The role of nodal roots in prostrate clonal herbs: ‘phalanx’ versus ‘guerrilla’

The influence of nodal rooting on branching was studied in three evolutionarily and morphologically diverse species of prostrate clonal herbs: Tradescantia fluminensis (a monocotyledonous extreme ‘phalanx’ species), Calystegia silvatica (a dicotyledonous extreme ‘guerrilla’ species) and Trifolium repens (a dicotyledonous intermediate species). In all three, branch development from axillary buds is regulated by a positive signal produced by roots together with inhibitory influences from both pre-existing branches and shoot apical buds (apical dominance). Responses to nodal roots are cumulative and increased root activity leads to more vigorous bud outgrowth. In the absence of nodal roots, a single basal root system is unable to maintain continued extension growth of the shoot. We suggest that as individual nodal roots and stem internodes are both short-lived in these nodally-rooting clonal species, the plants’ investment in them is minimal. Thus, in contrast to perennial species lacking nodal roots, individual root systems in prostrate clonal herbs are small and stems have little secondary thickening and development of long-distance transport tissues. Hence the decline in extension growth of the shoot in the absence of nodal roots could be linked to the weak development of long-distance transport tissues in their relatively thin horizontal stems and to resource sharing between primary stems and lateral branches (as suggested by the greater retardation of primary stem growth in the more profusely branched ‘phalanx’ species (Trifolium and Tradescantia) than in the weakly branched ‘guerrilla’ species, Calystegia). These findings are consistent with the view that the long-term persistence of genotypes of nodally-rooting prostrate species is dependent upon them encountering the moist conditions required to facilitate the continual development of new young nodal root systems.     

Relative importance of nodal roots and apical buds in the control of branching in Trifolium repens L.

Clonal species are characterised by having a growth form in which roots and shoots originate from the same meristem so that adventitious nodal roots form close to the terminal apical bud of stems. The nature of the relationship between nodal roots and axillary bud growth was investigated in three manipulative experiments on cuttings of a single genotype of Trifolium repens. In the absence of locally positioned nodal roots axillary bud development within the apical bud proceeded normally until it slowed once the subtending leaf had matured to be the second expanded leaf on the stem. Excision of apical tissues indicated that while there was no apical dominance apparent within fully rooted stems and very little in stems with 15 or more unrooted nodes, the outgrowth of the two most distal axillary buds was stimulated by decapitation in stems with intermediate numbers of unrooted nodes. Excision of the basal branches from stems growing without local nodal roots markedly increased the length and/or number of leaves on 14 distally positioned branches. The presence of basal branches therefore prevented the translocation of root-supplied resources (nutrients, water, phytohormones) to the more distally located nodes and this caused the retardation in the outgrowth of their axillary buds. Based on all three experiments we conclude that the primary control of bud outgrowth is exerted by roots via the acropetal transport of root-supplied resources necessary for axillary bud outgrowth and that apical dominance plays a very minor role in the regulation of axillary bud outgrowth in T. repens.     

A Comparison of Dry Matter Partitioning in Seminal and Adventitious Plants of Trifolium repens L

Trifolium repens has two types of root, one derived from theseed and the adventitious roots derived from the stolon nodes.It has been suggested that these two systems have differentpotentials for supporting growth. This paper presents a comparisonof plants grown on single seedling or adventitious roots anddemonstrates that although the shoot: root ratios for the twotypes differ this may be explained by differing shoot morphologies.Comparison of the lamina: root ratios for the two types of plantproduced no statistically significant differences and it isproposed that the two types of root system do not differ intheir relationship with leaf growth. A mechanism for large diameter‘tap’ root formation is suggested. white clover, Trifolium repens L., adventitious roots, seedling roots, shoot: root ratio     

Patterns of 14C-labelled Assimilate Partitioning in Red and White Clover During Vegetative Growth

