Root responses of Quercus ilex L. seedlings to drought and fire

Abstract

Drought treatments in holm-oak (Quercus ilex) seedlings induce variations in total root length, number of root apices, shoot/root dry weight, and root electrolyte leakage. When drought treatments last for more than 50 days a considerable number of fine lateral roots die, irrespective of branching order or distribution within the root system. Scorching of drought-treated seedlings induces a transient stimulation of root growth. These results indicate that root turnover is deeply affected during treatments, with survival of seedlings being entrusted to the tolerance of a number of roots situated in the deeper region of the root system. Activity of the meristematic tissue present within the apices of these surviving roots supports regeneration of above-ground lost organs during recovery. Knowledge of the mechanisms ensuring the survival of Mediterranean tree seedlings following drought and fire is useful for developing models of vegetation dynamics.     

Root Morphology, Stem Growth and Field Performance of Seedlings of Two Mediterranean Evergreen Oak Species Raised in Different Container Types

Outplanting container-grown oak seedlings with undesirable shoot and root characteristics result in poor establishment and reduced field growth. The objective of this study was to determine the influence of container type on both above-and below-ground nursery growth and field performance of one-year old tap-rooted seedlings Quercus ilex L. and Quercus coccifera L. The experiment was conducted in an open-air nursery and the seedlings were grown in three container types. At the end of the nursery, growth period seedlings’ shoot height, diameter (5 mm above root collar), shoot and root biomass, root surface area, root volume and total root length were assessed. Then the seedlings were planted in the field and their survival and growth were recorded for two growing seasons after outplanting. The results showed a difference between the Quercus species in the effect of container type. Q. ilex seedlings raised in paper-pot had significantly greater height, diameter, shoot and root biomass and root volume than those raised in the other two container types. Similarly, Q. coccifera seedlings raised in paper-pot, had significantly greater above-and below-ground growth than those raised in the other two container types. Both oak species showed relatively low survival in the field; the mortality was mainly observed the first year after outplanting, especially after the summer dry period. However, 2 years after outplanting, the paper-pot seedlings of the two oak species showed better field performance.     

Effects of sand burial depth on the root system of Salix cheilophila seedlings in Mu Us Sandy Land, Inner Mongolia, China

Salix cheilophila Schneid. is a naturally occurring Salix species in Mu Us Sandy Land, Inner Mongolia, China. We focused on the morphological adaptability of S. cheilophila to sand dune burial. For morphological measurements, 32 S. cheilophila seedlings were removed from a community which was in the process of being buried by a shifting sand dune. Each seedling collected included the entire root system. We measured the number, length, and biomass of the adventitious roots, primary lateral roots, and taproot, and compared the morphological characteristics of the root system, including adventitious roots, for seedlings buried to various levels in the sand. The growth range of adventitious roots increased as the length of the buried portion of the main shoot increased. In addition, the total dry weight of all current-year shoots tended to increase gradually with increasing total dry weight of the adventitious roots. These results suggest that S. cheilophila tends to make use of the sedimentary sand layer that accompanies shifting sand dunes. However, there was no correlation between biomass or number of adventitious roots and the length of the buried part of the main shoot. Thus, S. cheilophila does not grow adventitious roots proportional to the buried part. These morphological characteristics of the root system, including the adventitious roots, may indicate that S. cheilophila has poor morphological adaptability to sand dune burial.     

Modification of root architecture in woody plants is possible for the presence of two different mechanisms of lateral root production: The effect of slope in Spartium junceum L. seedlings

Abstract

This paper investigates the modification of root architecture of Spartium junceum L. seedlings grown in slope condition. It is reported that 50% of the total number of lateral roots are concentrated in few centimetres of the taproot near the collar. The anatomical analysis of transverse sections along the taproot axis reveals that this taproot zone is characterised by two types of lateral roots: one with a trace extending to the centre of the vascular cylinder by following the path of a medullar ray; one with a trace which ends in the vascular cambium. The first type may be lateral roots originated from the taproot primary structure; the second type seems to be lateral roots developing later when a secondary structure has completely substituted the primary structure. The emission of this second type of lateral roots seems to be strongly controlled by environmental conditions with considerable consequences upon the overall root architecture. In the example reported in this paper, young plants growing under mechanical stress due to a slope develop asymmetric root architecture with lateral roots elongating in two prevalent directions: up-slope or down-slope. This asymmetric architecture is produced in the zone of the taproot where a secondary structure is present and represents the plant response to the need of increasing its anchorage strength.     

