落叶松和水曲柳不同根序细根形态结构、组织氮浓度与根呼吸的关系

根系具有高度的形态和生理功能异质性, 在森林生态系统碳和养分循环中起重要作用。根系分枝的顺序构成根序,是根系最基本的构型特征, 根序代表根系不同的发育阶段。然而, 目前直接测定不同根序细根生理功能的研究很少。以落叶松(Larix gmelinii)和水曲柳(Fraxinus mandshurica)的细根为研究对象, 使用气相氧电极测定不同根序细根的呼吸速率, 探讨根系呼吸速率与其形态、结构和组织氮浓度的关系。结果表明: 落叶松和水曲柳细根的直径、根长和维管束直径均随着根序的增加(1–5级)而增加, 而比根长、组织氮浓度和呼吸速率随着根序的增加而降低, 各根序之间差异显著(P < 0.05); 1级根比根长最大、皮层组织发达、组织氮浓度最高且呼吸速率也最高, 其呼吸速率分别为17.57 nmolO₂·g^–1·s^–1(落叶松)和18.80 nmolO₂·g^–1·s^–1(水曲柳), 比5级根分别高148%(落叶松)和124%(水曲柳); 并且, 落叶松根的呼吸速率几乎有96%与根系组织氮浓度相关, 而水曲柳根的呼吸速率则有89%与根系组织氮浓度相关。上述结果说明, 细根的形态和生理功能异质性是紧密相连的, 低级根的形态、结构决定其功能是吸收养分和水, 而高级根的形态、结构决定其功能是运输和贮存养分。     

Relationships among shoot sinks for resources exported from nodal roots regulate branch development of distal non-rooted portions of Trifolium repens L

Two manipulative experiments tested hypotheses pertaining to the correlative control exerted by nodal roots on branch development of the distal non-rooted portion of Trifolium repens growing clonally under near-optimal conditions. The two experiments, differing in their pattern of excision to manipulate the number of branches formed at the first 9-10 phytomers distal to the youngest nodal root, each found that after 20 phytomers of growth the total number of lateral branches formed on the primary stolon remained between five and seven regardless of where the branches formed along the stolon. Additional treatments established that nodal roots influenced branch development via relationships among shoot sinks for the root-supplied resources rather than through variation in the supply of such resources induced by fluctuations in photosynthate supply to roots from branches. Regression analysis of data pooled from treatments of both experiments confirmed that shoot-sink relationships for root- supplied resources controlled the branching processes on the non-rooted portion of plants. A disbudding treatment, which removed all the apical and axillary buds present on basal branches, but left other branch tissues intact, increased branch development of the apical region in the same way as did complete excision of the basal lateral branches. The apical buds and the elongation processes occurring immediately proximal to the buds were thus identified as strong sinks for the root-supplied resources. Such results suggest that branch development on the non-rooted shoot portion distal to the youngest nodal root is regulated by competition among sinks for root-derived resources, of limited availability, necessary for the processes of elongation of axillary buds and the primary stolon apical bud.     

Root system architecture of Quercus pubescens trees growing on different sloping conditions

BACKGROUND AND AIMS: Plant roots' growth direction has important implications for plant development and survival; moreover it plays an effective and vital role in stabilizing weathered soil on a steep slope. The aim of this work was to assess the influence of slope on the architecture of woody root systems. METHODS: Five mature, single-stemmed Quercus pubescens trees growing on a steep slope and five on a shallow slope were excavated to a root diameter of 1 cm. A very precise numeric representation of the geometry and topology of structural root architecture was gained using a low-magnetic-field digitizing device (Fastrak, Polhemus). Several characteristics of root architecture were extracted by macros, including root volume, diameter, length, number, spatial position and branching order. KEY RESULTS: The diameter at breast height (dbh) was the best predictor of the root volume but had no correlation with length and number of roots. The slope affected the root volume for each branching order, and the basal cross-sectional area (CSA), number and length of the first-order roots. Number and length of the second- and third-order laterals were closely related in both conditions, although this relationship was closer in the shallow trees, suggesting the influence of a genetic control. Sloping trees showed a clustering tendency of the first- and second-order lateral roots in the up-slope direction, suggesting that the laterals rather than the taproots provide much of the anchorage. In a steep-slope condition, the taproot tapering was positively correlated with the asymmetry magnitude of first-order roots, indicating compensation between taproot and main lateral roots' clustering tendency. CONCLUSIONS: These results suggest that on a slope, on clayey soils, root asymmetry appears to be a consequence of several environmental factors such as inclination, shallow-slides and soil compactness. In addition, this adaptive growth seems to counteract the turning moment induced by the self-loading forces acting in slope conditions, and as a consequence improves the tree stability.     