A quantitative analysis of the ¹⁴C-labelled assimilate suppliedby the expanded leaves on the primary shoot to growing leaves,stem, lateral shoots (branches or stolons) and roots in redand white clover was conducted during vegetative growth. Stem growth of the primary shoot was inhibited in both cloversand utilized no energy resources. The growing leaves at theprimary shoot apex of white clover imported 4 per cent of theshoot's assimilate compared with 10 per cent in red clover.At the basal end of the primary shoot, the tap root of whiteclover imported 16 per cent of the shoot's assimilate comparedwith 22 per cent in red clover. Branches in red clover and stolonsin white clover were by far the largest sinks for primary shootassimilate, importing 39 per cent and 63 per cent of the labelledassimilate, respectively. Analyses of the translocation of assimilate from individualprimary shoot leaves demonstrated that in both clovers olderleaves exported more of their assimilate to branches or stolons,whereas younger leaves exported more of their assimilate toroots, and possibly in white clover, to growing leaves at thetip of the shoot. Of the labelled assimilate exported to branchesor stolons, each primary shoot leaf exported preferentiallyto the branch or stolon in its own axil, but in addition exportedsubstantial quantities of assimilate to all other axillary shoots,particularly those arising from basal axils where the subtendingleaf had died. Trifolium repens, Trifolium pratense, red clover, white clover, assimilate partitioning, perennation     

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.     

Effect of Light Quality (Red:Far-red Ratio) at the Apical Bud of the Main Stolon on Morphogenesis of Trifolium repens L.

A controlled environment experiment investigated whether thered:far-red (R:FR) ratio of light at the apical bud of the mainstolon could alter plant morphogenesis in clonal cuttings ofwhite clover (Trifolium repens L.) The apical bud included theapical meristem, five to six developing leaf primordia withassociated axillary bud primordia and stipules and the firstemerged folded leaf until development was greater than 0·3on the Carlson scale. Three light regimes were imposed on theapical bud by collimating light from R or FR light-emittingdiodes so that the R:FR ratio of light incident at the apicalbud was set at 0·25, 1·6 or 2·1, withoutsignificantly altering photosynthetically active radiation.The effect of these light regimes on white clover seedling growthwas also tested. At a low R:FR ratio seedling hypocotyl and cotyledon lengthswere significantly longer. However, with the cuttings, the lighttreatments did not alter node appearance rate or internode lengthof the main stolon, petiole length, area of leaves or totalshoot dry matter. There was one significant photomorphogeneticresponse in the cuttings, a delay of 0·5 of a phyllochronin the appearance of branches from axillary buds in the lowR:FR ratio treatment relative to the other treatments. Wherebranch appearance was delayed plants had fewer branches. Thisdifference could be ascribed solely to a delay in branch appearanceas there were no significant treatment effects on either theinitiation of axillary bud primordia within the apical bud,the probability of branching or on the rate of growth of branchesafter appearance. Because treatment of the apical bud inducedonly one of the many previously observed responses of whiteclover to a decrease in the R:FR ratio of light, we concludethat other plant organs must also sense the quality of incidentlight.Copyright 1994, 1999 Academic Press White clover, Trifolium repens, apical bud, light quality, red:far-red ratio, light-emitting diode, branching, axillary buds, photomorphogenesis     

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     

The Effects of Rootzone Salinity and Hypoxia on Shoot and Root Growth in Trifolium Species

The effect of concurrent salinity (0-60 mM NaCl) and rootzonehypoxia (flooding for up to 15 d) on shoot and root growth andshoot ion concentrations of six species of Trifolium (T. subterraneumL., T. fragiferum L., T. michelianum Savi., T. isthmocarpumBot., T. purpureum Lois., and T. repens L.), was studied intwo greenhouse experiments. There was a significant salinityx flooding effect for shoot yield but no significant salinityx flooding x species interaction although individual speciesdiffered significantly (P < 0·001) in their growthresponse to the saline or flooded conditions separately. Concentrationsof Na and Cl in the shoots of all species increased with increasingperiods of saline flooding and there was a significant salinityx flooding interaction. Sodium and Cl concentrations were significantlyhigher (P < 0·001) in T. purpureum, the species inwhich shoot growth was most depressed by saline flooding, thanother species. In T. michelianum, T. fragiferum and T. repens,fresh and dry weight of roots increased with flooding underboth saline and non-saline conditions while in T. subterraneumroot growth decreased. A significant proportion of the increasedroot growth in the first three species occurred as new adventitiousroots. These roots had higher percentages of internal gas spaceswithin the root tissue even in the presence of salinity comparedwith roots from non-flooded conditions. There were also significantlymore gas spaces in the total root tissue in T. fragiferum andT. repens under saline-flooding than in roots of T. subterraneum.Electron micrographs of the root cross sections illustratedthe presence of these gas spaces or aerenchyma. Trifolium fragiferum, T. repens and T. michelianum are morelikely to be suited to growth in soils prone to high salinityand to flooding than are T. subterraneum, T. purpureum and T.ishmocarpum.Copyright 1993, 1999 Academic Press Trifolium subterraneum, Trifolium fragiferum, Trifolium michelianum, Trifolium isthmocarpum, Trifolium purpureum, Trifolium repens, salinity, flooding, hypoxia, adventitious roots, aerenchyma, subterranean clover, white clover, strawberry clover, purple clover, balansa clover     

