Does ABA in the Xylem Control the Rate of Leaf Growth in Soil-Dried Maize and Sunflower Plants?

Maize (Zea mays L.) and sunflower (Helianthus annuus L.) plantswere grown in large volumes of soil and leaf growth rate wasmonitored on a daily basis. Half the plants were given a soildrying treatment and when they showed a significant restrictionof growth rate (compared to both their daily growth rate beforedrying and the average growth rate of well-watered plants onthe same day), leaf water relations were measured and xylemsap was extracted using several techniques. There was a significant negative log-linear relationship betweenthe rate of leaf growth and the concentration of ABA in thexylem for both species. There was no clear relationship betweenleaf growth rate and leaf water potential or turgor for eitherspecies. Assessment of different methods for sampling xylemsap suggests that exudates collected from stem stumps or samplescollected by pressurizing the whole root system are suitablefor estimating ABA concentration in xylem, at least with largeplants of maize or sunflower, provided the first few hundredcubic millimetres of collected sap are used for the assay. Centrifugationof sections of stems resulted in dilution of ABA in the xylemsap with sap squeezed from parenchyma tissue. This is because,at least in plants subjected to mild soil drying, the concentrationof the ABA in the xylem is far higher than that in the cellsap of stem tissue. Results support the proposal that ABA plays a major role asa chemical signal involved in the root-to-shoot communicationof the effects of soil drying. The non-hydraulic restrictionof leaf growth by a chemical signal can be explained by theextra root-sourced ABA in the xylem and may be an importantcomponent of the modification of growth and development whichresults from prolonged soil drought. Key words: Soil drying, ABA, leaf growth, Zea mays L., Helianthus annuus L.     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant II. Water relations, stomatal conductance, abscisic acid content in leaves and xylem sap of plants subjected to water deficiency

The response of w-1, a wilty sunflower (Helianthus annuus L.)mutant, to water stress is described in comparison with thecontrol line (W-1). Detached leaves of w-1 strongly dehydratedduring the first 30 min without significant changes in leafconductance, whereas W-1 responded rapidly to water loss byreducing stomatal aperture. After 2 h stress ABA increased slightlyin w-1, while W-1 leaves showed a 20-fold increase. When waterstress was imposed to potted plants by water withholding, w-1quickly dehydrated, and lost turgor, while W-1 maintained positiveturgor values for a longer period. Wild-type plants respondedto small changes in leaf water potential by accumulating ABAand by closing stomata, whereas in the mutant significant changesin ABA content and in stomatal conductance were found only atvery low water potentials. In another experiment in which waterwas withheld under high relative humidity, when soil water contentstarted to decrease W-1 rapidly closed stomata in the absenceof any change in leaf water status and the reduction in conductancewas paralleled by a rise in xylem sap ABA concentration. Bycontrast the mutant started to accumulate ABA in the xylem sapand to close stomata when soil water content and leaf waterpotential were dramatically reduced. The low endogenous ABAlevels and the inability to synthesize the hormone rapidly eitherin the leaves or in the roots seem to be responsible for thehigh sensitivity of w-1 to water stress. Key words: ABA, Helianthus annuus L, water relations, stomatal conductance, drought, wilty mutant     

Delivery rates of abscisic acid in xylem sap of Ricinus communis L. plants subjected to part-drying of the soil

Abscisic acid (ABA) moving from roots to shoots in the transpirationstream is a potential hormonal message integrating perceptionof a root stress with adaptive changes in the shoot. A twinroot system was used to study ways of estimating effects ofdroughting the upper roots of Ricinus communis L. on abscisicacid (ABA) transport to the shoot in the transpiration stream.Droughted plants transpired more slowly than controls. Droughtingalso increased concentrations of ABA up to I I-fold in sap inducedto flow from the roots of freshly decapitated plants at ratesof whole plant transpiration. However, because of dilution effectsarising from the different sap flows in control and droughtedplants, these changes in ABA concentration in the xylem sapdid not accurately reflect amounts of ABA transported. To overcomethis problem, delivery rates were calculated by multiplyingconcentration with sap flow rate to generate ABA delivery interms of µmol s–1 per plant. Droughting for 24 hor more increased ABA delivery from roots to shoots by 5-fold.Since droughting can alter the relative sizes of the roots andshoots and also the root:shoot ratio these delivery rates wererefined in several ways to reflect both the amount of root generatingthe ABA message and the size of the recipient shoot system. Key words: Abscisic acid, Ricinus communis L., soil drying, xylem sap     

