Comparison of Two Shoot--Root Partitioning Models with Respect to Substrate Utilization and Functional Balance

A transport resistance model for shoot—root partitioningby Thornley and a more aggregated partitioning model by Reynoldsand Thornley are compared. Three functional forms of substrateutilization are applied, corresponding to different assumptionson the ability of carbon and nitrogen to compensate one anotherin promoting structural growth. On the basis of simulationsat balanced exponential growth, it is shown that the Reynoldsand Thornley model (in optimal form) is embedded in the Thornleymodel. Davidson's functional balance is studied as a functionof the degree of carbon-nitrogen compensation. The applicabilityof the models and the utilization functions is discussed. Model, shoot-root partitioning, substrate utilization, functional balance     

Simulating Growth and Root-shoot Partitioning in Prairie Grasses Under Elevated Atmospheric CO2and Water Stress

We constructed a model simulating growth, shoot-root partitioning,plant nitrogen (N) concentration and total non-structural carbohydratesin perennial grasses. Carbon (C) allocation was based on theconcept of a functional balance between root and shoot growth,which responded to variable plant C and N supplies. Interactionsbetween the plant and environment were made explicit by wayof variables for soil water and soil inorganic N. The modelwas fitted to data on the growth of two species of perennialgrass subjected to elevated atmospheric CO₂and water stresstreatments. The model exhibited complex feedbacks between plantand environment, and the indirect effects of CO₂and water treatmentson soil water and soil inorganic N supplies were important ininterpreting observed plant responses. Growth was surprisinglyinsensitive to shoot-root partitioning in the model, apparentlybecause of the limited soil N supply, which weakened the expectedpositive relationship between root growth and total N uptake.Alternative models for the regulation of allocation betweenshoots and roots were objectively compared by using optimizationto find the least squares fit of each model to the data. Regulationby various combinations of C and N uptake rates, C and N substrateconcentrations, and shoot and root biomass gave nearly equivalentfits to the data, apparently because these variables were correlatedwith each other. A partitioning function that maximized growthpredicted too high a root to shoot ratio, suggesting that partitioningdid not serve to maximize growth under the conditions of theexperiment.Copyright 1998 Annals of Botany Company plant growth model, optimization, nitrogen, non-structural carbohydrates, carbon partitioning, elevated CO₂, water stress,Pascopyrum smithii,Bouteloua gracilis, photosynthetic pathway, maximal growth     

The Effects of Nitrogen Fertilization and the Growing Season on Carbon Partitioning in a Sward of Tall Fescue (Festuca arundinacea Schreb)

The effect of N fertilization on the relative carbon partitioningto the roots of tall fescue (Festuca arundinacea Schreb ), grownunder field conditions, was studied with a ¹⁴C-labelling techniqueon three regrowths representing contrasting growing seasonsUnder non-limiting N growing conditions, the relative carbonpartitioning to the roots averaged 17.0, 15 8, and 11 1% inthe summer, autumn, and spring regrowths, respectively The relativecarbon partitioning to the roots increased during the summerand autumn regrowths but decreased during the spring regrowthIn the absence of N fertilization, the relative carbon partitioningto the roots averaged 31 3, 26 5, and 26 7 in the summer, autumn,and spring regrowths, respectively The results were interpretedin terms of a functional equilibrium between the shoots andthe roots It was concluded that, for a dense canopy of a perennialgrass growing under fluctuating conditions of solar radiationand temperature, the relative growth of the roots compared tothe relative growth of the total biomass is primarily a functionof the shoot biomass Festuca arundinacea Schreb, carbon, partitioning, nitrogen, root growth, fertilization, grass     

Canopy Photosynthesis of Tomato, Cucumber and Sweet Pepper in Greenhouses: Measurements Compared to Models

Two models for canopy photosynthesis (modified versions of thoseof Acock et al. , 1978 and of Thornley, 1976) were examinedby comparison with experimental photosynthesis data of cucumber(Cucumis sativus L.), sweet pepper (Capsicum annuum L.) andtomato (Lycopersicon esculentum Mill.). The data were obtainedin six large-scale, long-term, semi-commercial cultivationsin greenhouses (Nederhoff and Vegter, 1994). Measured environmentalconditions and measured LAI were input to the model. The emphasiswas on the models' sensitivity to the prevailing CO₂ concentration. The (modified) Acock model with 'standard' (originally published)parameters underestimated the photosynthesis rate. This modeltuned to one of our experimental data sets did not fit verywell to the other data sets. As expected, if the model was tunedto each particular data set, it was fairly in agreement withthe measurements, but the fitted parameter values were sometimesquestionable. With the (modified) Thornley model it was obligatoryto estimate or tune the light extinction. The model performedreasonably if all parameters were tuned and also if only thelight extinction was tuned. The modified models were considered usable for practical applications,after parameter tuning. As the sensitivity to CO₂ was not alwaysequal among the models and the measurements, care should betaken when applying the models for CO₂ supply control.Copyright1994, 1999 Academic Press Canopy photosynthesis, Capsicum annuum L., carbon dioxide, cucumber, CO₂, Cucumis sativus L., glasshouse, greenhouse, Lycopersicon esculentum Mill., measurements, model, sweet pepper, tomato     

