How periodic growth pattern and source/sink relations affect root growth in oak tree seedlings

Seedlings of Quercus pubescens were grown in root boxes to study the growth pattern of the root system in relation to shoot development. Shoot growth was typically rhythmic. Root elongation was also periodic, in contrast to several previous reports on other Quercus species. Both taproot and lateral root elongation were depressed during expansion of the second leaf flush, with a more pronounced response of lateral root growth. Apical diameter of the taproot followed comparable but less prominent trends than taproot elongation. Modifying source/sink relationships through various defoliation treatments altered the root growth pattern. Ablation of source organs (mature leaves or cotyledons) amplified the decrease in root growth concomitant with leaf expansion. Root growth recovery was even more difficult when both cotyledons and mature leaves had been removed. Ablation of sink aerial organs (young leaves) initially suppressed competition for growth between the shoot and the root, and then caused a gradual decrease in lateral root growth. Antagonism between maximum leaf expansion and root growth reduction during the second flush, and various responses of seedlings with modified source/sink relationships, raise an hypothesis of mutual competition for carbohydrates. The gradual decrease in lateral root growth after ablation of young leaves suggests a long-term carbohydrate limitation, or auxin limitation as auxin sources have been removed.     

Competition within the root system of rubber seedlings (Hevea brasiliensis) studied by root pruning and blockage

The effects of taproot blocking and pruning on the developmentof the early secondary roots (ESR) of rubber seedlings werestudied in root observation boxes under controlled conditions.During shoot flush, both the mean elongation rate and mean apicaldiameter of the ESR decreased regardless of treatments. Thereafter,the elongation rate of the ESR increased greatly when the taprootwas blocked, slightly for the control and scarcely for the prunedsystems in which fast growing regenerated roots developed. Thedifferences between treatments were related to the proportionof ESR which ceased growing. Following shoot arrest, the apicaldiameter of ESR increased greatly for blocked seedlings andto a lesser extent for pruned seedlings. Branching density ofthe ESR and elongation of tertiary roots were also higher forseedlings without a growing taproot. The dynamics of ESR response was not consistent with activeinhibition of their development by the growing taproot. Moreover,this response was dependent on concurrent development of shootand regenerating roots, hence competition processes were morelikely to be determining. In such an hypothesis, root elongationcan be limited by assimilate availability, but also by eachroot's maximum growth rate in non-limiting conditions, i.e.growth potential. Since the latter is related to apical diameter,a significant acceleration of elongation required a parallelincrease in apical diameter and this may explain the relativeinertia of ESR to taproot alteration. Conversely, regeneratingroots could have a high growth potential because they were initiatedin a favourable context, thus their development competed stronglywith elongation of ESR. Key words: Hevea brasiliensis, root system, development, growth potential, root diameter, competition     

Root System Development of Oak Seedlings Analysed using an Architectural Model. Effects of Competition with Grass

A dynamic 3D model of root system development was adapted to young sessile oak seedlings, in order to evaluate the effects of grass competition on seedling root system development. The model is based on a root typology and the implementation of a series of developmental processes (axial and radial growth, branching, reiteration, decay and abscission). Parameters describing the different processes are estimated for each root type. Young oak seedlings were grown for 4 years in bare soil or with grass competition and were periodically excavated for root system observation and measurements (topology of the root system, length and diameter of all roots with a diameter greater than 0.3 mm). In the fourth year, 40 cm×20 cm×20 cm soil monoliths were excavated for fine root measurement (root density and root length). Root spatial development was analysed on a sub-sample of roots selected on four seedlings. The model was a guideline that provided a complete and consistent set of parameters to represent root system development. It gave a comprehensive view of the root systems and made it possible to quantify the effects of competition on the different root growth processes. The same root typology was used to describe the seedlings in bare soil and in grass. Five root types were defined, from large tap roots to fine roots. Root system size was considerably reduced by grass competition. Branching density was not affected but the branch roots were always smaller for the seedlings grown in competition. Reiteration capacity was also reduced by competition. Cross sectional areas before and after branching were linearly related with a scaling coefficient close to 1, as predicted by the pipe model theory. This relationship was not affected by grass competition.     

