The formation of adventitious roots on root axes is a widespread occurrence in field-grown dicotyledonous plants

The formation of adventitious branch roots in the secondary tissues of parental root axes is a widespread and frequent occurrence under field conditions. Anatomical features diagnostic for the recognition of adventitious roots were utilized to confirm the occurrence of adventitious roots on roots of 22 species from 12 families in nine orders of dicotyledonous plants. Adventitious roots may play an important role in generating the population of fine roots as part of root turnover in the soil.     

Lateral root development, including responses to soil drying, of maize (Zea mays) and wheat (Triticum aestivum) seminal roots

The spatial distribution of lateral roots in the soil is an important factor influencing water and nutrient absorption. However, lateral root development has rarely been studied in detail, especially concerning morphological variations, mainly because such examinations are both time-consuming and laborious. We measured the number and length of all first-order lateral roots on the seminal roots of maize ( Zea mays L.) and wheat ( Triticum aestivum L.) to investigate variations in linear frequency and length. This was conducted with reference to species, root types, and positions on their parental roots. Although the linear frequency of first-order lateral roots varied along the root axis in maize, the variation was not as great as in wheat. Variations were found in the length of lateral roots among plant species, root types, and positions on their parental root axes. Such variations in the length of lateral roots along the root axes were caused by differences in the elongation period of lateral roots rather than those in the elongation rate. Additionally, we examined the effects of soil drying on lateral root development. As a response to soil drying, the length of lateral roots varied depending on the period they were placed under the stressed condition. Moderate soil drying could also accelerate the elongation of some lateral roots. Variations in the length of first-order lateral roots and their responses to soil drying could help distribute their tips thoroughly throughout the soil. This might be adaptive for water absorption for root system development when resources are limited.     

Specific intercalary growth of rhizophores and roots in Selaginella kraussiana (Selaginellaceae) is related to unique dichotomous branching

Rhizophores - leafless axial organs, which apices are the only sites for root formation - are unique to the Selaginella genus. Both rhizophores and roots are dichotomously branched, forming a multibranch rhizophore-root system. In the examined Selaginella kraussiana, the first dichotomous division of a rhizophore results in the initiation of the twin root meristems, giving rise to the root primordia. These primordia are temporarily arrested in growth, but at the same time they are elevated, hidden inside the rhizophore apex due to the activity of a meristematic zone at the rhizophore axis. This meristematic region, located basally to the root meristems, apart from the rhizophore tissues consists of the derivative cells of root primordia and may be considered as a specific intercalary meristem.The growth mode of the roots is similar to that of the rhizophore, including dichotomous branching of the meristem and a temporary developmental arrest of the subsequent root primordia. The reiteration of the developmental program also involves the formation of new intercalary zones, thus through divisional activity, the root with dichotomized apex grows for some time as a single axis and, then the young roots emerge. At each step of the rhizophore-root branch system formation, tissues of the parental axis contribute significantly to the growth and emergence of the next axes. Therefore, dichotomous branching in Selaginella appears to be exceptional in comparison with other known dichotomies of axial organs.     

THE OCCURRENCE OF “ROOT GAPS” IN THE RHIZOME SOLENOSTELE AND LEAF TRACE OF DENNSTAEDTIA CICUTARIA

The vascular system of the rhizome axes of Dennstaedtia cicutaria consists of a solenostele with an amphicribral vascular bundle centrally located in the pith. Each leaf has a single trace which is an amphicribral vascular bundle. At each node of an axis there is a complex consisting of the main axis, leaf base, and a branch axis attached to the basiscopic margin of the leaf base. Numerous roots are present on the rhizomes and the abaxial side of the leaf bases. Parenchymatous gaps occur in the rhizome solenostele and the leaf trace directly above the departure of some of the root traces. These gaps are termed root gaps. In some instances the root gaps are confluent. However, not all of the root traces have an associated root gap. The leaf trace is inserted laterally on the main and branch axes at the node so that the acroscopic leaf trace margin anastomoses with the main axis of the vascular system and the basiscopic margin with that of the branch axis. Two leaf gaps are associated with each leaf trace, one occurring in the main axis solenostele and the other in the branch axis solenostele. The medullary bundle of each axis anastomoses with each leaf trace at its point of attachment to the rhizome solenostele. Thus, the medullary bundle forms a continuous vascular strand from leaf trace to leaf trace in any given rhizome axis.     