Carbon Balance of Tall Fescue (Festuca arundinacea Schreb.): Effects of Nitrogen Fertilization and the Growing Season

The C balance of a tall fescue sward grown under different ratesof N fertilization in summer, autumn, and spring was calculatedusing models derived from measurements of shoot growth, canopygross photosynthesis, shoot respiration and of C partitioningto the roots. Under the diverse growing conditions associatedwith the seasons and the N fertilization, C utilization forabove- and below-ground biomass accumulation never exceeded39 and 14% of the canopy gross photosynthesis, respectively.Carbon losses attributed to root respiration and exudation,which were estimated by difference between canopy net photosynthesisand total growth, ranged between 3 and 30% of canopy gross photosynthesis.Seasonal differences in shoot growth could be attributed tothe amount of intercepted radiation, the radiation-use efficiencyand the C partitioning to the roots. The effect of N deficiencyon shoot growth can be attributed to its effects on canopy photosynthesis(principally resulting from changes in intercepted photosyntheticallyactive radiation) and C partitioning. In comparison with theeffect on shoot growth, the effect of the N deficiency on thecanopy gross photosynthesis per unit of light intercepted overthe regrowth cycle was limited. It is concluded that most ofthe effect of N fertilization on shoot growth is due to changesin C partitioning which result in faster leaf area developmentand greater light interception.Copyright 1994, 1999 AcademicPress Tall rescue, Festuca arundinacea Schreb., carbon balance, nitrogen, grass, fertilization     

A Review of Evidence on the Control of Shoot: Root Ratio, in Relation to Models

Four basic models exist for the control of shoot: root ratio(S: R): (a) allometric models, proposing a fixed ratio of shootgrowth rate to root growth rate; (b) functional equilibriummodels, based on the ratio of shoot activity to root activity;(c) the Thornley model, based on carbon and nitrogen uptakeand transport, (d) hormone models, generally suggesting theroot produces a hormone that controls the shoot and vice versa.Models (a) and (b) are empirical, and therefore provide no testof the processes operating. Ontogenetic changes in S: R for fibrous-rooted herbs could befitted by a modified Thornley model. Ontogenetic effects mustbe excluded in judging other effects. Responses of S: R to deficits of water, major inorganic nutrients,light and carbon dioxide, and to defoliation and root pruning,usually conform to Thornley's model. With current knowledgeThornley's model cannot usefully be applied to minor nutrients,nutrient toxicity or temperature differences. S: R changes atreproduction usually conform to Thornley's model if it is assumedthat young reproductive structures are a strong sink, but thisbegs the question of what determines sink strength. There areapparent exceptions to most of these responses, which shouldbe studied further. Phytohormones can influence S: R, but may not be the controloperating in the normal, intact plant. Most of the availableevidence is compatible with a source-sink model of Thornley'stype, and therefore does not demand a hormonal theory of S:R control. There is a need for more critical tests. Shoot: root ratio, strategy, partitioning, models     

Accumulation and Partitioning of Dry Matter in Taro [Colocasia esculenta (L.) Schott]

A field study was conducted as part of an ongoing effort tocollect data on patterns of leaf area development and dry matteraccumulation and partitioning among various plant parts duringgrowth and development of two taro cultivars. Plants were harvestedfor biomass about every 6 weeks during the growing season. Ateach harvest, plants were separated into various plant parts,and their dry matter content was determined. The first 80 dafter planting were characterized by low rates of dry matteraccumulation, with only leaves, petioles, and roots showingsubstantial growth. Afterwards, increases in total dry matterwere mainly the result of corm and sucker growth. Corm bulkingoccurred after the attainment of maximal leaf area indices.The absence of an optimal leaf area index for a longer periodof time may have prevented the realization of higher dry matteryields. The partitioning of dry matter to the corms of bothcultivars remained almost constant especially after 150 d afterplanting. This process was in contrast to the partitioning ofdry matter to the suckers, which increased significantly untilthe end of the growing cycle.Copyright 1995, 1999 Academic Press Taro, Colocasia sp., growth, dry matter partitioning     