水曲柳和落叶松不同根序之间细根直径的变异研究

细根直径大小和根序高低对细根寿命和周转估计具有重要的影响,研究不同根序之间的直径变异对认识细根直径与根序的关系具有重要意义。该文根据Pregitzer等(2002)提供的方法,研究了位于东北林业大学帽儿山实验林场尖砬沟森林培育实验站17年生水曲柳(Fraxinus mandshurica)和落叶松(Larix gmelinii)人工林细根1~5级根序的平均直径的变化、直径的最小值和最大值范围、直径的变异系数。结果表明,水曲柳和落叶松细根直径<2 mm时,包含5个根序,随着根序由小到大的增加,细根直径也在增大。各根序平均直径之间,存在较大的差异。在同一根序内,细根直径范围很大,水曲柳和落叶松一级根最小直径均<0.20 mm,最大直径分别<0.50 mm(水曲柳)和<0.70 mm(落叶松)左右。2~3级根序直径最小值在0.20~0.30 mm之间,最大值≤1.0 mm。5级根直径最小值<1.0 mm,最大值超过2.0 mm。随着根序等级增加,直径变异系数增大。一级根序的直径平均变异系数<10%,2~3级根序直径平均变异系数在10%~15%左右,4~5级根序直径的平均变异系数在20%~30%之间。因此,在细根寿命与周转研究过程中,必须同时考虑直径和根序对细根的寿命估计的影响。     

帽儿山天然次生林20个阔叶树种细根形态

 细根在森林生态系统C分配和养分循环过程中发挥着重要作用。细根形态不但影响养分和水分的吸收, 而且与细根寿命和周转有密切关系。因此, 研究森林树种的细根形态对了解根系结构与功能、预测寿命与周转具有重要理论意义。该文根据细根分枝等级划分方法, 研究了东北帽儿山天然次生林20个阔叶树种1～5级根直径、根长和比根长等形态指标。结果表明, 20个树种中, 除5个树种1级根直径略大于2级和比根长略小于2级根外, 其余15个树种均表现为1级根直径和根长最小、比根长最高, 随着根序增加, 直径和根长增加, 而比根长降低。20个阔叶树种前3级根的累积根长均占前5级根总根长的80%以上。9个内生菌根侵染的树种的平均直径、根长和比根长均大于11个外生菌根侵染的树种。     

Fine root architecture, morphology, and biomass of different branch orders of two Chinese temperate tree species

We have limited understanding of architecture and morphology of fine root systems in large woody trees. This study investigated architecture, morphology, and biomass of different fine root branch orders of two temperate tree species from Northeastern China—Larix gmelinii Rupr and Fraxinus mandshurica Rupr —by sampling up to five fine root branch orders three times during the 2003 growing season from two soil depths (i.e., 0–10 and.10–20 cm). Branching ratio (R _b) differed with the level of branching: R _b values from the fifth to the second order of branching were approximately three in both species, but markedly higher for the first two orders of branching, reaching a value of 10.4 for L. gmelinii and 18.6 for F. mandshurica. Fine root diameter, length, SRL and root length density not only had systematic changes with root order, but also varied significantly with season and soil depth. Total biomass per order did not change systematically with branch order. Compared to the second, third and/or fourth order, the first order roots exhibited higher biomass throughout the growing season and soil depths, a pattern related to consistently higher R _b values for the first two orders of branching than the other levels of branching. Moreover, the differences in architecture and morphology across order, season, and soil depth between the two species were consistent with the morphological disparity between gymnosperms and angiosperms reported previously. The results of this study suggest that root architecture and morphology, especially those of the first order roots, should be important for understanding the complexity and multi-functionality of tree fine roots with respect to root nutrient and water uptake, and fine root dynamics in forest ecosystems.     