Respiration rate in maize roots is related to concentration of reduced nitrogen and proliferation of lateral roots

The relationship between specific rate of respiration (respiration rate per unit root dry weight) and concentration of reduced nitrogen was examined for maize ( Zea mays L.) roots. Plants with 2 primary nodal root axes were grown for 8 days in a split-root hydroponic system in which NO-₃ was supplied to both axes at 1.0 mol m⁻³, to one axis at 1.0 mol m⁻³ and the other axis at 0.0 mol m⁻³ or to both axes at 0.0 mol m⁻³ Respiration rates and root characteristics were measured at 2-day intervals. Specific rate of respiration was positively correlated in a nonlinear relationship with concentration of reduced nitrogen. The lowest specific rates of respiration occurred when neither axis received exogenous NO⁻³ and the concentration of reduced nitrogen in the axes was less than 9 mg g⁻¹. The greatest rates occurred in axes that were actively absorbing NO⁻³ and contained more than 35 mg g⁻¹ of reduced nitrogen. At 23 mg g⁻¹ of reduced nitrogen, below which initiation of lateral branches was decreased by 30–50%. specific rate of respiration was 17% greater for roots actively absorbing NO⁻³ than for roots not absorbing NO⁻³ Increases in specific rate of respiration associated with concentrations of reduced nitrogen greater than 23 mg g⁻¹ were concluded to be attributable primarily to proliferation of lateral branches.     

Nitrogen dynamics differed among the first six root branch orders of Fraxinus mandshurica and Larix gmelinii during short-term decomposition

Fine root (<2 mm) decomposition provides a substantial amount of available nitrogen (N) that sustains plant growth. The N release pattern during litter decomposition is generally controlled by initial N concentrations or C/N. Because root branch order and mycorrhizal colonization (related with branch order) are both highly related with different initial chemistry, a hypothesis was proposed that N dynamics during root decomposition varied among different branch orders. Using the litterbag method, decomposition of the first six order roots for Fraxinus mandshurica (an arbuscular mycorrhizal species) and Larix gmelinii (an ectomycorrhizal species) was studied in Northeast China during a 513-day period. Results showed a similar pattern for the two species with contrasting mycorrhizal type: lower-order roots (the lateral root tips), which had an initial C/N of 17–21, continuously released N without any immobilization and maintained a consistently low C/N (<20), whereas higher-order roots, which had an initial C/N of 28–48, periodically immobilized N, leading to a declining C/N over time. In addition, the magnitude of N dynamics is different between species for lower-order roots, but no different for higher-order roots. These results suggest that fine root N dynamics are heterogeneous among branch orders and that species-specific differences depend on the behavior of lower-order roots.     

GRAFTING AND ROOTING OF LEAVES OF GUAREA (MELIACEAE): EXPERIMENTAL STUDIES ON LEAF AUTONOMY

The pinnately compound, indeterminate leaves of G. glabra and G. guidonia were air layered, detached from their original shoots, and grown on their own adventitious root systems for up to 58 mo and 26 mo, respectively. The detached leaves grew in the same indeterminate manner and reached sizes similar to attached leaves. Although detached leaves grew autonomously, they never produced shoot buds. Leaves of both species were grafted onto their own stems and cut free of their original leaf bases. Leaf scions survived and grew for up to 29 mo and 20 mo, respectively, similar to ungrafted leaves. Axillary branches were grafted onto subtending leaves. Branch scions grew on their leaf stocks for over 30 mo and 24 mo, respectively, after being cut free from the branch bases. Secondary growth of the leaf axis (petiole) was promoted, and vascular tissues of leaf and branch axes were continuous. However, the unlignified basal region of the leaf, including the abscission zone, remained unchanged after grafting. The results indicate that proximity of roots and bypassing the abscission zone did not enhance leaf longevity or pinna production. The presence of a growing branch on a leaf did not modify the structure of the abscission zone, which suggests that the zone is strongly committed or developmentally fixed.     