Effects of soil mechanical impedance on root and shoot growth of Lolium perenne L., Agrostis capillaris and Trifolium repens L.

A method was used for applying a uniform mechanical impedanceto plant roots using sand packed at different bulk densitieswith depth within each growth cylinder. In a growth experiment,replicate cylinders were packed with sand to give the followingmechanical impedances: 0.25 MPa penetration resistance (negligibleimpedance), 1.40 MPa (moderate impedance) and 2.30 MPa (severeimpedance). Seedlings of Lolium perenne L., Trifolium repensL. and Agrostis capillaris were grown for 23 d in each impedancetreatment and effects on both roots and shoots were studied.Severe mechanical impedance affected both root and shoot growthrates for all three species resulting in smaller leaves andshorter roots. For the grasses, the root-to-shoot ratio at harvestwas the same for all the treatments, but a delay in the initiationof both shoots and roots was observed in the severe mechanicalimpedance treatment. The results are discussed in relation tothe possibility that roots penetrating the impeding treatmentsmay have caused signalling that kept shoot and root growth insynchrony. Differences observed in the response to mechanicalimpedance of T. repens compared to the other two species maybe a function of differences in the physiology between mono-and dicotyledonous species. Key words: Mechanical impedance, Lolium perenne, Trifolium repens, roots, shoots     

Evidence suggests plagiotropic clonal species have evolved a branching physiology emphasizing regulation by nodal roots

In the plagiotropic nodally rooting clonal herb, Trifolium repens,the development of branches on stems is primarily controlled by the presence of nodal roots, and apical dominance is of secondary importance; only six to ten branches form distal to the youngest nodal root on a horizontal stem. We assessed the hypothesis that this phenomenon is general for clonal herbs with prostrate nodally rooting stems, and that they all have the same physiological system regulating branching, by testing a selection of species from diverse angiosperm families that exhibit either phalanx (Leptinella (Asteraceae), Hydrocotyle (Apiaceae), Acaena (Rosaceae)) or guerilla (Vinca (Apocynaceae), Glechoma and Lamiastrum (Lamiaceae)) growth strategies. In all these species the establishment of a single nodal root on a prostrate stem, otherwise prevented from nodally rooting, induced the outgrowth of a limited number of axillary buds (the number of which was species specific) at the nodes immediately distal to the newly established root, thereby indicating a phenotypic response similar to that in T. repens. Furthermore, their branching responses to manipulative treatments were also similar to those of T. repens, indicating that their regulatory physiology of axillary bud outgrowth from their prostrate stems is similar. We conclude that, for the group of prostrate nodally rooting clonal herbs as a whole, the apical dominance phenotype arises predominantly from variation in the supply of resources from nodal roots rather than from repression of axillary buds by apical tissues (apical dominance). We suggest that evolution of such a physiological mechanism enhances the exploration for patchily distributed favourable nodal rooting sites by regulating shoot development so as to efficiently utilise the diminishing intra-plant availability of root-supplied resources. For the species examined, inter-specific variation in intensity of branching response to a nodal root is shown to be linked to a trade off in foraging strategy, with the allocation of resources primarily to explorative growth (long internodes, few branches) in guerilla species or to exploitive growth (short internodes, many branches) in phalanx species.Co-ordinating editor: J. Tuomi     

Uptake of Phosphate by White Clover: I. EVIDENCE FOR AN ELECTROGENIC PHOSPHATE PUMP

Uptake of phosphate from flowing solution and cell membraneelectrical potential differences (PD) have been followed simultaneouslyin the roots of Trifolium repens L. Intact plants were usedand it was found that uptake and PD were strongly influencedby the shoot. They were reduced by excision of the root, defoliationof the plant, and lowering the light intensity at the leaf surface.2, 4-Dinitrophenol caused the PD to decline by approximately40 mV. A close correlation between phosphate uptake and PD wasobtained over a wide range of conditions and removal of phosphatefrom the solution resulted in a decline in PD of about 35 mV.It was concluded that there is an electrogenic phosphate pumpin T. repens roots which is closely dependent on the carbohydratesupply from the shoot.     

