The significance of nodal rooting in Trifolium repens L.: ^sp distribution and local growth responses

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1 nt: Phytol. (1994), 127, The significance of nodal rooting in Trifolium repens L.: ^sp distribution and local growth responses BY W. D. KEMBALL^ AND C. MARSHALL School of Biological Sciences, University of Wales, Bangor, Gwynedd LL57 2UW, UK {Received 11 August 1993; accepted 9 January 1994) S L' M M A R Y The distribution of ^'^P from single nodal roots, and the consequences of nodal rooting on local growth characteristics were investigated in order to evaluate the iniponance of nodal rooting in Trifolium repens. The movement of radiophosphate was mostly acropetal. and the principal sinks were the closest components to the supplied root. The branch stolon originating from the same nodt as the root was the most significant sink, and its importance as a sink increased the older and larger it became. Nodal rooting on the mam stolon resulted in a localized increase in secondary branching on the primary hranch originating from the same node. The results are discussed in terms of phy.siojogicaj integration m T. repens and other clonal plants. Key words: Trifolium repens (white clover), nodal rooting, '^ integration. distribution, local growth response, clonal INTRODrCTION In white clover {Trifolium repens), the modular, stoloniferous pattern of growth allows the production of nodal roots at the base of the leaf from each node along the stolon axis. Such roots develop either as a pseudo-tap root structure or as a fibrous root system (Caradus, 1977). In the field only about 25 ".'o of nodes produce roots (Chapman, 1983), probably because of restricted water supply (Stevenson & Laidlaw, 1985). The growth and development of nodal root systems has important nutritional implications for the development of the stoloniferous shoot. Furthermore, there is a close relationship between the degree of nodal rooting and the growth and development of the branch stolon arising from the rooted node (Chapman, 1983). Chapman, Robson & Snaydon (1991a) also noted a close link between the leaves of a primary branch stolon and its associated main axis nodal roots in terms of the allocation of assimilate. The death of a rooted node can result in the death of a cohort of nodes acropetally up to the next rooted node (Sackville-Hamilton & Harper, 1989). Thus the ' Present address: AFRC In.stitUTe of Grassland and Environmental Re.search. Plas Gogerddan. Aberystwyth, Dyfed SY23 3EB, UK. production of nodal roots along an extending stolon is of prime importance in providing nutrients and water to sustain its long-term growth and development. There have been more studies on the partitioning of current assimilate in both T. repens (Chapman, Robson & Snaydon, 1991a, 1991&, 1992; Kemball, Palmer & AiarshalJ, 1992) and other,stoloniferous species (e.g, Forde, 1966; Atpert & Mooney, 1986; Price, Marshall & Hutchings, 1992) than of mineral nutrients (e.g. Headley, Callaghan & Lee, 1988; Marshall, 1990). Many clonal plants appear to have a high level of physiological integration, leading to a pattern of organization that is considered to optimize their foraging ability especially in relation to the exploitation of patchy environments (Pitelka & Ashmun, J985; Hutchings & Bradbury. 1986; Headley et al, 1988). In view of the relatively widespread movement of '*C-assimi]ate in T. repens (Chapman et al., 1991 a, 6, 1992; Kemball et al., 1992), it is important to have information also on the degree to which mineral nutrients acquired by nodal roots are distributed throughout the clona! system. In T. repens the acropetal movement of phosphate has been noted in growth studies (Nelson & Brady, 1953) and in the distribution of ^^P (Ueno & Williams, 1967; Hoshino, 1974). In seedlings, Ueno & Williams (1967) noted a wider distribution of 6-2