The significance of the Presence of Stomata on Seedling Roots

Slightly squashed and transversely sectioned sprouts (seedlingroot + hypocotyl to cotyledons) of Pisum arvense, Ornithopussativus and Helianthus annuus were examined. The presence ofstomata is described on seedling roots, including the root hairzone, in Pisum and Ornithopus and in the root hair zone of thehypocotyl of Helianthus. The stomata found in the root hairzones are almost always open, usually without chloroplasts andare not sensitive to the action of abscisic acid (ABA). TheABA sensitivity of the stomata appears first above the roothair zone and increases gradually towards the cotyledons. Thepossible role of stomata in the root hair zone is discussed. Seedling root, hypocotyl, root hair zone, stomata     

Effect of Mechanical Stress on Sunflower (Helianthus annuus L.) Hypocotyl Growth

Etiolated seedlings of Sunflower (Helianthus annuus L.) weresubjected to mechanical stress by longitudinally compressingthe hypocotyl with approx. 0·05 N force (equivalent to0·025 MPa for a 1·6 mm diameter hypocotyl). Thisrelatively low compressive stress effected an increase in relativegrowth rates (RGR) of the hypocotyl for a period of 1-2 h, followingwhich RGR returned to the pretreatment rate. RGR was also increasedby an equivalent compressive stress treatment (0·025MPa) for 4 h in water or in 10^-10 mol l^-1 IAA. These resultsare discussed in the context of a possible role for mechanicallyinduced stress in the initiation and maintenance of nutationalgrowth movements.Copyright 1994, 1999 Academic Press Helianthus annuus, Sunflower, hypocotyl growth, mechanical stress, seedling growth, nutational growth movements, circumnutation     

Adaptation and Functional Balance Between Shoot and Root Activity of Sunflower Plants Grown at Different Root Temperatures

Helianthus annuus L. plants were grown with the shoots at normalair temperature and with the roots in nutrient solution at 10,20 or 30 °C. The higher the root temperature the greaterthe growth of the leaves, resulting in higher production ofphotosynthates. Irrespective of growth conditions an equilibriumwas established between the maintenance respiratory activityof shoot and roots. A constant proportion of the photosynthateproduced was used in respiration. The results are discussedin relation to a thermodynamic theory of stability of biologicalsystems. It is suggested that changes in energy partition betweenmaintenance and growth, and then in relative growth rates ofshoots and roots during an adaptation period, represent a majorhomeostatic mechanism. shoots, roots, maintenance respiration, growth, relative growth rate, respiration, adaptation, sunflower, Helianthus annuus L.     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

Nutrient-limited growth rates: quantitative benefits of stress responses and some aspects of regulation