Effect of Root and Shoot Meristem Temperature on Shoot to Root Dry Matter Partitioning and the Internal Concentrations of Nitrogen and Carbohydrates in Maize and Wheat

Maize (Zea mays L.) and spring wheat (Triticum aestivum L.)were grown in nutrient solution at uniformly high air temperature(20 °C), but different root zone temperatures (RZT 20, 16,12 °C). To manipulate the ratio of shoot activity to rootactivity, the plants were grown with their shoot base includingthe apical meristem either above (i.e. at 20 °C) or withinthe nutrient solution (i.e. at 20, 16 or 12 °C). In wheat, the ratio of shoot:root dry matter partitioning decreasedat low RZT, whereas the opposite was true for maize. In bothspecies, dry matter partitioning to the shoot was one-sidedlyincreased when the shoot base temperature, and thus shoot activity,were increased at low RZT. The concentrations of non-structuralcarbohydrates (NSC) in the shoots and roots were higher at lowin comparison to high RZT in both species, irrespective of theshoot base temperature. The concentrations of nitrogen (N) inthe shoot and root fresh matter also increased at low RZT withthe exception of maize grown at 12 °C RZT and 20 °Cshoot base temperature. The ratio of NSC:N was increased inboth species at low RZT. However this ratio was negatively correlatedwith the ratio of shoot:root dry matter partitioning in wheat,but positively correlated in maize. It is suggested that dry matter partitioning between shoot androots at low RZT is not causally related to the internal nitrogenor carbohydrate status of the plants. Furthermore, balancedactivity between shoot and roots is maintained by adaptationsin specific shoot and root activity, rather than by an alteredratio of biomass allocation between shoot and roots.Copyright1994, 1999 Academic Press Wheat, Triticum aestivum, maize, Zea mays, root temperature, shoot meristem temperature, biomass allocation, shoot:root ratio, carbohydrate status, nitrogen status, functional equilibrium     

Growth of Near-isogenic Wheat Lines Differing in Development--Spaced Plants

The relationship between phenological development in wheat (TriticumaestivumL.) and growth was studied to determine if the switchfrom a vegetative to a reproductive apex increases plant growthrate. Plant partitioning and relative growth rates during vegetativeand pre-flowering reproductive periods were determined in twosets of near-isogenic lines differing in phenological development.Spaced plants were grown in two photoperiods (15 and 10 h) toincrease the range of development rates. Lines within each isogenicset and photoperiod treatment did not differ in whole plantgrowth rate despite large differences in developmental rate.In addition, the partitioning of biomass between roots and shootswas also similar. The transition of the apex from vegetativeto reproductive mostly affected the partitioning of shoot biomassinto leaf (blades) and stems (rest of the shoot). A longer timeto reach floral initiation was associated with the productionof more, and larger, leaves as well as more tillers. This resultedin large differences in leaf area between isolines. However,at the whole plant level, all lines accumulated biomass at thesame rate with time. The early flowering lines compensated fortheir reduced leaf area by having a higher net assimilationrate and were thereby able to maintain the same relative growthrate as their later flowering counterparts.Copyright 1998 Annalsof Botany Company Development, growth, partitioning,Triticum aestivumL., wheat, isolines.     

A Comparison of Two Approaches for Modelling Cassava (Manihot esculenta Crantz.) Crop Growth

Two approaches for modelling the growth and development of cassava,Manihot esculenta Crantz, are described and evaluated. The twomodels differ only in the hypotheses accounting for storageroot growth. In model 1, assimilate allocation to storage rootsis governed by the combined Chanter's (1976: Mathematical modelsin mushroom research and production. PhD Thesis, Universityof Sussex, UK) growth equation; and in model 2 the spill-overhypothesis for assimilate allocation to storage root governsstorage root growth. In both models, canopy photosynthesis generatesthe carbon substrate required for all growth processes. Thegrowth rates of leaves, stems and storage roots are definedby growth equations subject to substrate saturation kinetics.A key feature of both models is that the growth demands of thestem, fibrous roots and storage roots are related to leaf demandrates. Allocation to stems and branches was modelled by meansof a modified logistic growth equation which includes all theparameters and variables (number of nodes, internode lengths,stem density, stem modulus of elasticity and branch tensilestrength) that define the limits of the load bearing capacityof the shoot's supportive structures. The correlation coefficientsfor determination of yield prediction for the models werer =0.898,P =0.0385 (model 1) and r =0.954, P =0.0117 (model 2). For agrowth season of 290 d (after which leaf area index equals zeroand crop growth ceases), both models simulate the sigmoidaltransition from the lag to exponential phase of crop growth.Both models are equally well corroborated by observed data;however, model 1 has greater explanatory power. Copyright 2000Annals of Botany Company Allocation, simulation, model, crop growth, cassava, Manihot esculenta Crantz.     