Lower order roots more palatable to herbivores: a case study with two temperate tree species

Determining which kinds of roots are likely to be consumed by root herbivores may improve our understanding of the mechanistic control on fine root dynamics. Here, we tested the hypothesis that root herbivores prefer to consume the distal lower order roots in their branching networks. Insecticide was applied to soil to quantify effects of root herbivores on root biomass and production in the first five orders (the distal roots numbered as first-order) in Fraxinus mandshurica and Larix gmelinii plantations from May 2008 to July 2009. Root morphology, chemistry, anatomy and physiology were measured simultaneously across branching orders. Among the first five order roots, significant consumptions by herbivores were found only for the two distal lower order roots throughout growing seasons, with 62% of biomass and 57% of production for F. mandshurica, and 71% and 79% for L. gmelinii, respectively. Our results suggest that the distal lower order roots are more palatable and attractive to root herbivores in both plantations, probably because they have higher tissue N, greater respiration rates and lower cellulose. Thus, overlooking herbivore consumption may lead to large underestimation in root biomass and production, which are critical in determining C budget and nutrient cycles in forest ecosystems.     

The development of Archaeopteris: new evolutionary characters from the structural analysis of an Early Famennian trunk from southeast Morocco

A 5 m long trunk of a young Archaeopteris/Callixylon erianum tree from the Late Devonian of Morocco shows new branching patterns for early lignophytes. This progymnosperm tree produces a helical pattern of traces that we infer belonged to reduced, short-lived, primary (apical) branches (type A) as well as two types of adventitious traces (types B and H). We infer that type-B traces supplied branches that initiate close to the site of attachment on the trunk of some, but not all type-A branches in an irregular but nonrandom pattern. Unlike ephemeral type-A branches, those of type B persist and become long-lived, potentially permanent units of the architecture of Archaeopteris trees. Type-H adventitious traces are also short-lived and occur singly or in serial groups, but differ from traces of either type A or B branches by lacking differentiation into a readily identifiable organ category. We interpret type-H traces as supplying latent primordia that could develop into either adventitious roots or shoots depending on extrinsic factors. Our new data suggest that Archaeopteris had a wide range of branch primordium amplitude. Type-B branches compare with axillary lateral branch buds of some Early Carboniferous spermatophytes (Calamopitys) and are a major developmental departure from the strictly apical, pseudomonopodial shoot branching of older aneurophyte progymnosperms. Type-H traces suggest that Archaeopteris trees had some potential for formation of adventitious roots or shoots in response to environmental factors, such as partial burial by overbank sedimentation. Collectively, these novel methods of tree branching may partly explain the extraordinary success and worldwide dominance of Archaeopteris forests on fluvially dominated, Late Devonian floodplains.     

DEVELOPMENT OF THE WOODY PORTION OF THE ROOT SYSTEM OF BETULA PAPYRIFERA

Our ontogenetic analysis of paper birch root systems shows that the fate of a root tip is related to its relative primary xylem diam (PXD). Lateral root tips with an initial PXD less than about 25 % of the parent root PXD are ephemeral. Some tips having a PXD of more than about 25 % of the parent root PXD become the permanent portions of the root system, enlarging over time as they elongate. In seedling root systems, the primary root and first-formed laterals are initially about the same size, and their PXD's all enlarge with increasing distance from the stem as the tips elongate to form the initial horizontal woody framework. Permanent lateral root branches with a large relative PXD develop after root tip injury, when the root tip is forced to grow in a curve, or from other unaccountable causes. Our observations show the importance of using relative diameters when classifying roots and when applying the concept of heterorhizy to paper birch root systems. Evidence points to the existence of some form of apical dominance in roots.     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition.Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattern.     

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.     