2 84 W. D. Kemball and C. Marshall isotope from the tap root than from young nodal roots, and Hoshino (1974) found that most exported ^^P was accumulated at the stolon growing points especially at alternate branch apices along the stolon, indicating some restriction of mos'ement by the vascular system. Recently Chapman & Hay (1993) observed young nodal roots exported ^'P to the main stolon leaves, stolon internodes and apex whilst older noda] roots exported more to branch stolons. Furthermore, the most important sink was the branch stolon originating at the rooted main stolon node. The experiments reported in this paper were designed to provide basic information on the pattern of distribution of ^''P absorbed by newly induced noda! roots originating at a single node m relatively large stoloniferous plants of T. repens. Particular attention was given to the location of the rooted node in relation to the main stolon apex. The consequences of nodal rooting in local growth, i.e. on the performance of the primary branch arising at the rooted node, were also studied in plants rooted at different nodes of the main stolon. M.'VTERiALS.AND METHODS Design and analysis Plants of Trifobum repens L. were established from single node cuttings (consisting of one node, one leaf and adjacent internodes) from a genet stock maintained at the Uni\ersity of Wales, Bangor. Br(B) Br(A) Br(C) Figure 1. A diagrammatic representation of the position at which individual nodal roots were induced three weeks prior to being supplied with ^^P-phosphate in Experiment 1. The basai part of the plant is omitted from the diagram. MS, main stolon; Br, primary branch stolon originating from the rooted node (not apparent in treatment A at this time); Sr, secondary stolon originating from the rooted node; (A), (B), (C) refer to the treatment in which a particular node was rooted. They were grown in 11 cm diameter pots of John lnnes No. 1 compost in a glasshouse, at minimum day and night temperatures of 18 and 15 C respectively and with natural light supplemented by sodium lighting to give a minimum 14 h day. Water was continually available via capillary matting. Plants were grown until nodes had been produced on the original stolon (termed the main stolon), which extended beyond the confines of the pot. Rooting was then induced at a particular node by placing a 3 cm diameter pot of John lnnes No, 1 compost under that node and pinning the stolon to the compost using a rubber coated wire. Plants were left for three weeks, by which time a single large nodal root system originating from one of a pair of root primordia, situated either side of the axillary bud, had developed. This root system was excavated from the pots, washed free of all compost, and supplied with 75 kbq of ^-P-NaH.POj in 75 cm' of y^ Strength Long Ashton solution (Hewitt, 1966) in a 100 cm^ conical Bask. After 24 h, the root was removed from the solution and washed in distilled water. Plants were harvested immediately and subdivided into their component parts. The ^""P-treated root was discarded. Plant material was dried at 60 C for 48 h and weighed. Each plant component was placed in a glass scintillation via] and ashed in a muffle furnace at 500 C for 12 h, but samples > 1 g were subsampled prior to ashing. After coowng, 5 cnv' oi 0-1 M HCI and 6 cm'' of deionized water were added to each \'ial. The radioactivity in each vial was measured using a scintillation counter set to detect Cerenkov radiation (window setting 8-212) with inbuilt quench correction using the sample channel ratio method. Correction for decay of the ^""Pphosphate isotope was made when necessary. This general method of ^^P-analysis followed that of Atkinson (1974). Experiment 1. The distribution of'^'^p from nodal roois at different positions This experiment was designed to determine the distribution patterns of '"P from nodal roots, and whether the position of the rooted node and sink size or type affected the distribution. Plants were grown Table 1. The subdivision of plants in Experiment 1 for ^^P analysis Treatment Plant component A B C Main root Main root Main root Basal stolon Basal stolon Basal stolon 3 nodes 3 nodes 3 nodes of of of MS MS MS 3 nodes of MS Br Br: Apical Basal Sr Br 3 3 nodes nodes of MS of MS 3 nodes of MS 3 nodes of MS Apical stolon Apical stolon Apical stolon MS, main stolon; Br, primary branch stolon arising at the rooted MS node; Sr, secondary branch stolon arising at the rooted node of the primary stolon.