Genotypic Differences in the Production and Partitioning of Carbohydrates Between Roots and Shoots of Wheat Grown under Zinc or Manganese Deficiency

The partitioning of soluble carbohydrates and starch betweenroots and shoots was investigated in wheat genotypes differingin Zn or Mn efficiency. The plants were grown for 11 d in achelate-buffered nutrient solution with sufficient or deficientZn and Mn supply. The Zn-efficient cultivar Warigal had a greatershoot fresh weight under sufficient Zn compared with the Zn-inefficientcultivar Durati. When supplied with sufficient Zn, Warigal hada greater concentration and content of soluble carbohydratesin roots and shoots in comparison with Durati. Under deficientZn supply, Durati had a greater concentration and content ofstarch in roots and shoots compared with Warigal. In an experimentwith varying supply of Mn, the Mn-efficient genotype C8MM hada greater shoot fresh weight than the Mn-inefficient cultivarBayonet under sufficient or deficient Mn supply. The concentrationof soluble carbohydrates in roots and shoots was decreased bydeficient Mn supply in C8MM but not in Bayonet. Starch accumulatedin the roots of Bayonet under deficient Mn supply. The resultssuggest that synthesis of carbohydrates is decreased under Zndeficiency, while they are preferentially partitioned to theroots to increase growth and thus the surface area availablefor Zn uptake. In the case of Mn deficiency, carbohydrate productionwas limited, but partitioning between roots and shoots was notaltered.Copyright 1997 Annals of Botany Company Carbohydrate; deficiency; manganese; assimilate partitioning; starch; Triticum aestivum; zinc     

Effects of Edaphic Factors on the Soluble Carbohydrate Content of Roots of Lolium perenneL. and Trifolium repens L.

In a 3³x2² factorial experiment with Lolium perenne L. and Trifoliumrepens L. grown at three soil moisture tensions, three levelsof nitrogen and three levels of phosphorus, there was a widerange of percentage soluble carbohydrate (TSC) in the roots.The significant positive correlations between percentage ofTSC and root weight suggest that root growth-rate was proportionalto the concentration of TSC in the roots. In other experiments with Lolium perenne grown at five constantsoil temperatures and factorial combinations with three soilmoisture treatments with and without manure, there was declinein the percentage of TSC with increasing temperature. This impliedthat the respiration sink strength in the roots did not controlthe partitioning of photosynthate to the roots. No firm conclusions can be drawn from this work, but it is inferredthat there is a dynamic balancing mechanism in the foliage whichcontrols the partitioning of photosynthate between foliage androot growth. This mechanism appears to divert carbohydrate toroots in inverse proportion to root activity.     

The Effect of Clipping on Net Photosynthesis and Dark Respiration Rates of Plants from an Upland Grassland, with Reference to Carbon Partitioning in Festuca ovina

Four co-existing species (Deschampsia flexuosa, Festuca ovina,Juncus squarrosus and Nardus stricta) were subjected to clippingand the net photosynthetic and dark respiration rates were followedafter this treatment for 50 d. Concurrently carbon partitioningin F. ovina plants clipped initially and again at 50 and 100d was examined. An expansion of new leaf lamina was observed with F. ovina,which had a greater net photsynthetic rate per unit leaf areathan unclipped lamina. The remaining leaf lamina (stubble) afterclipping also showed net photosynthetic and dark respirationrates greater than unclipped lamina; these responses were uniqueto F. ovina plants. N. stricta was the only other species toattain a pre-clipping photosynthetic rate within 6 d. Clipped F. ovina plants showed a change in carbon allocationpattern, with a reduction in carbon allocated to roots. ¹⁴Caccumulated in roots and stubble was shown to have a role inregrowth, as was current assimilate via the production of newleaf lamina. Plants initially clipped before exposure to ¹⁴C,redistributed less ¹⁴C to new shoot growth and, therefore, lostless when subsequently clipped. Further redistribution of ¹⁴Ccame from leaf stubble tissue and not at the expense of roots.The variation between species in clipping response are discussedin terms of the implications for coexistence. Carbon partitioning, clipping, gas exchange, grasses     

The Role of Different Daily Irradiations on Shoot Growth and Root/Shoot Ratio in Lucerne (Medicago sativa L.)