Phosphate Availability Alters Lateral Root Anatomy and Root Architecture of Fraxinus mandshurica Rupr. Seedlings

Plants have evolved some mechanisms to maximize the efficiency of phosphorus acquisition. Changes in root architecture are one such mechanism. When Fraxinus mandshurica Rupr. seedlings were grown under conditions of low phosphorus availability, the length of cells in the meristem zone of the lateral roots was longer, but the length of cells in the elongation and mature zones of the lateral roots was shorter,compared with seedlings grown under conditions of high phosphorus availability. The elongation rates of primary roots increased as phosphorus availability increased, but the elongation rates of the branched zones of the primary roots decreased. The number of lateral root primordia and the length of the lateral roots decreased as phosphorus availability increased. The topological index (altitude slope) decreased as phosphorus availability increased, suggesting that root architecture tended to be herringbone-like when seedlings were grown under conditions of low phosphate availability. Herringbone-like root systems exploit nutrients more efficiently, but they have higher construction costs than root systems with a branching pattem.     

Citrus root responses to localized drying soil: A new approach to studying mycorrhizal effects on the roots of mature trees

Because fine roots tend to be concentrated at the soil surface, exposure to dry surface soil can have a large influence on patterns of root growth, death and respiration. We studied the effects of arbuscular mycorrhizas (AM) formation on specific root length (SRL), respiration and mortality of fine roots of bearing red grapefruit (Citrus paradisi Macf.) trees on Volkamer lemon (C. volkameriana Tan. & Pasq.) rootstock exposed to drying soil. For each tree, the fine roots were removed from two woody lateral roots, the roots were surface sterilized and then each woody root was placed in a separate pair of vertically divided and independently irrigated soil compartments. The two split-pot systems were filled with sterilized soil and one was inoculated with arbuscular mycorrhizal fungi (Glomus etunicatum/G. intraradices). New fine lateral roots that emerged from the woody laterals were permitted to grow inside the pots over a 10-month period. Irrigation was then removed from the top compartment for a 15-week period. At the end of the study, roots inoculated with AM fungi exhibited about 20% incidence of AM formation, whereas the uninoculated roots were completely void of AM fungi. Arbuscular mycorrhizal roots exhibited lower SRL, lower root/soil respiration and about 10% lower fine root mortality than nonmycorrhizal roots after 15 weeks of exposure to dry surface soil. This study demonstrates the feasibility of examining mycorrhizal effects on the fine roots of adult trees in the field using simple inexpensive methods.     

落叶松和水曲柳不同根序细根形态结构、组织氮浓度与根呼吸的关系

根系具有高度的形态和生理功能异质性,在森林生态系统碳和养分循环中起重要作用。根系分枝的顺序构成根序,是根系最基本的构型特征,根序代表根系不同的发育阶段。然而,目前直接测定不同根序细根生理功能的研究很少。以落叶松（Larix gmelinii）和水曲柳（Fraxinus mandshurica）的细根为研究对象,使用气相氧电极测定不同根序细根的呼吸速率,探讨根系呼吸速率与其形态、结构和组织氮浓度的关系。结果表明：落叶松和水曲柳细根的直径、根长和维管束直径均随着根序的增加（1–5级）而增加,而比根长、组织氮浓度和呼吸速率随着根序的增加而降低,各根序之间差异显著（P〈0.05）;1级根比根长最大、皮层组织发达、组织氮浓度最高且呼吸速率也最高,其呼吸速率分别为17.57nmolO2·g^–1·s^–1（落叶松）和18.80 nmolO2·g^–1·s^–1（水曲柳）,比5级根分别高148%（落叶松）和124%（水曲柳）;并且,落叶松根的呼吸速率几乎有96%与根系组织氮浓度相关,而水曲柳根的呼吸速率则有89%与根系组织氮浓度相关。上述结果说明,细根的形态和生理功能异质性是紧密相连的,低级根的形态、结构决定其功能是吸收养分和水,而高级根的形态、结构决定其功能是运输和贮存养分。     

Focus Issue on Roots: It’s Complicated: Intraroot System Variability of Respiration and Morphological Traits in Four Deciduous Tree Species