Modelling of the Hydraulic Architecture of Root Systems: An Integrated Approach to Water Absorption--Distribution of Axial and Radial Conductances in Maize

The ‘Hydraulic Tree Model’ of the root system simulateswater uptake through root systems by coupling a root architecturemodel with laws for water flow into and along roots (Doussan,Pagès and Vercambre,Annals of Botany81: 213–223,1998). A detailed picture of water absorption in all roots comprisingthe root system is thus provided. Moreover, the influence ofdifferent distributions of radial and axial hydraulic conductancesin the root system on the patterns of water uptake can be analysed.Use of the model with Varney and Canny's data (1993) for flowalong maize roots demonstrated that a constant conductance inthe root system cannot reproduce the observed water flux profiles.Taking into account the existing data on hydraulic conductancesin maize roots, we fitted the distribution of conductances inthe root system to the observed flux data. The result is that,during root tissue maturation, the radial conductivity decreasesby one order of magnitude while the axial conductance increasesby about three orders of magnitude. Both types of conductanceexhibit abrupt changes in their evolution. Due to the conductancedistribution in the root system, appreciable water potentialgradients may develop in the roots, in both the branch rootsand main axes. An important point is that the conductance distributionin the branch roots described by the model should be relatedto the age of the tissue (and not the distance from the branchroot tip) and is therefore closely related to the developmentprocess. Thus for branch roots, which represent about 90% ofthe calculated total water uptake in 43-d-old maize, water absorptionwill depend on the opening of the metaxylem in the axes, andon the time dependent variation of the conductances in the branchroots.Copyright 1998 Annals of Botany Company Water; absorption; root system; architecture; model; hydraulic conductance;Zea maysL.     

Allometry of root branching and its relationship to root morphological and functional traits in three range grasses

The study of proportional relationships between size, shape, and function of part of or the whole organism is traditionally known as allometry. Examination of correlative changes in the size of interbranch distances (IBDs) at different root orders may help to identify root branching rules. Root morphological and functional characteristics in three range grasses {bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) L?ve], crested wheatgrass [Agropyron desertorum (Fisch. ex Link) Schult.×A. cristatum (L.) Gaert.], and cheatgrass (Bromus tectorum L.)} were examined in response to a soil nutrient gradient. Interbranch distances along the main root axis and the first-order laterals as well as other morphological and allocation root traits were determined. A model of nutrient diffusivity parameterized with root length and root diameter for the three grasses was used to estimate root functional properties (exploitation efficiency and exploitation potential). The results showed a significant negative allometric relationship between the main root axis and first-order lateral IBD (P ≤ 0.05), but only for bluebunch wheatgrass. The main root axis IBD was positively related to the number and length of roots, estimated exploitation efficiency of second-order roots, and specific root length, and was negatively related to estimated exploitation potential of first-order roots. Conversely, crested wheatgrass and cheatgrass, which rely mainly on root proliferation responses, exhibited fewer allometric relationships. Thus, the results suggested that species such as bluebunch wheatgrass, which display slow root growth and architectural root plasticity rather than opportunistic root proliferation and rapid growth, exhibit correlative allometry between the main axis IBD and morphological, allocation, and functional traits of roots.     

Root age distribution: how does it matter in plant processes? A focus on water uptake

Aims and background

Root growth creates a gradient in age at both the scale of the single root, from distal to proximal parts, but also at the root system level when young branch roots emerge from the axis or new nodal roots are emitted that may reach same soil domain as older roots. It is known that a number of root functions will vary with root type and root tissue age (e.g. respiration, exudation, ion uptake, root hydraulic conductance, mucilage release…) and so will the resulting rhizosphere properties. The impact of the distribution of root demography with depth, and related functions, on the overall functioning of the root system is fundamental for an integration of processes at the root system scale.

Scope and conclusion

Starting from methods for measuring root demography, we discuss the availability of data related to root age and its spatial distribution, considering plant types (monocot/dicot, perennial/annuals) which may exhibit different patterns. We then give a detailed review of variation of root/rhizosphere properties related to root age, focusing on root water uptake processes. We examine the type of response of certain properties to changes in age and whether a functional relationship can be derived. Integration of changing root properties with age into modelling approaches is shown from 3D models at the single plant scale to approaches at the field scale based on integrated root system age. Functional structural modelling combined with new development in non-invasive imaging of roots show promises for integrating influence of age on root properties, from the local to whole root system scales. However, experimental quantification of these properties, such as hydraulic conductance variation with root age and root types, or impact of mucilage and its degradation products on rhizosphere hydraulic properties, presently lag behind the theoretical developments and increase in computational power.

     

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。     

Variation of carbon age of fine roots in boreal forests determined from 14C measurements

Background and aims

The main objectives of this study were to determine how the carbon age of fine root cellulose varies between stands, tree species, root diameter and soil depth. In addition, we also compared the carbon age of fine roots from soil cores of this study with reported values from the roots of the same diameter classes of ingrowth cores on the same sites.