The effect of two levels of daily irradiation on shoot growthand root/shoot partitioning was investigated on two lucernevarieties (Medicago sativa L.). Individual plants were studiedunder constant temperature and optimal water and mineral nutritionconditions. For both lucerne varieties, daily irradiation did not changebiomass partitioning between shoots and roots. It can be shownthat leaf area expansion occurs independently of daily irradiationand that the process of shoot dry matter production is strictlyproportional to daily irradiation, since the ratio leaf area/shootdry matter is inversely proportional to it. From a model of shoot production of the isolated plant, we showthat the relative shoot specific activity is not affected bydaily irradiation. A simple model of partitioning leads us todetermine why partitioning remains the same for the two at thelevels of daily irradiation. Finally, the allocation between shoot and root turns out tobe independent of the main stem extension rate, which is fasterat the higher daily irradiation. Medicago sativa L., lucerne, daily irradiation, shoot production, root/shoot ratio, leaf area expansion, partitioning model     

Partitioning of carbon in split root systems of barley: effect of temperature of the root

The time-course of partitioning of recently fixed photosynthateto a barley root was followed during a step change in the temperatureof the root. The induced change in partitioning had a responsetime of about 1 h, whilst the change in respiratory loss fromthe root of labelled and unlabelled CO was much faster. Theseobservations suggest that while a sudden temperature changehas an immediate effect upon the metabolic rate of the rootsit is not the metabolic rate as such which influences import(sink strength). The reported temperature responses are consistentwith the supposition that the size of the cytosolic pool ofsugar in the root, which is supplied directly by phloem importand used as a substrate for root metabolism, appears to controlimport into the root. Key words: Carbon partitioning, temperature, barley, Hordeum vulgare L., roots     

MassFlowDyn I: A Carbon Transport and Partitioning Model for Root System Architecture

Carbon partitioning is important for understanding root developmentbut little is known about its regulation. Existing models suggestthat partitioning is controlled by the potential sink strength.They cannot, however, simulate hierarchical uptake other thanby using absolute priorities. Moreover, they cannot explainthat the changes in photoassimilate partitioning result fromchanges in photosynthesis. In this paper we present a modelof phloem sieve circulation, based on the model of Minchin etal. (Journal of Experimental Botany44: 947–955, 1993).The root system was represented by a network of segments towhich meristems were connected. The properties of the segmentswere determined by the differentiation stage. Photoassimilateimport from each organ was assumed to be limited by a metabolicprocess and driven by Michaelis–Menten kinetics. The axialgrowth was proportional to meristem respiration, which drivesthe flux of new cells required for root elongation. We usedthe model to look at trophic apical dominance, determinate andindeterminate root growth, the effect of the activity of a rooton competition with its neighbours, and the effect of photoassimilateavailability on changes in partitioning. The simulated phloemmass flow yielded results of the same order of magnitude asthose generally reported in the literature. For the main wellvascularized axis, the model predicted that one single apicalmeristem larger than its neighbouring laterals, was enough togenerate a taprooted system. Conversely, when the meristem oflaterals close to the collar had a volume similar to that ofthe taproot, the predicted network became fibrous. The modelpredicted a hierarchical priority for organ photoassimilateuptake, similar to that described in the literature, duringthe decline in photosynthetic activity. Our model suggests thatdeterminate growth of the first laterals resulted from a localshortage of photoassimilate at their meristem, as a result ofthe limited transport properties of the developed roots. Copyright2000 Annals of Botany Company Münch theory, phloem transport model, photoassimilate-partitioning, root growth, root system architecture, translocation     

Dry Matter Partitioning in a Determinate and an Indeterminate Cultivar of Vicia faba L. Under Contrasting Plant Distributions and Densities