Young sunflower plants (Helianthus annuus L.) under stress oflow nitrate or phosphate availability exhibited increases inroot: shoot ratio and in kinetic parameters for uptake. Theyshowed no significant changes in photosynthetic utilizationof either nutrient. Increases in root: shoot ratio were achievedby early and persistent suppression of shoot growth, but notroot growth. Affinity for phosphate uptake, 1/K_m(P), increasedwith phosphate stress, as did affinity for nitrate uptake, 1/K_m(N),with nitrate stress. Maximal uptake rate, V_max, for phosphateuptake increased with phosphorus stress; V_max for nitrate didnot increase with nitrogen stress. Phosphate V_max was relatedstrongly to root nutrient status. Decreases in V_max with plantage were not well explained by changes in age structure of roots.Estimated benefits of acclimatory changes in root: shoot ratioand uptake kinetics ranged up to 2-fold increases in relativegrowth rate, RGR. The relation of RGR to uptake physiology followedpredictions of functional balance moderately well, with somesystematic deviations. Analyses of RGR using growth models implyno significant growth benefit from regulating Vmax, specifically,not from down-regulating it at high nutrient availability. Quantitativebenefits of increases in root: shoot ratio and uptake parametersare predicted to be quite small under common conditions whereinnutrient concentrations are significantly depleted by uptake.The root: shoot response is estimated to confer the smallestbenefit under non-depleting conditions and the largest benefitunder depleting conditions. Even then, the absolute benefitis predicted to be small, possibly excepting the case of heterogeneoussoils. Depleting and non-depleting conditions are addressedwith very different experimental techniques. We note that atheoretical framework is lacking that spans both these cases,other than purely numerical formulations that are not readilyinterpreted. Key words: Nutrient stress, nutrient uptake, nutrient use efficiency, relative growth rate, Helianthus annuus     

Characterisation of Non-Uniform Photosynthesis Induced by Abscisic Acid in Leaves Having Different Mesophyll Anatomies

The effects of abscisic acid (ABA) on photosynthesis in leavesof Helianthus annuus L. were compared with those in leaves ofVicia faba L. After the ABA treatment, the response of photosyntheticCO₂ assimilation rate, A, to calculated intercellular partialpressure of CO₂, P_i, (A(p_i) relationship) was markedly depressedin H. annuus. A less marked depression was also observed inV.faba. However, when the abaxial epidermes were removed fromthese leaves, neither the maximum rate nor the CO₂ responseof photosynthetic oxygen evolution was affected by the applicationof ABA. Starch-iodine tests revealed that photosynthesis was not uniformover the leaves of H. annuus treated with ABA. The starch contentwas diffferent in each bundle sheath extension compartment (thesmallest subdivision of mesophyll by veins with bundle sheathextensions, having an area of ca. 0.25 mm² and ca. 50 stomata).In some compartments, no starch was detected. The distributionof open stomata, examined using the silicone rubber impressiontechniques, was similar to the pattern of starch accumulation.In V.faba leaves, which lack bundle sheath extensions, distributionof starch was more homogeneous. These results indicate that the apparent non-stomatal inhibitionof photosynthesis by ABA deduced from the depression of A(pⁱ)relationship is an artifact which can be attributed to the non-uniformdistribution of transpiration and photosynthesis over the leaf.Intercellular gaseous environment in the ABA-treated leavesis discussed in relation to mesophyll anatomy. ¹ Present address: Department of Botany, Duke University, Durham,NC 27706, U.S.A. (Received September 30, 1987; Accepted January 13, 1988)     

Responses of antioxidant defense system of <Emphasis Type="Italic">Helianthus annuus</Emphasis> to abscisic acid treatment under drought and waterlogging

The effect of abscisic acid (ABA) treatment on growth pigments and antioxidant defense system were investigated in seedlings of Helianthus annuus (cvs. Nantio F1 and Özdemirbey) subjected to drought and waterlogging stress. In addition, seedlings were sprayed with 10 M ABA three times every other day. Relative growth rate (RGR) was significantly reduced in both genotypes under drought stress, however, this growth inhibition was less in ABA-treated plants. Total chlorophyll content increased by drought stress in both genotypes. Ascorbate was not influenced by drought, while α-tocopherol increased in cv. Nantio F1. Ascorbate and α-tocopherol increased with drought stress in cv. Özdemirbey. ABA treatment decreased ascorbate and β-carotene contents while it increased α-tocopherol and xanthophylls contents under drought stress. The activity of superoxide dismutase (SOD) in both genotypes increased under drought stress-ABA combinations. Catalase (CAT) activity decreased under drought stress and drought-ABA combinations while it increased under waterlogging stress. Glutathione reductase (GR) activity decreased under drought stress but recovered with ABA treatment. The results suggested that ABA treatments have different effects on the components of antioxidant defense system in H. annuus genotypes and ABA may contribute drought-induced oxidative stress tolerance but not effects under waterlogging stress.     