3 Nodal rooting in Trifolium repens 85 Main root - Remaining basal stoion (MS) Basal nodes 4-6 (MS) Basal 3 nodes (MS) Br primary branch stolon Apical 3 nodes (MS) Apical stolon (MS) (W Main root Remaining basal stoion (MS) Basal 3 nodes (MS) Br primary branch stolon sixth (treatment B) node from the main stolon apex, or the third basal node of the primary stolon branch positioned seven nodes from the main stolon apex (treatment C). The nodal roots produced in treatments A-C w^ere supplied with ^^P as described previously. At harvests plants were dissected into component parts with the MS rooted node (treatments A and B) or the MS node of origin of the rooted branch stolon (treatment C) taken as the point for subdivision for the main stolon in both the apical and basal direction (Fig. 1, Table 1). Sets of three MS nodes (including internodes and branches) were separated in both basal and apical directions, together with remaining apical and basal stolon (Table 1). The branch stolon (Br) arising at the rooted node was harvested as one component in treatments A and B; in treatment C the rooted primary stolon (Br) was separated at the rooted node into apical and basal parts, with the secondary stolon (Sr) arising at the rooted node analyzed separately. In all treatments the main root was also analyzed (Table 1), (c) Apical 3 nodes (MS) Apical nodes 4-6 (MS) Apical stolon (MS) Main roots - Remaining basai stoion - Basal 3 nodes (MS) [} Basai primary stolon Sr secondary stoion Apical primary stoion Experiment 2. The detailed distribution of ^'^P supplied to a nodal root on the main stolon The following experiment was designed to provide more detailed information on the distribution of the ^^P. Rooting was induced at the node hearing the youngest fully expanded leaf on the main stolon (12-15 nodes from the basal node). After 21 days the newly produced nodal root was supplied with '"'"Pphosphate in 5 replicate plants. Plants were subdivided for ^^P analysis into: individual leaves and individual internodes of the primary stoion arising at the rooted node (Br); individual internodes and individual branches of MS apically from the rooted node; four internodes and four branches of MS basipetally from the rooted node. Remaining plant material was discarded. Apicai 3 nodes (MS) Apical stolon (MS) Exported ^^P (%) Figure 2. The distribution of exported ''P from a nodal root in Experiment 1 (-I-SE, n = 5), {a) Treatment A, {b) Treatment B, (c) Treatment C. Plant components are named using the term 'node' to refer to node, internode and associated branch stolons; the terms apical and basal are used in relation to the rooted node, and also the rooted branch (Fig, lc only); the terms Br or Sr are used for rooted primary and secondary branch stolons respectively. Arrows indicate the branch stolon originating from the same node as the root supplied with ' P. until the main stolon (MS) was approximately 15 nodes in length. Single nodes were then rooted as follows (Fig. 1): the second (treatment A) or the Experiment 3. The infiuence of nodal rooting upon stolon growth and development The following experinnent was designed to investigate the effects of nodal rooting on localized branching and the growth of the main stolon system. Plants with approximately 12 nodes on the MS growing in 11 cm diameter pots of John Innes No. 1 compost were also rooted at selected apical nodes in another 11 cm diameter pot of J.I. No, 1 compost as follow s: Treatment 1. Node 10 (the tenth node from the main root system) rooted. Treatment 2. Nodes 10 and 11 rooted. Treatment 3. Nodes 9 and 10 rooted. Rooting was induced as previously described, whilst rooting of other nodes was prevented by placing a small plastic sheet under the appropriate

4 86 W. D. Kemball and C. Marshall Table 2. The movement of n = 5) in relation to the dry weight of the branch stolon subtending the fed root f ± SE, Treatment Stolon type Dry weight (mg) B A C Primary Primary Secondary ( + 23'1) 27-1 ( + 5-7) 14-3 ( + 2-6) Movemetit of ^^ into stolon {%) 71-0 ( + 9-6) 27-4 ( + 4-3) 14-4 ( + 2-6) Movement of acropetally ("o 88-0 ( + 6-9) 66-8 ( + 5-6) node. Weekly measurements of length, number of nodes and number of branches were made for the main stolon (from node 10), and for the primary stolons originating at nodes 9, 10, and 11 (termed B9-11). The experiment lasted 35 days when the plants were harvested into the following parts: (1) individual primary stolon branches B9, BIO, and Bll; (2) their individual sets of secondary stolons; (3) the remaining apica! MS stolon; (4) apical branches of MS; (5) nodal roots originating at nodes The basal part of the plant was discarded. Each treatment had ten replicates. Data were compared using one-way analysis of variance