Within branched root systems, a distinct heterogeneity of traits exists. Knowledge about the ecophysiology of different root types is critical to understand root system functioning. Classification schemes have to match functional root types as closely as possible to be used for sampling and modeling. Among ecophysiological root traits, respiration is of particular importance, consuming a great amount of carbon allocated. Root architecture differs between the four deciduous tree seedlings. However, two types of terminal root segments (i.e. first and second orders), white colored and brown colored, can be distinguished in all four species but vary in frequency, their morphology differing widely from each other and higher coarse root orders. Root respiration is related to diameter and tissue density. The use of extended root ordering (i.e. order and color) explains the variance of respiration two times as well as root diameter or root order classes alone. White terminal roots respire significantly more than brown ones; both possess respiration rates that are greater than those of higher orders in regard to dry weight and lower in regard to surface area. The correlation of root tissue density to respiration will allow us to use this continuous parameter (or easier to determine dry matter content) to model the respiration within woody root systems without having to determine nitrogen contents. In addition, this study evidenced that extended root orders are better suited than root diameter classes to picture the differences between root functional types. Together with information on root order class frequencies, these data allow us to calculate realistic, species-specific respiration rates of root branches.In response to environmental parameters, whole-root systems exhibit high plasticity at different hierarchical scales, such as physiology, anatomy, morphology, and/or biomass (Deak and Malamy, 2005; Gruber et al., 2011). However, within branched systems of single-root units, a distinct heterogeneity of root traits is also present, especially in but not limited to woody root systems (Rewald et al., 2011). Thus, a key to the understanding of the functioning of root systems is knowledge of the traits of individual root segments, especially those related to water and nutrient uptake as well as carbon invested. Currently, several systems are used to classify roots within a branching root system. It is important that classes, when used as a basis for root sampling, are not arbitrary but matched as closely as possible to functional categories (Pregitzer et al., 1998); thus, classification schemes that are able to represent different types of root segments best have to be identified.The most commonly used classification system in woody plants is based on root diameters (Böhm, 1979), usually distinguishing rather ephemeral fine roots (diameter = 0–2 mm) from woody coarse roots (diameter ≥ 2 mm). Root diameter is one of the most important input parameters for root and rhizosphere modeling (Himmelbauer et al., 2004), likely because of the convenient determination by image analyses programs or during hand sorting. In contrast, two principal ways to number segments are used: centrifugal (i.e. basal to distal) or centripetal (i.e. distal to basal) ordering (Uylings et al., 1975). Today, the centrifugal ordering scheme is most commonly used (Fitter, 1996), especially by researchers describing root system development and root phenology of crop species (Chen et al., 2011; Clark et al., 2013; Leitner et al., 2014), but it has also been applied on tree roots (Pregitzer et al., 1997; Majdi et al., 2001). In contrast, ordering in centripetal systems (Strahler, 1957) is initiated at most distal segments, and order number is increased when two root segments of equal order meet (Pregitzer, 2002). Although the dynamics of centripetal systems are opposite to those of root system development, they group functionally similar terminal (most distal) tree segments, such as root tips, into the same order. In addition to those approaches, several researchers have successfully used individual classification parameters to distinguish root segments according to color (Goldfarb et al., 1990; Bouma et al., 2000) or characteristic branching patterns (e.g. cluster roots; Neumann and Martinoia, 2002). With respect to color, tree roots are predominantly regarded to be white when first produced, turning brown with age (Wells and Eissenstat, 2003).The respiration of CO₂ from fine roots is an important component of the terrestrial carbon cycle. Root respiration (RR) accounts for 25% to 60% of total soil respiration (Pregitzer et al., 1997; Epron et al., 1999; Dannoura et al., 2006a, 2006b) and consumes up to 75% of carbon allocated to roots (Majdi et al., 2007). Predictions on whole-plant carbon budget estimate that total plant respiration is about one-half of gross primary production (Chapin et al., 2012). In temperate forests, belowground net primary production is about 40% of total net primary production. However, large uncertainties remain in quantifying the allocation of carbon to tree root systems in general and the amount of RR of the whole-tree carbon budget in particular. Therefore, factors that describe the metabolic activity of roots and associated microbes are an important component of determining plant carbon budgets and allocation pattern more precisely. Physiologically, fine RR is critical for root maintenance and growth and one important variable determining the uptake efficiency of roots (George et al., 2003) alongside construction costs (Poorter, 1994). Ion transport across membranes may account for 25% to 50% of RR (Lambers et al., 2008). Previous measurements emphasized that fine RR rates can be highly variable (Pregitzer et al., 1998; Makita et al., 2009). This variability is probably partially because of the arbitrary classification of fine roots based on diameter rather than an anatomical or physiological basis (Bouma et al., 2000; Makita et al., 2009). Although positive correlations between fine RR and nitrogen contents have been established, with young roots having greater nitrogen contents, much of the variation in RR rate within a diameter class must be caused by other factors (Pregitzer et al., 1998). In the last decade, evidence increased that root traits often vary according to the position of individual roots segments among the root branching hierarchy (Pagès and Kervella, 1990; Pregitzer et al., 2002; Wang et al., 2006; Guo et al., 2008a, 2008b; Valenzuela-Estrada et al., 2008; Beyer et al., 2013) and that analysis by root order is one powerful approach to understand complex woody root systems under stress (Rewald et al., 2012). Thus, it is surprising that few attempts (Jia et al., 2013) have been made to test if root order-based classification is covering root system heterogeneity in respiration in a meaningful way and morphological parameters on which it may be based.The aim of this study is to describe the relationship between RR of four deciduous European tree species and continuous root morphological traits and root classes. Our hypotheses are that: (1) root architectural and morphological traits of woody tree species can be best described by extended centripetal root order classification and (2) RR is highly reflected by morphological traits. (3) Additionally, if sampling classes are used, extended root order classification has a significantly greater explanatory value than root diameter classes. Furthermore, we evaluate if upscaling of respiration rates per root order class can be used to compare species-specific RR rates—information that is scarce for European tree species.     