Cellular Differentiation and Tissue Partitioning in Caps of Primary and Lateral Roots of Helianthus annuus

Cellular and tissue volumes in caps of primary and lateral rootsof Helianthus annuus have been measured in order to determinequantitatively how tissues and their functions are partitionedin root caps. Patterns of change in cellular dimensions andvolumes are similar in caps of primary and lateral roots. Significantincreases in cellular dimensions and volume occur during thedifferentiation of columella cells and the innermost peripheralcells. There are no significant changes in cellular dimensionsas either (i) the production and secretion of mucilage begins,or (ii) cells are sloughed from the cap. Tissues are partitionedsimilarly in caps of primary and lateral roots. indeed, rootcaps allocate 7–8 per cent of their volume for regeneration(i.e. calyptrogen tissue), 16–19 per cent of their volumefor graviperception (i.e. columella tissue), and approx. 38per cent of their volume for the production and secretion ofmucilage. These results are discussed relative to patterns ofcellular differentiation and tissue function in root caps. Helianthus annuus, root caps, primary root, lateral root, calyptrogen, columella, peripheral cells, tissue partitioning     

The effects of methyl jasmonate on sunflower (Helianthus annuus L.) seed germination and seedling development

Some effects of methyl jasmonate (Me-Ja) on sunflower (Helianthus annuus L.) seed germination and seedling development are described and compared with those of ABA. Both growth regulators have very similar action. They inhibit germination, but high concentrations of O₂ in the atmosphere suppress this inhibitory action. Depending on the concentration, Me-Ja inhibits root and hypcotyl growth, however the root is more sensitive to Me-Ja than to ABA. Me-Ja also strongly reduces oxygen uptake during germination and inhibits chlorophyll biosynthesis in isolated cotyledons.     

Nutrient-Limited Growth Rates: Roles of Nutrient-Use Efficiency and of Adaptations to Increase Uptake Rate

When stressed by low nutrient availability, young sunflowerplants (Helianthus annuus) showed responses seen in many otherspecies: increases in root uptake capacity (V^max, l/K_m), root:shoot ratio, and putative nutrient-use efficiency, nUE=l/(tissuenutrient content). A straightforward mechanistic model is derivedfor relative growth rate (RGR) in solution culture in termsof these factors. A linear regression based on the model indicatesa negative role for nUE, which violates a premise of the model.A revised model proposes that primary adaptations are only inuptake rate and growth or nutrient allocations, and these actthrough the photosynthetic utility of nutrient. The tissue nutrientcontent and associated nUE become dependent quantities. Thepredictions for RGR, as tested by linear regression, are improved.The model predicts that nUE can increase as external solutionconcentration decreases, but decreases with increased uptakeadaptations in one given environment. The decrease in nUE compromisespotential gains in RGR from uptake adaptations, and makes increasesin root: shoot ratio a nearly insignificant contributor to earlyRGR. The model and associated regression analyses are generalizedfor additional adaptations such as increased root fineness andfor different quantitative ways that a nutrient may limit photosynthesis.The model and analyses are also generalized to plant growthin soil and growth without functional balance between root andshoot. Key words: Relative growth rate, Helianthus annuus, nutrient stress, nutrient use efficiency, functional balance     

Studies in the Physiological Action of 2, 2-Dichloropropionic Acid: I.: MECHANISMS CONTROLLING THE INHIBITION OF ROOT ELONGATION