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.     

Root apical diameter and root elongation rate of rubber seedlings (Hevea brasiliensis) show parallel responses to photoassimilate availability

The hypothesis that root apical diameter may be used to evaluate root growth potential was tested. Temporal variations in the apical diameter of individual roots of rubber seedlings ( Hevea brasiliensis ) were studied together with their elongation patterns, using root observation boxes under controlled conditions. This study confirmed the overall positive correlation between apical diameter and growth rale. Moreover, the two parameters, varied in the same way during the life of a given root. For roots with short growth duration, there was a parallel quick decrease in both apical diameter and elongation rate, whereas roots that grew for longer periods showed synchronous fluctuations for both parameters. Since the mean values for the secondary roots within a root system exhibited the same trends, variations in apical diameter and elongation rates should depend on factors influencing the whole root system. When related to shoot rhythmic growth, both apical diameter and elongation rates were depressed during the periods of leaf growth. These effects were enhanced and/or prolonged by shading, hence reinforcing the hypothesis that this development depends on assimilate availability. Such results can be interpreted in terms of a source-sink relationship within the whole plant by considering the apical diameter, representing the size of the meristem related to the number of rneristematic cells, as an indicator of each root's growth potential.     

Root cooling strongly affects diel leaf growth dynamics,water and carbohydrate relations in Ricinus communis

In laboratory and greenhouse experiments with potted plants, shoots and roots are exposed to temperature regimes throughout a 24 h (diel) cycle that can differ strongly from the regime under which these plants have evolved. In the field, roots are often exposed to lower temperatures than shoots. When the root‐zone temperature in Ricinus communis was decreased below a threshold value, leaf growth occurred preferentially at night and was strongly inhibited during the day. Overall, leaf expansion, shoot biomass growth, root elongation and ramification decreased rapidly, carbon fluxes from shoot to root were diminished and carbohydrate contents of both root and shoot increased. Further, transpiration rate was not affected, yet hydrostatic tensions in shoot xylem increased. When root temperature was increased again, xylem tension reduced, leaf growth recovered rapidly, carbon fluxes from shoot to root increased, and carbohydrate pools were depleted. We hypothesize that the decreased uptake of water in cool roots diminishes the growth potential of the entire plant – especially diurnally, when the growing leaf loses water via transpiration. As a consequence, leaf growth and metabolite concentrations can vary enormously, depending on root‐zone temperature and its heterogeneity inside pots.     

The consequence of peroxidase overexpression in transgenic plants on root growth and development

Transgenic tobacco plants that overproduce the tobacco anionic peroxidase wilt upon reaching maturity, although having functional stomata and normal vascular anatomy and physiology. These plants were examined further to determine the cause for wilting, and thus better understand how the anionic peroxidase functions in plant growth and development. Shoots from young peroxidase overproducing plants were grafted onto wild-type tobacco root stock to determine if the roots could absorb and transmit sufficient water to maintain leaf turgidity. These grafted plants never wilted when grown in the greenhouse though shoot peroxidase activity remained ten-fold greater than in control plants, thus indicating that wilting is a consequence of peroxidase expression in the roots. Close examination of root systems revealed considerably less root mass in the transformed plant, primarily exhibited through a decrease in branching. At flowering, root growth rate and total root mass in transformed plants were less than 50% of control plants although shoot mass and growth rate were unchanged. This is in contrast to root growth in young seedlings where transformed plants performed equivalently to controls. Root hydraulic conductivity was measured to evaluate the effect of elevated peroxidase expression on water absorption and transport; however, no significant change in hydraulic conductivity was found in transformed plants. The consequence of anionic peroxidase overexpression on indoleacetic acid (IAA) metabolism was also examined. No significant difference in IAA levels was observed; however, root elongation in plants overexpressing peroxidase was insensitive to exogenous IAA. It can be concluded that the overexpression of the tobacco anionic peroxidase in transformed plants results in diminished root mass from fewer root branches, which contributes to the wilting phenomenon seen in these plants. Further, this developmental change in transformed plants may be a consequence of the metabolism of IAA by the anionic peroxidase.     