Root morphology and architecture respond to N addition in Pinus tabuliformis, west China

Belowground dynamics of terrestrial ecosystems are responding to global increases in anthropogenic N deposition with important consequences for productivity and ecosystem health. We compared root characteristics across five root orders in Pinus tabuliformis plantations treated for 3 years to a gradient of N addition (0–15 g m^?2 year^?1). In reference plots, the roots of P. tabuliformis were finer and with higher specific root length than reported for other pine species, suggesting severe N limitation. Addition of N resulted in slightly reduced fine root biomass and significant changes in root morphology, responses that were associated primarily with first and second order roots. In particular, root number, cumulative root length, individual root length, and specific root length all declined with increasing N addition for first and second order roots, with most of the responses elicited at <9 g m^?2 year^?1 N addition. These responses (1) support the concept of ephemeral root modules consisting of first and second orders and (2) are consistent with a change in functional demand from uptake to transport with increasing soil resource availability. Traditionally, fine roots have been identified by a somewhat arbitrary diameter cut-off (e.g., 1 or 2 mm); as an index of fine root function, diameter would fail to reveal most of the functional response.     

Patterns of vegetative growth and reproduction in relation to branch orders: the plant as a spatially structured population

The patterns of vegetative growth and reproduction in relation to orders of terminal branches were examined in the evergreen woody plant, Eurya japonica. The branch order number was determined centrifugally. The trunk was given order number 1; branches issuing directly from the trunk were order 2; branches growing on order 2 branches were order 3, and so on. The results of this study show the differential patterns of vegetative growth and reproduction in relation to the branch orders. Lower-order shoots of terminal branches grew more, but produced few flowers. On the other hand, for the higher-order terminal branches, shoot growth was very limited but flowering was more intense. The results show that a tree can be interpreted not as a mere population of equivalent modules but as a spatially structured population. Thus, it is essential to consider the position of modules within the branch system when patterns of vegetative growth and reproduction are examined. It is hypothesized that the difference in the opportunity cost in relation to the branch orders is the main cause of the spatial structure for patterns of vegetative growth and reproduction. Furthermore, for same-order terminal branches, current-year shoot elongation was independent of flowering intensity. These results suggest that this species only invests resources in reproduction that are surplus to its requirements for the functions associated with survival, such as growth and/or storage. Received: 22 July 1999 / Accepted: 12 January 2000     