EFFECTS OF SOIL WATER DISTRIBUTION ON THE LATERAL ROOT DEVELOPMENT OF THREE SPECIES OF CALIFORNIA OAKS

The responses of seedling root systems of three species of oaks in California to two experimental soil moisture regimes were studied by comparing lateral root development, root and shoot weights, and root: shoot ratios. In the first soil moisture treatment the taproot was allowed to extend into moist soil throughout the duration of the experiment (control), while in the second treatment (shallow) the taproot grew into a dry substrate below 30 cm of moist soil. The treatments were intended to approximate soil moisture conditions experienced by oak seedlings in the field when deep soil water sources vary in their accessibility (control: accessible, shallow: inaccessible). Lateral root growth of Quercus agrifolia did not increase significantly when the primary root tip died in the shallow treatment, resulting in an overall decrease in the percent of the root system composed of lateral roots. Q. douglasii and Q. lobata increased lateral root weights by 80% and 70%, respectively, on the upper 30 cm of the primary root when the primary root tip died. Q. lobata was the only species that decreased in shoot and root weight (25% and 21%, respectively) with the loss of the root tip, indicating that, unlike the other species, it was dependent on the primary root for maximum growth. The morphological responses of these species correspond with their distributions and also may be a factor that influences their interactions with other species.     

Compared root system architectures in seedlings and in vitro plantlets of Hevea brasiliensis,in the initial years of growth in the field

In vitro culture of Hevea was undertaken to propagate selected clones on their own roots. The challenge was to overcome the failure of cuttings due to the poor conformity of regenerated root systems. Trees of several juvenile or mature genotypes were propagated either by in vitro microcutting, or by somatic embryogenesis, and planted in the field. Certain static and dynamic components of the root system were observed at different growth stages, from 0 to 3 years, and compared to those of seedlings of the same age used in the trial as a reference. A simple method was designed for measuring the vigour and balance of the root system. The in vitro plantlets had a well-developed taproot and lateral root system, with an architecture similar to that of plants obtained from seed. Moreover, clear differences occurred between selected clones for the relative vigour of the tap roots, lateral roots and trunk. This revised version was published online in June 2006 with corrections to the Cover Date.     

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     

The arbuscular mycorrhizal fungus Diversispora spurca ameliorates effects of waterlogging on growth,root system architecture and antioxidant enzyme activities of citrus seedlings

Citrus plants are often exposed to heavy rain and subsequent periods of soil waterlogging which severely restrict tree growth. We assessed the effect of one arbuscular mycorrhizal fungus species (Diversispora spurca) on growth, root system architecture (RSA), and antioxidant enzyme activities of young citrus (Citrus junos) seedlings. Waterlogging for 37 d significantly restricted mycorrhizal colonization but increased the number of entry points and vesicles. Compared with non-mycorrhizal controls, mycorrhizal seedlings had significantly greater plant height, fresh mass, total root and taproot lengths, projected and surface root areas, root volume, and numbers of lst, 2nd and 3rd order lateral roots regardless of waterlogging treatment. D. spurca significantly increased root catalase (CAT) activity in non-stressed seedlings and increased root soluble protein concentration and leaf CAT activity in waterlogged seedlings, thereby inducing lower oxidative damage. These results suggest that D. spurca ameliorates effects of waterlogging on growth, RSA and antioxidant enzyme activities.     

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.     

Post-fire seedlings of Nothofagus alpina in Southern Chile show strong dominance of a single ectomycorrhizal fungus and a vertical shift in root architecture

We investigated belowground responses of Nothofagus alpina seedlings to post-fire conditions during natural regeneration after a wildfire in Chile, focusing on mycorrhizal community and root architecture. The complete root systems of 2-year-old N. alpina seedlings were extracted from a post-fire site with natural regeneration and compared to roots of seedlings from undisturbed forest nearby. Mycorrhizal morphotype richness was determined in each seedling. Morphometric parameters of tertiary root structure and dry biomass of whole root systems were determined in 5 cm vertical intervals and in four lateral root classes. With 43.5% of colonized vital mycorrhizal root tips, the Basidiomycete Descolea antarctica was the most abundant fungal symbiont on post-fire seedlings. Tertiary root morphology of these seedlings was distinct from control plants and characterized by a deep-reaching tap root with rather evenly distributed lateral branches whereas seedlings from the undisturbed site had shallower root systems with most lateral roots concentrated in the upper soil layers. Post-fire seedlings had more mycorrhizal rootlets and mycorrhiza-bearing third order lateral roots than control plants which was expressed in a 34% higher total root number but only a 10% higher total root biomass, although both values were not statistically significant. A major part of these fine roots in seedlings from burnt forest was found in deeper soil horizons, compared to the seedlings from undisturbed forest. According to our results, post-fire conditions clearly favour Descolea antarctica as an early ectomycorrhizal colonizer of Nothofagus seedlings at the studied site. As no significant changes in soil chemistry could be observed at the burnt site, the deep-reaching tertiary root architecture of these seedlings may be interpreted as a response to other abiotic factors like reduced moisture in surface soil.     