Allometric relationships in young seedlings of faba bean (Vicia faba L.) following removal of certain root types

Sink-source relationships and allometric ratios were studied in young seedlings of faba bean (Vicia faba L.) following pruning of some root types. The plants were grown in an aeroponic system allowing an easy access to each part of the root system, throughout the experiment, without disturbing the others. Root, leaf and stem growth as well as their mineral content were determined in one group of undisturbed plants (CTRL) and in four groups of plants treated as follows: TAP – the distal-free portion of the taproot was removed; HALF – half the laterals were removed; ALL – all lateral roots were removed, and TAP+HALF – both the distal part of the taproot and half of the laterals were removed. Removal of all the lateral roots (ALL) induced the development of a longer taproot but severely arrested shoot growth. Phosphorus, potassium and calcium contents were lower in the plants of the ALL treatment. However, the content of Mg was practically unaffected. The effect of the HALF treatment was hardly noticeable but the effects of TAP+HALF treatment were cumulative. The allometric relationships between the surface area of the roots and that of the leaves were restored within the experimental period, apparently due to reduction in shoot growth. Removal of the distal parts of the taproot did not cause an increase in shoot growth. This indicates that the strength of the sinks (mostly of lateral roots) rather than that of the source determines these relationships.     

Plant development controls leaf area expansion in alfalfa plants competing for light

Background and Aims

The growth of crops in a mixture is more variable and difficult to predict than that in pure stands. Light partitioning and crop leaf area expansion play prominent roles in explaining this variability. However, in many crops commonly grown in mixtures, including the forage species alfalfa, the sensitivity and relative importance of the physiological responses involved in the light modulation of leaf area expansion are still to be established. This study was designed to assess the relative sensitivity of primary shoot development, branching and individual leaf expansion in alfalfa in response to light availability.

Methods

Two experiments were carried out. The first studied isolated plants to assess the potential development of different shoot types and growth periods. The second consisted of manipulating the intensity of competition for light using a range of canopies in pure and mixed stands at two densities so as to evaluate the relative effects on shoot development, leaf growth, and plant and shoot demography.

Key Results

Shoot development in the absence of light competition was deterministic (constant phyllochrons of 32·5 °Cd and 48·2 °Cd for primary axes and branches, branching probability of 1, constant delay of 1·75 phyllochron before axillary bud burst) and identical irrespective of shoot type and growth/regrowth periods. During light competition experiments, changes in plant development explained most of the plant leaf area variations, with average leaf size contributing to a lesser extent. Branch development and the number of shoots per plant were the leaf area components most affected by light availability. Primary axis development and plant demography were only affected in situations of severe light competition.

Conclusions

Plant leaf area components differed with regard to their sensitivity to light competition. The potential shoot development model presented in this study could serve as a framework to integrate light responses in alfalfa crop models.     

Differences in root and shoot development of tomato (Lycopersicon esculentum L.) varieties across contrasting soil environments

Summary The root and shoot growth of three tomato (Lycopersicon esculentum L.) varieties, VF-10, VF-1908 and Paste 56 were compared in either a river-washed sand or an alluvial silty loam, with or without added nutrients, to determine factors influencing root growth and development. VF-10 had consistently higher shoot weights than the other two varieties, particularly by 45 days from germination. Although the root-to-shoot ratio varied with soil treatment, no significant differences in this ratio occurred among varieties. The roots were characterised by a taproot, thickened in the uppermost 3 to 5 cm, with 86 to 190 secondary roots when harvested 30 days after germination. The variety VF-1908 had a higher proportion of fine roots than the other two varieties over all treatments. On the basis of root weight and length, VF-1908 was also more stable across the imposed environmental gradient than the varieties VF-10 or Paste 56. This arose from less variability across soil treatments in secondary branching in VF-1908; over the four soil treatments the number of secondary roots varied from 104 to 131 in VF-1908 compared to 86 to 139 in VF-10 and 95 to 190 in Paste 56. VF-1908 and Paste 56 also had less variation in tertiary branching than VF-10, which had a consistently greater number of major tertiary roots than the other two varieties over all soil treatments. With the exception of Paste 56 in the fertilized silt, shoot growth was correlated with root length.     