Root-to-shoot primordium conversion on Sesbania rostrata Brem. stem explants

The morphogenetic responses of cultured stem explants of Sesbaniarostrata Brem. from various positions along the stem axis wereanalysed after treatment with four growth regulators (BAP, NAA,kinetin, and GAJ. Internodal explants formed adventitious shootbuds when cultured on a Murashige and Skoog basal medium withoutadded growth regulators. Histological studies of regenerated shoot buds revealed thatapproximately 30% of the buds resulted from the conversion ofa preformed root primordium (characteristic of this species)into a shoot bud without a callogenesis phase. Each bud whichoriginated from a single root primordium grew into a leafy shoot.Preformed root primordia of stem explants of Sesbania rostratamay constitute an excellent model for physiological researchon plant differentiation. Key words: Organogenesis, adventitious bud, preformed root primordium, conversion, Sesbania rostrata     

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.     

THE PHYSIOLOGICAL BASIS OF MORPHOLOGICAL POLARITY IN REGENERATION. II

1. The experiments show that the mass of air roots formed in a stem increases with the mass of the leaf attached to the stem, though it has not been possible to establish an exact mathematical relation between the two masses, owing to unavoidable sources of error. 2. Darkened leaves do not increase the mass of roots formed. 3. In stems suspended horizontally air roots appear on the lower side of the stem, with the exception of the cut end where they usually appear around the whole circumference of the stem. When the lower half of a stem suspended horizontally is cut off, roots are formed on the upper side. It is shown by experiments on leaves suspended horizontally that the more rapidly growing roots and shoots on the lower side inhibit the root and shoot formation in the upper half of such a leaf; and likewise the more rapid formation of roots on the lower side of a horizontally suspended stem seems to account for the inhibition of root formation on the upper side of such a stem. Likewise the more rapid growth of shoots on the upper side of a stem suspended horizontally is likely to inhibit the growth of shoots on the lower side. 4. Each leaf contains in its axil a preformed bud capable of giving rise to a root, which never grows out in the normal stem on account of the inhibitory influence of the normal roots at the base of the plant. These dormant root buds are situated above (apically from) the dormant shoot bud. The apical root buds can be caused to develop into air roots when a piece of stem is cut out from a plant from which the leaves except those in the basal node of the piece are removed. The larger these basal leaves the better the experiments succeed. 5. These apical air roots grow out in a few days, while the roots at the basal end of the stem (which in our experiments dip into water) grow out about a week later. As soon as the basal roots grow out in water they cause the air roots in the more apical region of the stem to dry out and to disappear. 6. In addition to the basal roots, basal nodes have also an inhibitory effect on the growth of the dormant root buds in the apical region of a stem. This is indicated by the fact that a stem with one pair of leaves near the base will form apical air roots more readily when no node is situated on the stem basally from the leaf than if there is a node basally from the leaf.     

Vascular Architecture of a Large-leafed Genotype ofTrifolium repens

The objectives of this study were to identify the vascular connectionsfrom roots to upper axial bundles in one genotype ofTrifoliumrepensL. ‘Grasslands Kopu’, identify pathways followedby the transpiration stream, and establish whether these pathwayscould account for previously-observed patterns of clonal integration.The study provides new information on vascular connections betweenroot and parent and branch stolons at nodes possessing botha root and a branch, and to the first two leaves on branch stolons.A nodal root is connected to the lower nearside axial bundleof the parent stolon but to both lower and upper nearside axialbundles of the branch. Upper sympodia provide a long-distancetransport pathway from a parent stolon to the apex of branchstolons. Lower sympodia are functionally different, providingshort-distance transport to structures in close proximity tothe source root. This is consistent with observed patterns ofclonal integration inT. repensand may provide a simple architecturalmechanism facilitating foraging.Copyright 1998 Annals of BotanyCompany Acid fuchsin, clonal integration, foraging, physiological integration, serial sections, white clover,Trifolium repens(L.), vascular architecture, xylem transport.     