and Tukey's test for the comparison of means. Rant component Basal i nodes Main apex Exported ^^P (%) Figure 3. The detailed distribution of exported ^^P from a nodal root in Experiment 2 (+ SE, n = 5). The term node is used as in Fig. 2 with node 1 closest to the fed root and node 4 adjacent to the main apex, with each node component split into main stolon internode and associated branch stolon; Br is the primary branch stolon originating from the same node as the root supplied with *^P (this branch is also shown by two arrows). Q, main stolon internode; S, main stolon branch; B. branch (Br) arising at rooted node I (oldest node) leaves or stolon internode; EH, Br node 2; ^, Br, node 3;, Br node 4 (youngest node). RESULTS Experiment 1 When the most apically positioned nodal root (Treatment A) was supplied witb ^"P-phosphate, 88 % of the isotope was found in the distal parts, 35 'VJ was in the apical three nodes and branches, and 25'^o in the remaining apical stolon (Fig. 2a). The primary stolon branch (Br) originating from the rooted node had a considerable proportion of ^^P (27 "o). probably the greatest amount of radioactivity of any individual branch stolon. Of the small proportion of '^^P-phosphate moving basipetally, virtually all was found in the three nodes next to the treated root. When ^"P was supplied to a more basally positioned nodal root (Treatment B) more was exported to the subtending primary stolon branch (Br) tban for 'IVeatment A (71 % compared with 27"o) (fig- 26). Consequently a smaller percentage of the export was found in other fractions, w ith 21 % in the next three apical nodes, internodes and branches, and only 4"o in the basal parts of the plant. When a nodal root on a primary branch stolon (Treatment C) was supplied with '^^P 88 "o remained within that branch (Fig. 2 c). Within tbe branch, only 14 "o of ^^P was exported to its subtending secondary stolon (Sr), with the majority (52"o) found in the apical region and 21 % in the basal part of the stolon. Most of the ^"P-phosphate which was exported from the primary brancb stolon was found in tbe three nodes and branches situated towards the main stolon apex from the primary stolon (8"'o). Very little was exported basipetally dow-n the niain stolon. In Table 2 the percentage import of ^^P-phosphate by the branch stolon subtending the ^"P-treated nodal root is sbown in relation to its dry weight for tbe three treatments. In all cases a high percentage of '^P moved acropetally towards the apices of botb the primary and secondary stolons. The smaller secondary stolon received only 14 4 "-'o of exported isotope, with 67% moving acropetally, whilst the largest primary stolon received the greatest percentage of tbe isotope (71 %) with over 95 "o moving acropetally. Thus as tbe dry weigbt of tbe subtending branch increased, and correspondingly the dry weight of apical tissue increased, the proportion of

5 Nodal rooting in Trifolium repens 87 (a) 40 (W B I ] 5 - r 7 \, 1, 1, (c) (d) Time (days) Figure 4. The effects of position of nodal roots upon stolon elongation (±SE, n = 10), {a) Main stolon, (6) B9 primary stolon, {c) BIO primary stolon, {d) Bll primary stolon. D, Treatment 1 (rooted at node 10); A. Treatment 2 (rooted at node 10 and 11); D, Treatment 3 (rooted at node 9 and 10), 35 ^^P-phosphate exported to the subtending branch stolon increased, and also the extent of acropetal movement increased. Experiment 2 In experiment 2, 97'^i, of the isotope was exported acropetally (Fig. 3). The internodes of the primary branch stolon (Br) originating from the rooted node and its leaves were the major sinks for '"^^P accounting for 26 "o and 17% of the exported isotope respectively, considerably more than the total exported to other branch stolons (23"<>), Within this branch stolon (Br), more isotope was exported to the most apical stolon internode (including the apex) and to the youngest leaf than to any other stolon internodes and leaves. There was a stepwise fall in the amount of isotope exported to both apical internodes and branches as they increased in distance from the supplied root. Main stolon internodes received more isotope than their branches, both in total and when comparing individual components. Experiment 3 With one exception there was no significant difference between rooting treatments in the