The inter-relationships between time and concentration and thedegree of inhibition of root elongation have been examined forSorghum vulgare, Zea mays, Helianthus annuus, and Pisum sativum.For all species the inhibitory effect is cumulative but thereis a tenfold difference in the concentration required to halvethe elongation of the most sensitive (S. vulgare) and most resistantspecies (P. sativum). From a comparison of the growth of intactsroots and isolated segments, together with estimates of cellnumber, it has been established that the primary effect is tointerfere with meristematic activity in the root tip, wherethe mitotic cycle is arrested at prophase. Using 2, 2-dichloropropionic acid, containing chlorine-36, thecourses of uptake by both intact roots and isolated segmentshave been followes. In every instance uptake is cumulative withthe greatest accumulation in the root tip. There are again largespecific differences but of a reverse order; uptake is leastfor P. sativum and greatest for S. vulgare. For these two speciesand Z. mays, it is concluded that the magnitude of the equi-effectiveconcentration required to halve root elognation is dependenton the level of accumulation rather than on the reaction atcell level: the cells of H. annuus are more sensitive.     

Root and Shoot Growth of Plants Treated with Abscisic Acid

Young seedlings of Capsicum annum L., Commelina communis L.and maize (Zea mays L.) were subjected to a mild water-stressingtreatment and/or treated with abscisic acid (ABA). Plants rootedin soil received a soil-drying treatment and their leaves weresprayed with a 10–⁴ M solution of ABA. Plants grown insolution culture were stressed by the addition of polyethyleneglycol (PEG) to the rooting medium and ABA was also added tothe rooting medium, either with or without PEG. The effectsof both treatments on the growth of roots and shoots and theultimate root: shoot dry weight ratio were very similar. Shootgrowth was limited both by water stress and by ABA application;while there was some evidence that mild water stress and/orABA application may have resulted in a stimulation of root growth.More severe water stress reduced the growth of roots but theoverall effect of stress was to increase the ratio of rootsto shoots. Capsicum annum L., Commelina communis L., Zea mays L., water stress, abscisic acid     

Characterization of a wilty sunflower (Helianthus annuus L.) mutant: I. Abscisic acid content, light-dark changes in the stomatal conductance and genetic analysis

The present study was conducted to characterize geneticallyand physiologically a spontaneous mutation in sunflower whichconfers a wilty phenotype. The wilting condition of the mutantis due to abnormal stomatal behaviour. The mutant stomata resistclosure in darkness. This abnormality is associated with lowlevels of endogenous abscisic acid (ABA). By artificially elevatingthe ABA content of the mutant plants by spray treatments with10^– and 10^– M solutions it proved possible to effecta phenotypic reversion of the mutant. It has, therefore, beenproposed that the primary effect of this spontaneous mutationis to reduce the level of ABA. The genetic analysis has shownthat the willy phenotype is due to recessive nuclear mutationat a single locus. Key words: ABA, Helianthus annuus L, stomatal conductance, wilty mutant     

Relationship Between Root Morphogenesis and Period of Predominant Cell Arrest in Intact and Aseptically-cultured Roots of Helianthus annuus L.

Root morphogenesis and cell cycle kinetics of intact and aseptically-grownexcised roots of Helianthus annuus L. were studied. Intact rootsshow predominant cell arrest in G1 with an absence of polyploidcells coincident with secondary vascularization. Exposure ofthe cut ends of aseptically grown excised roots to known concentrationsof indol-3-yl acetic acid, benzyladenine, and myo-inositol for8 weeks initiated the production of secondary vascular tissuesand predominant cell arrest in G2 concommitant with poiyploidization.Excised roots grown in the absence of these substances producedroots with only primary vascularization and predominant cellarrest in G1 coincident with an absence of polyploidization.These results indicate that (a) root cells of H. annuus havethe ability to undergo polyploidization that may be inducedby exogeneously applied chemicals, (b) a general relationshipbetween predominant cell arrest in G1 coincident with the absenceof secondary vascularization does not hold true and (c) althoughsecondary vascularization occurs in cultured roots exposed toall three additives similar to secondary vascularization inintact roots, the two roots should not be considered identicalin all respects. Helianthus annus L., sunflower, root, morphogenesis, cell cycle kinetics, polyploidy, cell differentiation, vascularization     