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.     

Patterns of root architecture adaptation of a phreatophytic perennial desert plant in a hyperarid desert

Plant root systems can respond to nutrient availability and distribution by changing the three-dimensional deployment of their root architecture. The year after year variation of root architecture was investigated in a perennial phreatophyte in the controlled condition vegetation situated in the oasis in the Chinese Taklamakan desert with the goal to elucidate their adaptation to hyperarid environment. The whole plants of an indigenous perennial legume Alhagi sparsifolia Shap. with intact root systems were excavated at the end of each growing season from 2007 to 2009 and analyzed for architecture, above and belowground biomass, root/shoot ratio, root depth, seed yields and ramet. Changes in water availability were found to have stupendous effects on taproot depth, lateral root length and ramet quantity. Relative to shoot dry weight, taproot depth decreased with increasing water availability. In contrast, lateral root elongation was promoted by higher water availability.We tested the hypothesis that (1) irrigation increases root biomass and the quantity of ramets, and (2) A. sparsifolia Shap. develops an efficient root architecture that could absorb soil water and nutrition.     

Root System Asymmetry of Mediterranean Pines

Three groups of Mediterranean pines were examined to describe the development of root symmetry on sites characterized by shallow soils and low water availability. Sampling included: (1) 3-year-old planted seedlings of Pinus halepensis Mill. taken from Sithonia Halkidiki, northern Greece, (2) 5-year-old natural regenerated seedlings of Pinus brutia Ten. taken from Kedrinos Lofos, Thessaloniki and (3) 65-year-old trees of Pinus brutia taken from Kedrinos Lofos, Thessaloniki. Root system symmetry was examined by measuring the number, the diameter, the cross-sectional area (CSA), the root area index (RAI) and the length of the lateral roots of each root system, and by analyzing their distribution around the stem. Above-ground plant symmetry was also estimated. The findings of the study indicated that there was an asymmetric root system in all three groups that is characterized by the concentration of the main laterals along the contour lines instead of uphill or downhill; however, the asymmetry was much higher in the young plants. This asymmetry was not correlated with the above-ground plant growth form, which was found to be symmetric. The asymmetric development of root can be attributed to the shallow soil and the high mechanical resistance of the underground bedrock that stopped the taproot growth, restricted the root penetration in the deeper layers and obliged the roots to elongate towards the surface soil layers, where there is more available water.     

Root hydraulic conductance,gas exchange and leaf water potential in seedlings of Pistacia lentiscus L. and Quercus suber L. grown under different fertilization and light regimes

Differences in morphology, biomass allocations and physiological responses were investigated in seedlings of Mastic tree (Pistacia lentiscus L.) and Cork oak (Quercus suber L.) submitted to contrasting fertilization and light regimes during early growth. These species are two evergreen sclerophyllous Mediterranean species frequently used in Mediterranean reforestation programmes. Fertilization was the treatment that affected most of the morphological and physiological variables evaluated in P. lentiscus and Q. suber seedlings. Leaf area and specific leaf area (SLA) were affected by shading treatment in both species, showing higher values in seedlings grown under shade. P. lentiscus seedlings showed a high capacity to modify root morphological variables and root hydraulic conductance (K_R) with the fertilization treatment. In contrast, Q. suber showed low to moderate root system changes with the treatments applied, although the fertilization level affected biomass allocation (i.e., root to shoot ratio) in both species. Under high water demand, P. lentiscus seedlings with high K_R allowed transpiration (E) to increase without increasing the water potential gradient between soil and leaves. In Q. suber, high fertilization induced significant increases in photosynthesis (A), as well as a tendency to increase E with significantly lower leaf water potential (ψ_L).     

Is it possible to manipulate root anchorage in young trees?