Bud Structure in Relation to Shoot Morphology and Position on the Vegetative Annual Shoots of Juglans regia L. (Juglandaceae)

The length and basal diameter of all lateral and terminal budsof vegetative annual shoots of 7-year-oldJuglans regia treeswere measured. All buds were dissected and numbers of cataphylls,embryonic leaves and leaf primordia were recorded. Each axillarybud was ranked according to the position of its associated leaffrom the apex to the base of its parent shoot. Bud size andcontent were analysed in relation to bud position and were comparedwith the size and number of leaves of shoots in equivalent positionswhich extended during the following growing season. Length andbasal diameter of axillary buds varied according to their positionon the parent shoot. Terminal buds contained more embryonicleaves than any axillary bud. The number of leaves was smallerfor apical and basal axillary buds than for buds in intermediatepositions on the parent shoot only. All new extended shootswere entirely preformed in the buds that gave rise to them.Lateral shoots were formed in the median part of the parentshoot. These lateral shoots derived from buds which were largerthan both apical and basal ones. Copyright 2001 Annals of BotanyCompany Juglans regia L., Persian walnut tree, branching pattern, preformation, bud content, shoot morphology     

Distinct Influence of Root and Shoot Temperature on Nitrogen Fixation by White Clover

White clover (Trifolium repens L.) was grown in controlled environmentsto determine the distinct effects of root and shoot temperatureon the accumulation of total and fixed (¹⁵ N dilution) nitrogenat two levels of nitrate (10 and 75 mM). Nitrogen fixation(BNF) showed a positive response to higher shoot temperature(23 vs. 13 C day temperature), irrespective of whether or notroot temperature was increased in parallel. Low root temperature(5 C) caused a marked reduction in the accumulation of totalnitrogen at both nitrate levels, and led to a lower proportionof N derived from BNF. The temperature response of BNF was attributedfor the major part to an adaptation to the demand for fixedN. It is therefore concluded that BNF is not primarily responsiblefor the reduced clover growth at low temperatures. White clover, Trifolium repens L., temperature, nitrogen fixation, nitrate, root, shoot     

Environment-induced Modifications to Root Longevity in Lolium perenne and Trifolium repens

Understanding how environmental factors affect the longevityof roots is essential if root mortality linked nutrient cyclingprocesses within ecosystems are to be understood, and the impactof natural and anthropogenic climate change properly evaluated.In this study the longevity of roots at two geographically andclimatically distinct sites were compared to identify the scaleof change that can occur due to environmental differences. Minirhizotrontubes were inserted into swards sown with the same variety ofLolium perenne and Trifolium repens at sites in the UK and Italy.Roots were viewed using a video camera and digitized imagesof roots generated at intervals. From these images the lifehistory of individual roots was determined and compared. Therewere few differences in patterns of longevity between differentspecies at the same site. Major differences, however, were observedbetween roots of the same species at different sites. For example,73% of Trifolium repens roots failed to survive for 21 d inItaly compared to only 29% at the UK site. Similarly, over 84%of Lolium perenne roots failed to survive for more than 21 din Italy compared to 38% in the UK. These data suggest thatenvironmental factors can have a major impact on root longevity.Copyright 2000 Annals of Botany Company Climate change, Lolium perenne L., minirhizotron, perennial ryegrass, root longevity, Trifolium repens L., white clover     

Axillary bud growth in relation to adventitious root formation in cuttings

Single-node leaf-bud cuttings of Schefflera arboricola Hayata and Stephanotis floribunda Brongn. were set and root formation, onset of axillary bud growth and plant height were measured. An increase in the number of roots in Schefflera, which was achieved with increasing cutting position on the stock plant (measured from top to base) or with increasing stem length below the node, accelerated the onset of axillary bud growth and resulted in an increase in plant height. Increasing the number of roots per cutting in Stephanotis through an increase in basal temperature also accelerated bud and shoot growth. Positional effects on root formation in Stephanotis showed no relationship with axillary bud growth and plant height. A positive relationship between number of roots per cutting and axillary bud growth was found among clones of Stephanotis . In general the results suggest that, with some exceptions, the onset of axillary bud growth is accelerated in cuttings as a result of accelerated root formation and a higher number of roots per cutting.