6 W. D. Kemball and C. Marshall 12 MS / ^ 5 10 re XI o 8 v o? X! 4 Z 2 E 01 r (c) Time (days) Figure 5. The effects of position of nodal roots upon the production of nodes and branches (± se, «= 10). (a) Main stolon, (6) B9 primary stolon, (c) BIO primary stolon, (d) BH primary stolon, Q, Treatment 1 node number;, Treatment 1 and branch number; A, Treatment 2 node number; A, Treatment 2 branch number; n, Treatment 3 node number;, Treatment 3 branch number. elongation of, or the production of nodes by the main stolon, or of the primary stolons arising at nodes 9 (B9), 10 (BIO), and 11 (BU) (Figs 4 and 5). Only in the case of the Bl 1 stolon was stolon length affected, where elongation was greatest when rooted at nodes 10 and 11 (treatment B) and least when only rooted at node 10 (treatment A). The production of secondary branches on BIO which was always rooted was unaffected by the rooting of adjacent nodes (Fig. 5 c). However, the production of secondary branch stolons on the B9 stolon (Fig. Sb) was doubled when it was rooted (P = 0-05) (Treatment 3), with this difference apparent after two weeks. Similarly rooting node 11 (Treatment 2) resulted in a significant (P = 0-05) increase in branching on Bil compared to both Treatment 1 (P^O'Ol) and Treatment 3 (P = 0-05) (Fig. 5 d). The production of branches upon the main stolon was not affected by the rooting treatment (Fig. 5 a). The production of biomass by the main stolon and the nodal roots was not significantly affected by the rooting treatments, but the biomass of the apical branches was greatest in Treatment 2 where both nodes 10 and 11 were rooted (P = 0-05) (Fig. 6a). The biomass of the BIO branch and its secondary stolons, which always originated from a rooted main

7 Nodal rooting in Trifolium repens 89 (a} 400 id) OJ S Treatment Figure 6. Biomass of plant components after 35 days (4-SE, n = 10). (a) Main stolon; DD, main stolon axis; ^, apical branches; M< nodal roots; (b) B9 primary stolon; (c) BIO primary stolon; (d) Bil primary stolon; Si primary stolon; ^, secondary stolons. stolon node, was not significantly affected by the rooting treatments (Fig. be). The biomass of B9 and Bl 1 primary branch stolons was virtually unaffected by rooting treatments, although there was a considerable increase (significant at P ^ 0-05) in the biomass of their secondary branch stolons when that branch originated from a rooted node (Treatments 3 and 2 respectively) (Figs 66 & d). DISCUSSION The predominant flow of radiophosphate absorbed by a nodal root system was acropetal, with a distinct priority in allocation to the subtending branch stolon. A similar pattern has been shown or inferred in other experiments on T. repens (Nelson & Brady, 1953; Ueno & Wtlliams, 1967; Hoshino, 1974; Chapman & Hay, 1993) and several other clonal plants e.g. Care.x arenaria (Noble & Marshall, 1983); A. stohmifera (Anderson-Taylor, 1982) and L. annotinum (Headley et al., ] 988). In the present series of experiments the majority of the exported isotope was found in close proximity to the ^^Ptreated nodal root. The branch stolon originating at the rooted node was the major sink receiving a greater degree of support as it increased in size. This observation was recently reported by Chapman & Hay (1993). This suggests a specific source-sink link between a developing branch and the nodal root originating from the same parental node, as indicated in the studies of the vascular architecture of T. repens (Erith, 1924; Devidas & Beck, 1972). This view is further supported by the field observations of Chapman (1983) and the distribution pattern of *^C-

8 90 W. D. Kemball and C. Marshall assimilate to stolon branches (Chapman et al., 1992). The results of Experiment 2 showed that ail component parts of the subtending hranch stolon were supplied with ^'P, especially the extending apical region, implying some transport of ^^P in the phloem, as well as the xylem. This is supported by the observation that phosphate can readily transfer from xylem to phloem during longitudinal transport (Pate, 1975; Headley et al, 1988). Although the branch stolon at the rooted node was the major sink, all acropetally positioned components of the main stolon relative to the rooted node, especially the closest ones, were supplied with ^'^P. This differential support can be interpreted as a consequence of source-sink distance and also in terms of sink size. Younger branches were smaller, with less leaf area and hence less transpiration, and were also further from the rooted node. Thus such branches received less P than older, larger and closer branches. These observations on the allocation of ^^P to branches differ from those of Hoshino (1974) in which ^"P-phosphate was localized in alternate branches, but this may reflect differences between cuitivars in their vascular anatomy (Sackville- Hamilton personal communication). However Chapman & Hay (1993) reported similar findings to this study, although less acropetal movement of P was observed. The limited rooting adopted in this report (at a single node) may result in a more acropetai distribution of P with priority given to the extending apical region. The linkage