Fine-root system development and susceptibility to pathogen colonization

Root development may exert control on plant–pathogen interactions with soil-borne pathogens by shaping the spatial and temporal availability of susceptible tissues and in turn the impact of pathogen colonization on root function. To evaluate the relationship between root development and resistance to apple replant disease (ARD) pathogens, pathogen abundance was compared across root branching orders in a bioassay with two rootstock genotypes, M.26 (highly susceptible) and CG.210 (less susceptible). Root growth, anatomical development and secondary metabolite production were evaluated as tissue resistance mechanisms. ARD pathogens primarily colonized first and second order roots, which corresponded with cortical tissue senescence and loss in second and third order roots. Defense compounds were differentially allocated across root branching orders, while defense induction or stress response was only detected in first order and pioneer roots. Our results suggest disease development is based largely on fine-root tip attrition. In accordance, the less susceptible rootstock supported lower ARD pathogen abundance and altered defense compound production in first order and pioneer roots and maintained higher rates of root growth in both the ARD soil and pasteurized control compared to the more susceptible. Thus, this rootstock’s ability to maintain shoot growth in replant soil may be attributable to relative replant pathogen resistance in distal root branches as well as tolerance of infection based on rates of root growth.     

The effect of abscisic acid on root and shoot growth of cauliflower plants

Addition of abscisic acid (ABA) to the nutrient solution increased the root to shoot ratio of hydroponically-grown cauliflower plants by reducing the dry weight of the shoot and increasing that of the root. At concentrations higher than 10^–7 M, ABA increased root branching and root hair formation. Root extension was inhibited in plants kept continuously in solutions containing high ABA concentrations but following removal from the ABA solution root elongation was increased in comparison with plants given no ABA treatment. This elongation was greatest in plants with increased root branching caused by higher ABA concentrations.     

Gibberellin biosynthesis mutations and root development in pea

Dwarf mutants of pea (Pisum sativum), with impaired gibberellin (GA) biosynthesis in the shoot, were studied to determine whether the roots of these genotypes had altered elongation and GA levels. Mutations na, lh-2, and ls-1 reduced GA levels in root tips and taproot elongation, although in lh-2 and ls-1 roots the reduction in elongation was small (less than 15%). The na mutation reduced taproot length by about 50%. The roots of na plants elongated in response to applied GA(1) and recombining na with mutation sln (which blocks GA catabolism) increased GA(1) levels in root tips and completely restored normal root development. In shoots, Mendel's le-1 mutation impairs the 3beta-hydroxylation of GA(20) to the bioactive GA(1), resulting in dwarfism. However, GA(1) and GA(20) levels were normal in le-1 roots, as was root development. The null mutation le-2 also did not reduce root GA levels or elongation. The results support the theory that GAs are important for normal root elongation in pea, and indicate that a 3beta-hydroxylase gene other than LE operates in pea roots.     

Seasonal variation of growth and development of the roots of five second year conifer species in the nursery

Every other week over their second growing season, stem height, collar diameter, shoot and root dry masses, number of lateral roots and length of the tap root were measured on nursery grown seedlings ofAbies balsamea L. Mill.,Pinus banksiana Lamb.,Pinus resinosa Ait.,Picea mariana Mill. BSP andPicea glauca Moench Voss. Root elongation, branching and mycorrhizal development were also recorded.Given species showed distinct seasonal growth patterns. The rate and timing of maximum root growth (mg/dry weight/week) differed markedly between species.Except for the increase in height ofPinus banksiana, root and shoot growth were not negatively correlated.The results are discussed in relation to the performance of tree seedlings in the nursery.     