Methods

We used natural abundance of ¹⁴C to estimate root carbon age in four boreal Norway spruce and Scots pine stands in Finland and Estonia.

Results

Age of fine root carbon was older in 1.5–2 mm diameter fine roots than in fine roots with <0.5 mm diameter, and tended to be older in mineral soil than in organic soil. Fine root carbon was older in the less fertile Finnish spruce stands (11–12 years) than in the more fertile Estonian stand (3 and 8 years), implying that roots may live longer in less fertile soil. We further observed that on one of our sites carbon in live fine roots with the 1.5–2 mm diameter was of similar C age (7–12 years) than in the ingrowth core roots despite the reported root age in the ingrowth cores – being not older than 2 years.

Conclusions

From this result, we conclude that new live roots may in some cases use old carbon reserves for their cellulose formation. Future research should be oriented towards improving our understanding of possible internal redistribution and uptake of C in trees.     

Control of root growth: effects of carbohydrates on the extension, branching and rate of respiration of different fractions of wheat roots

Rates of extension, numbers of laterals and rates of respiration were measured in different fractions of wheat ( Triticum aestivum L. cv. Alexandria) roots following changes in carbohydrate supply. The supply of carbohydrate was varied by selective pruning and exogenously fed sugars. Pruning shoots to a single leaf (leaf-pruning) reduced the rate of O₂ uptake by intact roots. Rates were not stimulated by shortterm feeding of sucrose (25 m M ), but were stimulated by the uncoupler p -trifluoro-methoxy(carbonylcyanide)phenylhydrazone (FCCP). Feeding glucose to roots of leaf-pruned and non-pruned plants for 16–24 h increased the rate of O₂ uptake. It is concluded that respiration is under fine control by adenylates and coarse control by carbohydrate supply, with carbohydrates regulating directly the rate of some energy consuming process(es). These energy consuming processes are located in growing tissue fractions. Feeding glucose to leaf-pruned and non-pruned plants increased rates of O₂ uptake in seminal root tips, the zone of developing lateral primordia and mature root sections with elongating laterals, but had no effect on mature sections from which the laterals had been excised. Leaf-pruning reduced the extension rate of seminal axes and first-order laterals when measured over 24 h. Feeding glucose to roots from the time of pruning increased the rate, but did not fully restore it to control values. Pruning roots to a single seminal axis (root-pruning) and feeding glucose to non-pruned plants had no effect on the extension rate of the seminal axis or its laterals over this time period, although rates were increased by root-pruning when measured over 3 days. The number of lateral root primordia was reduced by leaf-pruning and increased by root-pruning and feeding glucose. The results are discussed in terms of the role of carbohydrates in the control of root growth and branching.     

Variation of the Linkage of Root Function with Root Branch Order

Mounting evidence has shown strong linkage of root function with root branch order. However, it is not known whether this linkage is consistent in different species. Here, root anatomic traits of the first five branch order were examined in five species differing in plant phylogeny and growth form in tropical and subtropical forests of south China. In Paramichelia baillonii, one tree species in Magnoliaceae, the intact cortex as well as mycorrhizal colonization existed even in the fifth-order root suggesting the preservation of absorption function in the higher-order roots. In contrast, dramatic decreases of cortex thickness and mycorrhizal colonization were observed from lower- to higher-order roots in three other tree species, Cunninghamia lanceolata, Acacia auriculiformis and Gordonia axillaries, which indicate the loss of absorption function. In a fern, Dicranopteris dichotoma, there were several cortex layers with prominently thickened cell wall and no mycorrhizal colonization in the third- and fourth-order roots, also demonstrating the loss of absorptive function in higher-order roots. Cluster analysis using these anatomic traits showed a different classification of root branch order in P. baillonii from other four species. As for the conduit diameter-density relationship in higher-order roots, the mechanism underpinning this relationship in P. baillonii was different from that in other species. In lower-order roots, different patterns of coefficient of variance for conduit diameter and density provided further evidence for the two types of linkage of root function with root branch order. These linkages corresponding to two types of ephemeral root modules have important implication in the prediction of terrestrial carbon cycling, although we caution that this study was pseudo-replicated. Future studies by sampling more species can test the generality of these two types of linkage.     

Response of millet and sorghum to a varying water supply around the primary and nodal roots

Background and Aims

Cereals have two root systems. The primary system originates from the embryo when the seed germinates and can support the plant until it produces grain. The nodal system can emerge from stem nodes throughout the plant''s life; its value for yield is unclear and depends on the environment. The aim of this study was to test the role of nodal roots of sorghum and millet in plant growth in response to variation in soil moisture. Sorghum and millet were chosen as both are adapted to dry conditions.