between the leaf at a node and the branch originating from that node, in terms of movement of '^C-assimilate. appears to be different from the movement of ^^P from the root at such a node. The highest proportion of '''C-assimilate moving into such a branch from its axillary leaf was 3(M-0 "o (Kemball et al., 1992). as compared to 70 ^, ^^P from a nodal root on the mam stolon (this study). Assimilate translocation was widespread moving to all parts of the plant, with the direction of movement depending upon the leaf's position relative to the main sinks and showed considerable basipetal movement (40-60%) (Chapman ei a/., 1991a; 1992). This contrasts with the predominantly acropetal movement of ^^P observed in this study, although Chapman & Hay (1993) observed less pronounced acropetal movement of P. These differences between ^^P and ^^C probably reflect the isotope's transport pathway with assimilate moving within the phloem and ^'P moving mainly in the xylem. Nodal rooting was associated with a local growth response, and it was clear that there was a significant increase in secondary branching on the primary stolon when rooting occurred at its node of origin on the main stolon (Expt 3). In contrast there was little effect upon stolon length, number of nodes and biomass on the main stolon. This very local growth response to local rooting was complemented by the short term patterns of acropetal transport of ^^P absorbed by nodal roots, especially in terms of its local utilization. Thus the production of nodal roots may be expected to have a significant effect on the materials imported by the subtending branch stolon and to its pattern of bud development. These inferences stress the importance of nodal rooting for the development of the stolon system especially branch stolons, and furthermore to guard against severance from the parental root stock through either trampling damage or stolon die back. The localized distribution of resources from fed roots, and the localized growth response may be considered good foraging strategy (de Kroon, 1990). It maximizes the colonization of a resource rich site and the exploitation of its mineral resources. Some cional foraging plants display morphological plasticity in hoth the degree of internode elongation and the frequency of branching in response to the level of local resources, e.g. Glechoma hederacea (Slade & Hutchings, 1987a, ;»; Hutchings & Slade 1988). In T. repens there was no trade off between increased branching and internode length in response to increased nutrient availability brought about by nodal rooting, and this follows the theoretical foraging response analyzed by Sutherland & Stillman (1988, 1990). Other observations on T. repens demonstrate a greater plasticity in branching rather than internode length, and with a variable response in the latter (Solangaarachchi & Harper. 1987; Thompson & Harper, 1988; Kemball et al., 1992; Thompson, 1993). The acropetal movement of resources to the stolon apices ensures that elongation is maintained, allowing the stolon to develop into new territory with the potential to find new sites for nodal rooting. Once nodal roots have developed along that stolon, the role of the more basal roots in supplying resources to the apical stolon is reduced. Thus nodal rooting gives a ramet or a group ot raniets the potential for independence from the parental plant in terms of its supply of water, nutrients and root produced hormones, even though the branch may still be involved in the considerahle assimilate exchange with other parts of the plant. Furthermore rootmg influences branch development and survival (Chapman, 1983), and thus has important agronomic implications for growing point numher and clover content of the sward. ACKNOWLEDGEMENTS This work was supported by the award of a SERC postgraduate studentship to W.D.K. We are indebted to Gareth Williams for assistance with the analysis of

9 Nodal rooting in Trifolium repens 91 REFERENCES Aipert P, Mooney HA Resource sharing among ramets in a clonal herb, Fragaria chiloensis. Oecologia 70; , Anderson-Taylor G Physiological aspects of rool-tiller interrelationshipsinhordeumdifiuchum and Agrostisstoloniit^ra. Ph.D. thesis, U.C.N.W., Bangor. Atkinson D Some observations on the distrihution of root activity in apple trees. Plant and Soil 40: Caradus JR Structural variation of white clover root systems. New Zealand Journal of Agricultural Research 20; 21.