Influence of temperature and water potential on root growth of white oak

Root growth of white oak ( Quercus alba L.) was observed under field conditions using a rhizotron. The effects of temperature, soil water potential, and leaf water potential were evaluated on three measures of root growth and development: root elongation rate, number of growing roots, and root growth intensity (sum of projected root area compared to the total root viewing area). Root elongation rate was linearly related to changes in soil temperature and soil water potential. At soil temperatures less than 17deg;C, temperature was the dominant factor affecting rate of growth, bat at temperatures greater than 17°C soil water potential became the important factor. Unlike root elongation rate, the number of growing roots and root growth intensity increased at cold soil temperatures (8°C) and at soil water potentials of-0.3 to -0.8 MPa. At high soil water potentials (-0.1 MPa) root elongation rate reached a maximum while the number of growing roots and root growth intensity were low. These differences showed that root growth and development were not exclusively affected by the soil environment. In addition, the relationship between root growth and predawn leaf water potential suggested that root growth was a contributing factor to the drought resistance of white oak.     

Maize root system growth and development as influenced by phosphorus deficiency

Effects on leaf growth, biomass accumulation and root morphogenesis associated with the establishment of phosphorus (P) deficiency were studied on maize in order to test the hypothesis that the root system response can be accounted for by the effect of P deficiency on the carbon budget of the plant. P deprivation had a large and rapid negative effect on leaf expansion. For 7 d after P deprivation, the total dry matter production per plant was almost fully accounted for by the effect of P starvation on leaf growth and its subsequent effect on photosynthetically active radiation (PAR) interception. No strong effect of P deficiency was observed on the radiation use efficiency during this first period, although it was reduced thereafter. Root growth was slightly enhanced a few days after P starvation, but strongly reduced thereafter. The elongation rate of axile roots was maintained throughout the experiment, whereas emergence of new axile roots and elongation of first-order laterals were drastically reduced. The density of first-order laterals was not severely affected. These morphological responses are very similar to what is observed when root growth is limited by the availability in carbohydrates. The results are therefore compatible with the hypothesis that P deficiency mainly affects the root system morphology through its effect on the carbon budget of the plant with no additional specific effect of P deficiency on root morphogenesis. The drastic and early reduction of shoot growth after P deprivation may explain that more carbohydrates were available for root growth which was observed a few days after P starvation and reported by several authors. Later on, however, because of the reduced leaf area of P-deprived plants, their capacity to intercept light was severely reduced so that root growth was finally reduced.Keywords: Zea mays L., maize, phosphorus, root, root morphogenesis.      

Root elongation rate is accounted for by intercepted PPFD and source-sink relations in field and laboratory-grown sunflower

The existence of relationships between intercepted photo-synthetic photon flux density (PPFD) and growth of individual organs is somewhat controversial. We have tested whether such relationships could account for the natural variability in elongation rates of taproot and secondary roots of sunflower (from 2 to 135 mm d⁻¹), in field and laboratory conditions. Elongation of taproot and secondary roots was recorded daily through windows in the field. A range of PPFD was obtained by following day-to-day natural fluctuation for three contrasting growing periods, and by shading part of the plants under study. A parallel experiment was carried out in a growth chamber with contrasting light intensities and with a ¹⁴CO₂ labelling experiment. After the two-leaf stage, i.e. when the contribution of photosynthetic carbon became appreciable in root growth, daily root elongation rate was closely linked to the PPFD intercepted from 36 to 12 h before the measurement of root elongation. Curvilinear relationships applied to plants grown in the field as well as in a growth chamber, and to shaded plants as well as to plants subjected to day-to-day changes in intercepted PPFD. For a given intercepted PPFD, the taproot elongated faster than secondary roots, and secondary roots originating near the base of the taproot elongated faster than those originating near the apex. The elongation rate of any secondary root apex was accounted for (r= 0.77) by the ratio of intercepted PPFD to the distance between the apex and the base of the taproot. No relationships between intercepted PPFD and elongation rate were observed before the two-leaf stage, when the CO₂ labelling experiment suggests that carbon essentially originates from the seed. Therefore, this study suggests a role for source-sink relations in the distribution of elongation between apices and a role for carbon nutrition in day-to-day variations of root elongation rate. Precise mechanisms explaining this behaviour remain to be investigated.     