Methods

Sorghum and millet were grown in a split-pot system that allowed the primary and nodal roots to be watered separately.

Key Results

When primary and nodal roots were watered (12 % soil water content; SWC), millet nodal roots were seven times longer than those of sorghum and six times longer than millet plants in dry treatments, mainly from an 8-fold increase in branch root length. When soil was allowed to dry in both compartments, millet nodal roots responded and grew 20 % longer branch roots than in the well-watered control. Sorghum nodal roots were unchanged. When only primary roots received water, nodal roots of both species emerged and elongated into extremely dry soil (0·6–1·5 % SWC), possibly with phloem-delivered water from the primary roots in the moist inner pot. Nodal roots were thick, short, branchless and vertical, indicating a tropism that was more pronounced in millet. Total nodal root length increased in both species when the dry soil was covered with plastic, suggesting that stubble retention or leaf mulching could facilitate nodal roots reaching deeper moist layers in dry climates. Greater nodal root length in millet than in sorghum was associated with increased shoot biomass, water uptake and water use efficiency (shoot mass per water). Millet had a more plastic response than sorghum to moisture around the nodal roots due to (1) faster growth and progression through ontogeny for earlier nodal root branch length and (2) partitioning to nodal root length from primary roots, independent of shoot size.

Conclusions

Nodal and primary roots have distinct responses to soil moisture that depend on species. They can be selected independently in a breeding programme to shape root architecture. A rapid rate of plant development and enhanced responsiveness to local moisture may be traits that favour nodal roots and water use efficiency at no cost to shoot growth.     

Influence of Position and Number of Nodal Roots on Outgrowth of Axillary Buds and Development of Branches in Trifolium repens (L.)

The implications of the presence of a root, either at the parentnode or at neighbour nodes, on branch formation of Trifoliumrepens (white clover) was investigated. Plants were freely rootedor rooting was restricted to every sixth or every twelfth nodealong the parent axis. The absence of a root at the parent nodehad little influence on the probability of the subtending axillarybud forming a branch but, on average, delayed the outgrowthof the bud. The probability that an axillary bud, emerging froma non-rooted parent node, developed to a lateral branch (branchwith elongated internodes) decreased with decreasing proximityof the parent node to a rooted node. Lateral branches emergingfrom non-rooted parent nodes which were two nodes distal toa rooted node had a higher rate of node appearance, a greatermean internode length and area per leaf, and were more branchedthan lateral branches emerging from other non-rooted parentnodes. The dry mass of each single root and of branches grownat rooted parent nodes were significantly higher in plants withrestricted rooting than in freely rooted plants. Restrictionin the number of rooted nodes per plant increased the numberof inflorescences. It is concluded that the whole plant responseto restricted root formation was continuous growth of the parentaxis and compensatory growth of the branch at the rooted node.In general, growth was slow for axillary buds whose developmentwas dependent on the basipetal movement or cross-transport withinthe stolons of resources exported from roots. Trifolium repens (L.); white clover; axillary bud outgrowth; branch development; clonal growth; nodal root     

A study on the population dynamics of the perennial plant species Chamaenerion angustifolium (L.) scop.

Summary From a study of differently aged populations of the perennial plant species Chamaenerion angustifolium a concept was deduced concerning the population dynamics of the species. The age structure was determined by counting the rings of periderm in the roots. After the establishment of seedlings vegetative propagation is the main factor in population development. The longevity of populations depends on exogenic factors, since older root samples—if isolated from the population—appeared to be as viable as were younger specimens. The oldest population studied had been able to persist for about thirty years due to the mineral cycle brought about by the population itself. In still developing populations a greater number of shoots tend to sprout from younger roots than from older roots, when compared per unit root weight. The shoot density in full-grown populations (in which further root expansion does not occur) is related to the root biomass per unit area. It was suggested that the allocation of reserve assimilates within the roots is an endogenic factor determining the shoot density. After the early sprouting phase the shoot development depends mainly on environmental factors.     