The optimal root system architecture for increased tree anchorage has not yet been determined and in particular, the role of the tap root remains elusive. In Maritime pine (Pinus pinaster Ait.), tap roots may play an important role in anchoring young trees, but in adult trees, their growth is often impeded by the presence of a hard pan layer in the soil and the tap root becomes a minor component of tree anchorage. To understand better the role of the tap root in young trees, we grew cuttings (no tap root present) and seedlings where the tap root had (?) or had not (+) been pruned, in the field for 7 years. The force (F) necessary to deflect the stem sideways was then measured and divided by stem cross-sectional area (CSA), giving a parameter analogous to stress during bending. Root systems were extracted and root architecture and wood mechanical properties (density and longitudinal modulus of elasticity, E _L ) determined. In seedlings (?) tap roots, new roots had regenerated where the tap root had been pruned, whereas in cuttings, one or two lateral roots had grown downwards and acted as tap roots. Cuttings had significantly less lateral roots than the other treatments, but those near the soil surface were 14% and 23% thicker than plants (+) and (?) tap roots, respectively. Cuttings were smaller than seedlings, but were not relatively less resistant to stem deflection, probably because the thicker lateral roots compensated for their lower number. Apart from stem volume which was greater in trees (+) tap roots, no significant differences with regard to size or any root system variable were found in plants (?) or (+) tap roots. In all treatments, lateral roots were structurally reinforced through extra growth along the direction of the prevailing wind, which also improved tap root anchorage. Predictors of log F/CSA differed depending on treatment: in trees (?) tap roots, a combination of the predictors stem taper and %volume allocated to deep roots was highly regressed with log F/CSA (R ² = 0.83), unlike plants (+) tap roots where the combined predictors of lateral root number and root depth were best regressed with log F/CSA (R ² = 0.80). In cuttings, no clear relationships between log F/CSA and any parameter could be found. Wood density and E _L did not differ between roots, but did diminish with increasing distance from the stem in lateral roots. E _L was significantly lower in lateral roots from cuttings. Results showed that nursery techniques influence plant development but that the architectural pattern of Maritime pine root systems is stable, developing a sinker root system even when grown from cuttings. Anchorage is affected but the consequences for the long-term are still not known. Numerical modelling may be the only viable method to investigate the function that each root plays in adult tree anchorage.     

Local and Systemic Regulation of Plant Root System Architecture and Symbiotic Nodulation by a Receptor-Like Kinase

In plants, root system architecture is determined by the activity of root apical meristems, which control the root growth rate, and by the formation of lateral roots. In legumes, an additional root lateral organ can develop: the symbiotic nitrogen-fixing nodule. We identified in Medicago truncatula ten allelic mutants showing a compact root architecture phenotype (cra2) independent of any major shoot phenotype, and that consisted of shorter roots, an increased number of lateral roots, and a reduced number of nodules. The CRA2 gene encodes a Leucine-Rich Repeat Receptor-Like Kinase (LRR-RLK) that primarily negatively regulates lateral root formation and positively regulates symbiotic nodulation. Grafting experiments revealed that CRA2 acts through different pathways to regulate these lateral organs originating from the roots, locally controlling the lateral root development and nodule formation systemically from the shoots. The CRA2 LRR-RLK therefore integrates short- and long-distance regulations to control root system architecture under non-symbiotic and symbiotic conditions.     

Deep Phenotyping of Coarse Root Architecture in R. pseudoacacia Reveals That Tree Root System Plasticity Is Confined within Its Architectural Model

This study aims at assessing the influence of slope angle and multi-directional flexing and their interaction on the root architecture of Robinia pseudoacacia seedlings, with a particular focus on architectural model and trait plasticity. 36 trees were grown from seed in containers inclined at 0° (control) or 45° (slope) in a glasshouse. The shoots of half the plants were gently flexed for 5 minutes a day. After 6 months, root systems were excavated and digitized in 3D, and biomass measured. Over 100 root architectural traits were determined. Both slope and flexing increased significantly plant size. Non-flexed trees on 45° slopes developed shallow roots which were largely aligned perpendicular to the slope. Compared to the controls, flexed trees on 0° slopes possessed a shorter and thicker taproot held in place by regularly distributed long and thin lateral roots. Flexed trees on the 45° slope also developed a thick vertically aligned taproot, with more volume allocated to upslope surface lateral roots, due to the greater soil volume uphill. We show that there is an inherent root system architectural model, but that a certain number of traits are highly plastic. This plasticity will permit root architectural design to be modified depending on external mechanical signals perceived by young trees.     