The rhizosphere signal molecule lumichrome alters seedling development in both legumes and cereals

The stimulatory role of lumichrome, a rhizosphere metabolite, was assessed on the growth of legume and cereal seedlings. At a very low nanomolar concentration (5 nm), lumichrome elicited growth promotion in cowpea, soybean, sorghum, millet and maize, but not in common bean, Bambara groundnut and Sudan grass. In soybean and cowpea only, 5 nm lumichrome caused early initiation of trifoliate leaf development, expansion in unifoliate and trifoliate leaves, increased stem elongation and, as a result, an increase in shoot and plant total biomass relative to control. Lumichrome (5 nm) also increased leaf area in maize and sorghum, and thus raised shoot and total biomass but there was no effect on the leaf area of the other cereals. Root growth was also stimulated in sorghum and millet by the supply of 5 nm lumichrome. By contrast, the application of a higher dose of lumichrome (50 nm) depressed development of unifoliate leaves in soybean, the second trifoliate leaf in cowpea, and shoot biomass in soybean. The 50 nm concentration also consistently decreased root development in cowpea and millet, but had no effect on the other species. These data show that lumichrome is a rhizosphere signal molecule that affects seedling development in both monocots and dicots.     

Effects of applying etnylene to the root system of Zea mays on growth and nutrient concentration in relation to flooding tolerance

Previous studies have shown increases in the concentration of ethylene in the soil and roots of plants when the soil is water saturated (flooded). In Zea mays L. this occurs in association with an overall reduction in growth but without extensive foliar senescence and in conjunction with the development of an adventitious root system. We have assessed the possibility that ethylene may be involved in these responses to flooding. Mixtures of the gas in air were therefore supplied to the roots and stem-base of Z. mays growing in nutrient solution.
Seven or 14 d exposure to ethylene (1 or 5 νl 1⁻¹) inhibited seminal root elongation and growth in dry weight and accelerated the emergence of adventitious roots, although their final length and dry weight were depressed. Leaf extension was inhibited by 0.1,1.0 or 5.0 μl 1⁻¹ ethylene around the roots; leaves extending rapidiy at the start of treatment were the most sensitive. Final shoot fresh and dry weights were depressed by the gas but tie shootrroot dry weighl ratio and percentage dry matter were not affected greatly. Leaf chlorosis was not observed but the concentration of phosphorus in the shoots was 26 to 31% below normal.
When aeration of the nutrient solution was stopped, the concentration of dissolved oxygen declined and the concentration of ethylene in the roots increased. Similar changes occur in response to soil flooding. Root and shoot growth was slowed by non-aeration although the shootroot dry weight ratio remained unchanged. The phosphorus concentration of the shoots was depressed but there was little chlorosis or leaf death. The similarity in these respects between the effects of ethylene and non-aeration suggests that in flooded Z. mays , ethylene contributes to their development by accelerating the emergence of adventitioos roots, inhibiting phosphorus accumulation in the shoots and by a non-toxic inhibition of plant growth.     

Patterns of variability in the diameter of lateral roots in the banana root system

The relative importance of root system structure, plant carbon status and soil environment in the determination of lateral root diameter remains unclear, and was investigated in this study. Banana (Musa acuminata) plants were grown at various moderate levels of soil compaction in two distinct experiments, in a field experiment (FE) and in a glasshouse experiment (GE). Radiant flux density was 5 times lower in GE. The distribution of root diameter was measured for several root branching orders. Root diameters ranged between 0.09 and 0.52 mm for secondary roots and between 0.06 and 0.27 mm for tertiary roots. A relationship was found between the diameter of the parent bearing root and the median diameter of its laterals, which appears to be valid for a wide range of species. Mean lateral root diameter increased with distance to the base of the root and decreased with branching density [number of lateral roots per unit length of bearing root (cm(-1))]. Typical symptoms of low light availability were observed in GE. In this case, lateral root diameter variability was reduced. Although primary root growth was affected by soil compaction, no effects on lateral root diameter were observed.