^ 219. Chapman DF Growth and demography of Trifolium repens stolons in grazed hill pasture. Journal of Applied Ecology 20: , Chapman DF, Hay MJM Transiocation of phosphorus from nodal roots in two contrasting genotypes of white clover {Trifolium repens). Physiologia Plantarum 89: , Chapman DF, Robson MJ, Snaydon RW a The influence of leaf position and defoliation on the assimiiation and distribution of carbon in white clover {Trifolium repens L,), 1, Carbon distrihution patterns. Annals of Botany 67: , Chapman DF, Robson MJ, Snaydon RW , The influence of leaf position and defoliation on the assiniilarion and distribution of carbon in wbite clover {Trifotium repens L,), 2, Quantitative carbon movement, Aniials of Botany fil: , Chapman, DF, Robson MJ, Snaydon RW. 1992, Physiological integration in the clonal. perennial herb Trifo/iurr repens L. Oecologia 89; , Devitjas C, Beck CB. 1972, Comparative morphology of the primary vascular system in some species tif Rosaceae and Leguminosae. American Journal of Botany S Erith AG. 1924, White clover (Trifolium repens L,); «monograph. London: Duckworth & Co, Eorde BJ. 1966, Translocation in Grasses. 1, Bermuda Grass. AVw Zealand Journal of Bolanv 4; 479 fy5, Headley AD, Callaghan TV,'Lee JA Phosphate and nitrate movement in the clonal plants Lycopodium annotinum and Diphasiastrum complanatum L, Holub, Neu- Phytologist 110; , Hewitt EJ. 1966, Sand and tvater methods used in the study of plant nutrition. Farnham Royal, Berks: Commonwealth Agricultural Bureau, HoshinoM Translocation and atcumulation of assimilate and phosphate in Ladino clover. Bulletin of the National Grassland Research Institute (Japan/ 5: 35-84, Hutchings MJ, Bradbury IK. 1986, Ecological perspectives on cjonal pttcnnial herb,s. Bw-Science 36: , Hutchings MJ, Slade AJ. 1988, Morphological plasticity, foraging and integration in clonal perennial herbs, in: Davey AJ, Hutcbings MJ, Watkinson.AR, eds. Plant population ecology. Oxford: Elackwf 11, Kemball WD. 1992, Source-sink interrelationships in white clover (Trifolium repens L,). Ph,D. thesis, Uni\ersiry of Wales, Kemball WD, Palmer MJ, Marshall C. 1992, Tbe eftect of local shading and darkening on branch grovvth, development and survival in Trifolium repens and Galium aparint. Oikos 63: Kroon H de In search of a foraging plant. The clonal growth of Bracbypodium pinnatum and Carex flacca. Ph.D, thesis, University of Utrecht, Marshall C Source-sink relations of interconnected ramets. In: van Groenendael J, Kroon H de, eds, Clonalgrotuth in plants. The Hague; SPB Academic Publishing, 23-41, Nelson LE, Brady NC. 1953, Translocation of potassium through Ladino clover stolons. Agronomy Journal 45 : , Noble JC, Marshall C, 1983, The population biology of plants with clonal growth, 2, The nutrient strategy and modular physiology of Carex arenaria. Journal of Ecology 71: , Pate JS. 1975, Exchange of solutes between phloem and xylem and circulation in the whole plant. In: Zimmerman MH, Milburn JA, eds. Transport in plants. I. Phloern transport. Encyclopedia of Plant Physiology. Berlin: Springer Verlag, 451^73, Pitelka LF, Ashmun JW. 1985, Physiology and integration of ramets in clonal plants. In: Jackson JBC. Buss LW, Cook RE, ed.s. Population biology and ei'olution of clonal organisms. New- Haven: Vale LTniversity Press, 399^35, Price EAC, Marshall C, Hutchings MJ Studies of the growth of rhe clonal herb Glechoma hrderacea. 1, An analysis of patterns of physiological integration. Journal of Ecology 80: Sackville-Hamilton NR, Harper JL. 1989, The dynamics of Trifolium repens ir\ a pi:tma.niir>\ pasture. 1, The population dynamics of leaves and nodes per shoot axis. Proceeding.' of the Royal Society London B 237: , Slade AJ, Hutchings MJ 1987 o. Clonal integration and plasticity in foraging bfhaiior in Gltchoma hederacea. Journal of Ecology 75; , Slade AJ, Hutchings MJ b..\r\ analysis of the influence of clone size and stolon connections between ramets on the growth of Gleflinma hi'deracea I., T^eii Phytologist 106: i, Solangaarachchi SM, Harper JL. 1987, The effect of canopy filtered light on the growth of white clover (7", repens). Oecologia 72: , Stevenson CA, Laidlaw AS. 1985, The effect of moisture stress on stolon and adventitious root development in white clover {Trifolium repens L,). Plant and Soil 85: , Sutherland WJ, Stillman RA. 1988, The foraging tactics of plants, Oikos 52: Sutherland WJ, Stillman RA. 1990, Clonal growth: insights from models. In: van Groenendaei J, Kroon H de, eds, Clonal growth in plants. The Hague: SPB Academic Publishing, , Thompson L The influence of natural canopy density on the growth of white clover, Trifolium repens. Oikos ft7 : , Thompson L, Harper JL. 1988, The effect of grasses on the quality of transmitted radiauon and its influence upon the growth of white clover Trifolium repens. Oecologia 75 : , Ueno M, Williams RD Absorption of radioactive phosphorus from tap and nodal roots of white clover (Trifolium repens ]...). Journal of the British Grassland Society 22:

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