Hormonal responses to partial drying of the root system of Helianthus annuus

Plants of Helianthus annuus were pot-grown in soil, with approximately30% of the root system protruding through the base. After 7d, the upper part of the root system of half of the plants wasexposed to drought (internal roots) while the lower part waskept in aerated nutrient solution (protruding root). The treatmentrapidly reduced the internal roots' water content from 26.1to 21.9 g g^–1 dry weight (DW), while in protruding rootsof stressed plants it slowly and continuously decreased from31.9 to 25.2 g g^–1 DW. Leaf water content rapidly decreasedin treated plants from 7.4 to 6.4 g g^–1 DW in the first2d and then reached a plateau. In stressed plants leaf stomatalresistance was significantly higher in the first 3 d while leafwater potential was lower only on the last day. Abscisic acid (ABA) concentration in treated plants increasedsignificantly compared to the controls. In treated internalroots, ABA rose from the first day, reaching a maximum of 1.48±0.49nmol g^–1 DW after 3 d. In treated protruding roots a maximumof 0.99±0.09 nmol g^–1 DW was reached after 1 d.ABA concentration in the xylem sap increased 2 d and 3 d afterthe start of soil drying, with a maximum of 113±12nmoll^–1 during the third day. The ABA rise in the leaves oftreated plants was less significant. Indol-3yl-acetic acid (IAA) concentration in internal rootsof treated plants reached a maximum of 22.54±3.34 nmolg^–1 DW on the third day, then decreased dramatically.The protruding root system of control plants showed a maximumvalue of 16.05±1.77 nmol g^–1 DW on the sixth day. Little difference in cytokinin content of xylem sap was notedbetween control and treated plants. Hormonal variations in different parts of the plant are discussedin relation to drought stress. Key words: Soil drying, roots, ABA, IAA, cytokinins     

A pH Gradient across the Root

A pH gradient across the intact root of Helianthus annuus hasbeen measured using pH microelectrodes. The results indicatea steady increase in pH from epidermis to protoxylem. Thereis evidence for the activity of a proton extrusion pump in someof the cells. The possibility that the pH gradient may be thebasis of centripetal ion transport across the root is discussed.     

Compensatory Roles of Nitrogen Uptake and Photosynthetic N-use Efficiency in Determining Plant Growth Response to Elevated CO2: Evaluation Using a Functional Balance Model

We used a modified functional balance (FB) model to predictgrowth response of Helianthus annuus L. to elevated CO₂. Modelpredictions were evaluated against measurements obtained twiceduring the experiment. There was a good agreement between modelpredictions of relative growth rate (RGR) responses to elevatedCO₂and observations, particularly at the second harvest. Themodel was then used to compare the relative effects of biomassallocation to roots, nitrogen (N) uptake and photosyntheticN-use efficiency (PNUE) in determining plant growth responseto elevated CO₂. The model predicted that a rather substantialincrease in biomass allocation to root growth had little effecton whole plant growth response to elevated CO₂, suggesting thatplasticity in root allocation is relatively unimportant in determininggrowth response. Average N uptake rate at elevated comparedto ambient CO₂was decreased by 21–29%. In contrast, elevatedCO₂increased PNUE by approx. 50% due to a corresponding risein the CO₂-saturation factor for carboxylation at elevated CO₂.The model predicted that the decreased N uptake rate at elevatedCO₂lowered RGR modestly, but this effect was counterbalancedby an increase in PNUE resulting in a positive CO₂effect ongrowth. Increased PNUE may also explain why in many experimentselevated CO₂enhances biomass accumulation despite a significantdrop in tissue nitrogen concentration. The formulation of theFB model as presented here successfully predicted plant growthresponses to elevated CO₂. It also proved effective in resolvingwhich plant properties had the greatest leverage on such responses.Copyright 2000 Annals of Botany Company Elevated CO₂, functional balance model, Helianthus annuus L., N uptake, photosynthetic nitrogen use efficiency, root:shoot ratio