Influence of phosphate-stress on phosphate absorption and translocation by various parts of the root system of Hordeum vulgare L. (barley)

Plants of Hordeum vulgare (barley) were grown initially in a solution containing 150 M phosphate and then transferred on day 6 to solutions with (+P) and without (-P) phosphate supplied. After various times plants from these treatments were supplied with labelled phosphate. Analysis of plant growth and rates of labelled phosphate uptake showed that a general enhancement of uptake and translocation was found, in plants which had been in the-P solution, several days before the rate of dry matter accumulation was affected. Subsequently a detailed analysis of phosphate uptake by segments of intact root axes showed that the enhancement of phosphate uptake by P-stress occurred first in the old and mature parts of the seminal root axis and last in the young zones 1 cm from the root apex. During this transition period there were profound changes in the pattern of P absorption along the length of the root. Most of the additional P absorbed in response to P-stress was translocated to the shoot, particularly in older zones of the axis. Enhancement of phosphate uptake in young zones of nodal axes occurred at an earlier stage than in seminal axes. The results are related to the P-status of shoots and root zones and discussed in relation to the general control by the shoot of phosphate transport in the root.     

Cluster Roots: A Curiosity in Context

Cluster roots are an adaptation for nutrient acquisition from nutrient-poor soils. They develop on root systems of a range of species belonging to a number of different families (e.g., Proteaceae, Casuarinaceae, Fabaceae and Myricaceae) and are also found on root systems of some crop species (e.g., albus, Macadamia integrifoliaandCucurbita pepo). Their morphology is variable but typically, large numbers of determinate branch roots develop over very short distances of main root axes. Root clusters are ephemeral, and continually replaced by extension of the main root axes. Carboxylates are released from cluster roots at very fast rates for only a few days during a brief developmental window termed an ‘exudative burst’. Most of the studies of cluster-root metabolism have been carried out using the crop plant L. albus, but results on native plants have provided important additional information on carbon metabolism and exudate composition. Cluster-root forming species are generally non-mycorrhizal, and rely upon their specialised roots for the acquisition of phosphorus and other scarcely available nutrients. Phosphorus is a key plant nutrient for altering cluster-root formation, but their formation is also influenced by N and Fe. The initiation and growth of cluster roots is enhanced when plants are grown at a very low phosphate supply (viz. ≤1 μM P), and cluster-root suppression occurs at relatively higher P supplies. An important feature of some Proteaceae is storage of phosphorus in stem tissues which is associated with the seasonality of cluster-root development and P uptake (winter) and shoot growth (summer), and also maintains low leaf [P]. Some species of Proteaceae develop symptoms of P toxicity at relatively low external P supply. Our findings with Hakea prostrata (Proteaceae) indicate that P-toxicity symptoms result after the capacity of tissues to store P is exceeded. P accumulation in H. prostrata is due to its strongly decreased capacity to down-regulate P uptake when the external P supply is supra-optimal. The present review investigates cluster-root functioning in (1) L.albus (white lupin), the model crop plant for cluster-root studies, and (2) native Proteaceae that have evolved in phosphate-impoverished environments.     

黄河三角洲滨海盐碱地11个造林树种细根解剖性状对土壤条件的响应

细根作为植物与土壤连接的重要部位,能够反映植物对生存环境的适应性。以黄河三角洲滨海盐碱地不同立地条件下11个造林树种为对象,基于细根分支等级划分1-4级根序并进行解剖特征测定,分析细根解剖性状对滨海盐碱地不同土壤条件的响应规律。结果表明：（1）不同根序的细根直径存在显著差异,细根直径随根序升高呈增大趋势,而同根序的细根直径在不同树种间表现出显著的种间差异（P < 0.05）。1-2级细根皮层厚度、3-4级细根导管密度在树种间的差异均达显著水平（P < 0.05）。（2）在较为严重盐渍化土壤条件下（立地1）,细根皮层厚度较其他立地显著增大,但细根导管密度较小;在轻度盐碱立地条件下（立地3）,细根导管密度较大;较为严重的盐碱立地具有更为发达的细根直径及维管柱直径。（3）树种1-2级细根解剖结构与土壤环境关系最为密切,其中1级根直径与土壤pH值显著正相关（P < 0.05）,与土壤硝态氮含量呈显著负相关（P < 0.05）。对土壤理化性质与细根解剖性状的冗余分析表明,前两个轴的特征值达0.640和0.196,土壤速效养分含量与轴一（RDA1）呈正相关,低级根解剖性状则与轴二（RDA2）呈显著负相关。低级根解剖结构以及土壤的pH值能解释较多树种的差异性,其中低级根直径与皮层厚度对盐碱环境表现出较强的响应。