Deuterium labelling of roots provides evidence of deep water access and hydraulic lift by Pinus nigra in a Mediterranean forest of NE Spain

We studied the tree access to deep water sources and the possibility of hydraulic lift from the deep roots of a Pinus nigra tree to the shallow soil layers in a Mediterranean forest of NE Spain. We also studied the use of hydraulically lifted water by neighboring trees, shrubs, and sprouts. We enriched the roots of a large P. nigra (10 m tall) with deuterium by accessing them from a below ground cave. During the next 3 days we measured stable deuterium isotopic composition of xylem sap, shoot predawn and midday water potentials, and the leaf δ¹³C and δ¹⁵N of the P. nigra tree, neighboring Quercus ilex ballota trees and sprouts, and Juniperus oxycedrus shrubs. The study was conducted both in dry summer and in wet spring. In summer, deuterated water absorbed by deep roots of P. nigra appeared in the stem water of neighboring plants and in surface soil. The most δD-enriched plant xylem sap was found in the enriched P. nigra tree, followed by the Q. ilex sprouts, the small Q. ilex trees and the surface soil (15 cm). All these trends disappeared in the wet spring season, when HDO only slightly appeared in the surface soil. The results show that the studied P. nigra tree accesses deep water source and conducts hydraulic lift in this Mediterranean forest in dry summer but not necessarily in wet spring.     

Characterization of CYCLOPHILLIN38 shows that a photosynthesis-derived systemic signal controls lateral root emergence

Photosynthesis in leaves generates fixed-carbon resources and essential metabolites that support sink tissues, such as roots. Two of these metabolites, sucrose and auxin, promote growth in root systems, but the explicit connection between photosynthetic activity and control of root architecture has not been explored. Through a mutant screen to identify pathways regulating root system architecture, we identified a mutation in the Arabidopsis thaliana CYCLOPHILIN 38 (CYP38) gene, which causes accumulation of pre-emergent stage lateral roots. CYP38 was previously reported to stabilize photosystem II (PSII) in chloroplasts. CYP38 expression is enriched in shoots, and grafting experiments show that the gene acts non-cell-autonomously to promote lateral root emergence. Growth of wild-type plants under low-light conditions phenocopies the cyp38 lateral root emergence defect, as does the inhibition of PSII-dependent electron transport or Nicotinamide adenine dinucleotide phosphate (NADPH) production. Importantly, these perturbations to photosynthetic activity rapidly suppress lateral root emergence, which is separate from their effects on shoot size. Supplementary exogenous sucrose largely rescued primary root (PR) growth in cyp38, but not lateral root growth. Auxin (indole-3-acetic acid (IAA)) biosynthesis from tryptophan is dependent on reductant generated during photosynthesis. Consistently, we found that wild-type seedlings grown under low light and cyp38 mutants have highly diminished levels of IAA in root tissues. IAA treatment rescued the cyp38 lateral root defect, revealing that photosynthesis promotes lateral root emergence partly through IAA biosynthesis. These data directly confirm the importance of CYP38-dependent photosynthetic activity in supporting root growth, and define the specific contributions of two metabolites in refining root architecture under light-limited conditions.

Lateral root emergence is regulated via systemic signaling that incorporates photosynthesis-dependent redox control and auxin biosynthesis.     

Growth and branching of the taproot of young oak trees--a dynamic study

The growth and branching of the taproot of young oak trees werestudied on seedlings grown in root observation boxes over aperiod of 30–45 d. The development of shoots and rootswere recorded daily on a set of eight plants, and additionalobservations on the initiation of primordia were made on anotherset of 18 plants. Taproot growth was typically indeterminateand linear, with growth rates in the range of 1.5–2.5cm d^–1. In some cases, however, growth slowed down orstopped, and resumed a few days later. The growth of shootsand roots were not synchronized. Taproots branched in two ways:acropetal branches emerged from 4–8-d-old taproot tissues,and late branches emerged from older tissues (up to 30 d inour experiment). The latter appeared especially when taprootgrowth slowed down or stopped. New primordia were initiatedon tissues older than 1.4 d, and lateral roots emerged aftera minimal development duration of 2.3 d. These time-relatedparameters described the emergence of branching very effectivety,since they were quite stable over a wide range of taproot growthrates. However, emergence duration decreased slightly in fastgrowing taproots. Branching density tended to increase withthe taproot growth rate. Physiological significance and consequences for modelling rootsystem development are discussed. Key words: Quercus robur, root system, growth, branching, primordium initiation