Models for root: shoot, vegetative: generative and stem: leafpartitioning are presented to quantify dry matter partitioningof two contrasting genotypes of Vicia faba, an indeterminateand a ‘topless’. A third plant type, a determinateof which two to three inflorescences had been removed, was alsoincluded but behaved similarly to the intact plant. The root:shoot partitioning model predicts linear relationships betweenroot proportion and the product of air vapour pressure deficitand relative growth rate. Data from field experiments were consistentwith model prediction and coefficients estimated were similarfor both genotypes. Partitioning into pods was modelled as proportionalto the number of actively growing pods younger than 1000°Cd. Coefficients estimated were similar in both genotypes butdifferent between densities and years. In the indeterminategenotype, stem: leaf ratio was allometric throughout, whereasleaf growth ceased but stem growth continued in the determinateafter formation of the terminal inflorescence. Relatively moredry matter was allocated to stems than to leaves in high thanin low densities. In conclusion, the main differences in drymatter partitioning between genotypes concern leaf: stem partitioningduring early pod filling and pod partitioning due to pod numberdifferences. Partitioning, root, shoot, leaf, stem, Vicia faba L.     

Modelling the Components of Plant Respiration: Representation and Realism

This paper outlines the different ways in which plant respirationis modelled, with reference to the principles set out in Cannelland Thornley (Annals of Botany85: 55–67, 2000), firstin whole-plant ‘toy’ models, then within ecosystemor crop models using the growth-maintenance paradigm, and finallyrepresenting many component processes within the Hurley Pasture(HPM) and Edinburgh Forest Models (EFM), both of which separateC and N substrates from structure. Whole-plant models can beformulated so that either maintenance or growth respirationtake priority for assimilates, or so that growth respirationis the difference between total respiration and maintenanceassociated with the resynthesis of degraded tissues. All threeschemes can be converted to dynamic models which give similar,reasonable predictions of plant growth and respiration, butall have limiting assumptions and scope. Ecosystem and cropmodels which use the growth-maintenance respiration paradigmwithout separating substrates from structure, implicitly assumethat maintenance respiration is a fixed cost, uncoupled to assimilatesupply, and use fixed rate coefficients chosen from a rangeof measured values. Separation of substrates in the HPM andEFM enables estimates to be made of respiration associated withlocal growth, phloem loading, ammonium and nitrate N uptake,nitrate reduction, N₂fixation and other mineral ion uptake,leaving a ‘residual maintenance’ term. The lattercan be explicitly related to C substrate supply. Simulated changesin grassland respiration over a season and forest respirationover a rotation show that the ratio of total respiration togross canopy photosynthesis varies within the expected limitedrange, that residual maintenance accounts for 46–48% oftotal respiration, growth 36–42%, phloem loading 10–12%and the other components for the small remainder, with the ratiosbetween components varying during a season or forest rotation.It is concluded that the growth-maintenance approach to respiration,extended to represent many of the component processes, has considerablemerit. It can be connected to reality at many points, it givesmore information, it can be examined at the level of assumptionsas well as at the level of predictions, and it is open to modificationas more knowledge emerges. However, currently, there are stillparameters that require adjustment so that the predictions ofthe model are acceptable. Copyright 2000 Annals of Botany Company Respiration, photosynthesis, growth, maintenance, substrate, N uptake, mineral uptake, phloem loading, model.     

Growth and partitioning of C and fixed N in the shrub legume Acacia littorea in the presence or absence of the root hemiparasite Olax phyllanthi

Nodulated plants of Acacia littorea were pot cultured singlyin minus nitrogen sand culture in the presence or absence ofa transplanted seedling of the root hemiparasite Olax phyllanthiand harvests of cultures made 4 and 8 months after introducingthe parasite. Parasitism decreased host shoot growth while increasingroot growth to a similar extent. Final shoot:root dry weightratio was 2.2 for parasitized versus 4.3 for unparasitized Acacia.Partitioning of fixed N showed 4-fold larger N increments inshoots than roots of unparasitized plants, whereas parasitizedplants lost a small amount of shoot N, made a root gain of Ndouble that of unparasitized plants and lost over half of theirN to Olax. The increment of fixed N in the host:parasite associationwas similar to that of unparasitized Acacia. Data on dry mattergain per unit foliage area and mean CO₂ assimilation rates pershoot of Olax and Acacia (parasitized or unparasitized) werediscussed in relation to an estimated heterotrophic gain ofxylem C from the host equivalent to 40% of the increment ofdry matter C made by the parasite. Growth of Olax was accompaniedby large increases in numbers of haustoria, 9% of which wereattached to root nodules as opposed to roots. Structural andnutritional features of direct parasitism of nodules are described.Models of flow and utilization of C and N in the Acacia:Olaxassociation and unparasitized Acacia are discussed in relationto published data for other host:parasite associations. Key words: Olax phyllanthi, host-parasite relationships, C and N partitioning, Acacia, N₂ fixation