compared with the relation of L_t=2n predicted theoretically for randomly oriented lines (Melhuish and Lang, 1968). Root length density was related to the intersection density by the equation L_t=2.43n_v (r=0.940) for the vertical faces and L_t=1.88n_h (r=0.984) for the horizontal faces. L_t/n_v was greater than L_t/n_h because of the preferential vertical orientation of the main root axes. The Melhuish and Lang (1968) equation does not generally give accurate prediction of root length density from field experiment data. Under field conditions, values have been reported in the ranges of 1.4 to 16 for L_t/n_h, and 3.8 to 9 for L_t/n_v. The most likely explanation for this difference is that only a small proportion (e.g. about 20–30%) of the actual number of roots are counted using the core-break and root mapping (including the trench wall) methods, due to the practical experimental difficulties of identifying individual fine roots under field conditions. Detailed experimental studies are needed to identify what portion of the root system is recorded using these field techniques (e.g. whether the main root axes are counted while the fine lateral roots remain undetected). Three-dimensional models of root growth provide a new method of studying the relations between L_t, n_v and n_h for root systems generated stochastically according to known geometrical rules. Using these models it will be possible to determine the effects of the degree of gravitropism and of root branching on the value and on the variability of L_t/n_h and L_t/n_v. The effectiveness of the statistical corrections that have been developed to correct for non-random root orientation can also be evaluated, as can the effects of sample position.     

Genotypic Variation in Root Growth Angle in Rice (Oryza sativa L.) and its Association with Deep Root Development in Upland Fields with Different Water Regimes

Deep root development, which is important for the drought resistance in rice (Oryza sativa L.), is a complex trait combining various root morphologies. The objective of this study was to elucidate genotypic variation in deep root development in relation to morphological indicators such as vertical root distribution and root growth angle. Two experiments were conducted: one on upland fields, and one in pots and fields. In experiment 1, the root systems of six rice cultivars on upland fields were physio-morphologically analyzed under different water regimes (irrigated and intermittent drought conditions during panicle development). In experiment 2, cultivar differences in root growth angles were evaluated with 12 cultivars using the basket method under irrigated conditions. No cultivar × environment interactions were found for total root length or deep root length between irrigated and drought conditions in experiment 1. This suggests that constitutive root growth, which is genetically determined, is important for deep root development under intermittent drought conditions during reproductive stage. Among root traits, the deep root ratio (i.e., deep root weight divided by total root weight) was most closely related to deep root length under both water regimes. This suggested that vertical root distribution constitutively affects deep root length. Significant genotypic variation existed in the nodal root diameter and root growth angle of upland rice in experiment 2. It was considered that genotypes with thick roots allocated more assimilates to deep roots through root growth angles higher to the horizontal plane on upland fields. This is the first report on genotypic variation in the root growth angle of rice on upland fields. It should prove useful for rough estimations of genotypic variation in the vertical root distribution of upland rice because root growth angle is rapidly and easily measured.     

Why are laterals less affected than main axes by homogeneous unfavourable physical conditions? A model-based hypothesis

When plants develop in strong soils, growth of the root system is generally depressed. However, branching and elongation of branches are often less affected than growth of the main axes, whenever the whole root system encounters even-impeded conditions. On the basis of a model simulating root growth and architecture as related to assimilate availability, we propose a simple hypothesis to explain such behaviour. In the model, growth of each root depends on its own elongation potential, which is estimated by its apical diameter. The potential elongation rate–apical diameter relationship is the same for all the roots of the system and is described by a monomolecular function. Our hypothesis is that the effect of soil strength can be simulated by introducing an impedance factor in the definition of root maximum potential elongation rate, common to the whole root system. When such impedance factor is applied, it affects more the potential of larger roots (main axes) than that of thinner roots (secondary and tertiary branches). Simulations provided in high impedance conditions led to root systems characterised by short taproots, whereas growth of secondary roots was unaffected and growth of tertiary roots was enhanced. Actual branching density was also higher, although branching rules have been unchanged. Such simulated systems where similar to that observed in strong soils. Friction laws or pore size can be involved in the larger reduction of the potential growth of main axes. Moreover, when growth of main axes is restricted, assimilate availability becomes higher for branches and that could explain that their growth could be increased in a homogeneous strong soil. This revised version was published online in June 2006 with corrections to the Cover Date.