A radicle solution: morphology and biomechanics of the Eucalyptocrinites ‘root’ system

Eucalyptocrinites is one the most common and familiar mid‐Palaeozoic crinoids and is the exemplar of a form with dendritic radicular holdfasts. American museums have hundreds of specimens of Eucalyptocrinites holdfasts from the Silurian Waldron Shale of Indiana and Kentucky, USA. The radix (‘root’) system of Eucalyptocrinites can be described as comprised of links (branches) that meet at nodes. Measured values include the x‐ and y‐coordinates of the nodes, the distances of the nodes from the stem, the angles between branches, branch length and branch width. Rose diagrams show clearly that the root systems are not isotropic but have preferred orientations. Branch angles are highly variable, but cluster around 60 degrees. Branch lengths and distal branch widths are relatively constant, but branch width increases variably towards the column. The branching pattern can be modelled as a self‐similar (‘fractal’) structure. Several specimens labelled as Eucalyptocrinites show a distinct fivefold symmetry without branching and very likely represent a different taxon. The Eucalyptocrinites radix system, along with the probably stiff dististele, most likely functioned as a rigid plate that resisted rotational forces due to currents acting on the crown. Upstream radicles experienced tension, whereas downstream roots were compressed. This force distribution may explain the observed anisotropies in radix morphology. The ‘roots’ of Eucalyptocrinites and other crinoids have long been compared with the root systems of plants. Although there are superficial similarities, there are fundamental differences.     

Existing branches correlatively inhibit further branching in Trifolium repens: possible mechanisms

In Trifolium repens removal of any number of existing branches distal to a nodal root stimulates development of axillary buds further along the stem such that the complement of branches distal to a nodal root remains constant. This study aimed to assess possible mechanisms by which existing branches correlatively inhibit the outgrowth of axillary buds distal to them. Treatments were applied to basal branches to evaluate the roles of three postulated inhibitory mechanisms: (I) the transport of a phloem-mobile inhibitory feedback signal from branches into the main stem; (II) the polar flow of auxin from branches into the main stem acting to limit further branch development; or (III) the basal branches functioning as sinks for a net root-derived stimulatory signal (NRS). Results showed that transport of auxin, or of a non-auxin phloem-mobile signal, from basal branches did not influence regulation of correlative inhibition and were consistent with the possibility that the intra-plant distribution of NRS could be involved in the correlative inhibition of distal buds by basal branches. This study supports existing evidence that regulation of branching in T. repens is dominated by a root-derived stimulatory signal, initially distributed via the xylem, the characterization of which will progress the generic understanding of branching regulation.     

Root anatomy, morphology, and longevity among root orders in Vaccinium corymbosum (Ericaceae)

Understanding root processes at the whole-plant or ecosystem scales requires an accounting of the range of functions within a root system. Studying root traits based on their branching order can be a powerful approach to understanding this complex system. The current study examined the highly branched root system of the ericoid plant, Vaccinium corymbosum L. (highbush blueberry) by classifying its root orders with a modified version of the morphometric approach similar to that used in hydrology for stream classification. Root anatomy provided valuable insight into variation in root function across orders. The more permanent portion of the root system occurred in 4th- and higher-order roots. Roots in these orders had radial growth; the lowest specific root length, N:C ratios, and mycorrhizal colonization; the highest tissue density and vessel number; and the coarsest root diameter. The ephemeral portion of the root system was mainly in the first three root orders. First- and 2nd-order roots were nearly anatomically identical, with similar mycorrhizal colonization and diameter, and also, despite being extremely fine, median lifespans were not very short (115-120 d; estimated with minirhizotrons). Our research underscores the value of examining root traits by root order and its implications to understanding belowground processes.