Jasmonic acid treatment to part of the root system is consistent with simulated leaf herbivory, diverting recently assimilated carbon towards untreated roots within an hour

It is known that shoot application of jasmonic acid (JA) leads to an increased carbon export from leaves to stem and roots, and that root treatment with JA inhibits root growth. Using the radioisotope ¹¹C, we measured JA effects on carbon partitioning in sterile, split-root, barley plants. JA applied to one root half reduced carbon partitioning to the JA-treated tissue within minutes, whereas the untreated side showed a corresponding – but slower – increase. This response was not observed when instead of applying JA, the sink strength of one root half was reduced by cooling it: there was no enhanced partitioning to the untreated roots. The slower response in the JA-untreated roots, and the difference between the effect of JA and temperature, suggest that root JA treatment caused transduction of a signal from the treated roots to the shoot, leading to an increase in carbon allocation from the leaves to the untreated root tissue, as was indeed observed 10 min after the shoot application of JA. This supports the hypothesis that the response of some plant species to both leaf and root herbivores may be the diversion of resources to safer locations.     

The shoot controls zeatin riboside export from pea roots. Evidence from the branching mutant rms4

The rms4 mutant of pea ( Pisum sativum L.) was used in grafting studies and cytokinin analyses of the root xylem sap to provide evidence that, at least for pea, the shoot can modify the import of cytokinins from the root. The rms4 mutation, which confers a phenotype with increased branching in the shoot, causes a very substantial decrease (down to 40-fold less) in the concentration of zeatin riboside (ZR) in the xylem sap of the roots. Results from grafts between wild-type (WT) and rms4 plants indicate that the concentration of cytokinins in the xylem sap of the roots is determined almost entirely by the genotype of the shoot. WT scions normalize the cytokinin concentration in the sap of rms4 mutant roots, whereas mutant scions cause WT roots to behave like those of self-grafted mutant plants. The mechanism whereby rms4 shoots of pea cause a down-regulation in the export of cytokinins from the roots is unknown at this time. However, our data provide evidence that the shoot transmits a signal to the roots and thereby controls processes involved in the regulation of cytokinin biosynthesis in the root.     

How does nitrogen availability alter rhizodeposition in Lolium multiflorum Lam. during vegetative growth?

The objective of this work was to determine if the impact of nitrogen (N) on the release of organic carbon (C) into the soil by roots (rhizodeposition) correlated with the effect of this nutrient on some variables of plant growth. Lolium multiflorum Lam. was grown at two levels of N supply, either in sterile sand percolated with nutrient solution or in non-sterile soil. The axenic sand systems allowed continuous quantification of rhizodeposition and accurate analysis of root morphology whilst the soil microcosms allowed the study of ¹⁴C labelled C flows in physico-chemical and biological conditions relevant to natural soils. In the axenic sand cultures, enhanced N supply strongly increased the plant biomass, the plant N content and the shoot to root ratio. N supply altered the root morphology by increasing the root surface area and the density of apices, both being significantly positively correlated with the rate of organic C release by plant roots before sampling. This observation is consistent with the production of mucilage by root tips and with mechanisms of root exudation reported previously in the literature, i.e. the passive diffusion of roots solutes along the root with increased rate behind the root apex. We proposed a model of root net exudation, based on the number of root apices and on root soluble C that explained 60% of the variability in the rate of C release from roots at harvest. The effects of N on plant growth were less marked in soil, probably related to the relatively high supply of N from non-fertiliser soil-sources. N fertilization increased the shoot N concentration of the plants and the shoot to root ratio. Increased N supply decreased the partitioning of ¹⁴C to roots. In parallel, N fertilisation increased the root soluble ¹⁴C and the ¹⁴C recovered in the soil per unit of root biomass, suggesting a stimulation of root exudation by N supply. However, due to the high concentration of N in our unfertilised plants, this stimulation was assumed to be very weak because no significant effect of N was observed on the microbial C and on the bacterial abundance in the rhizosphere. Considering the difficulties in evaluating rhizodeposition in non sterile soil, it is suggested that the root soluble C, the root surface area and the root apex density are additional relevant variables that should be useful to measure along with the variables that are commonly determined when investigating how plant functioning impacts on the release of C by roots (i.e soil C, C of the microbial biomass, rhizosphere respiration).     

The Influence of Shoots, Roots, and Hormones on Sucrose Distribution

The effects of hormones on sucrose distribution were studiedin relation to the control of the relative growth of the shootand the root. The roots and the shoots above the cotyledonsof bean seedlings were removed and the remaining explants allowedto regenerate so that they consisted of growing buds, a segmentof the hypocotyl, and short growing roots. [¹⁴C]Sucrose wasapplied to the middle of the hypocotyl and its distributionwas measured in relation to the presence of the shoots and rootsand to their replacement by auxin and a cytokinin. It was foundthat radioactivity moved towards both the shoot and the root,and that removing one growing region greatly reduced the transportin its direction and increased the relative transport in theopposite direction. Both hormones restored a large part of themovement of radioactive sucrose when they replaced any one ofthe growing regions. However, there was some preferential actionof benzyladenine when it replaced the buds and of auxin whenit replaced the roots. The results indicate that in the intactplant the development of the shoot and the root is correlatedby a positive feedback involving auxin and cytokinins, and themovement of sucrose is controlled indirectly by the sink activityof these growing regions. This control of the movement of sucrose,and presumably other metabolites, accounts for the short-termcompetitive relations between the shoot and the root.     

Suboptimal temperature favors reserve formation in biennial carrot (Daucus carota) plants

In response to suboptimal temperatures, temperate annual plants often increase root:shoot ratios, build-up carbohydrates and display typical morphological and anatomical changes. We know less about the responses of biennials such as carrot. As a model plant, carrot has the additional feature of two functionally and morphologically distinct root parts: the taproot, which stores carbohydrate and other compounds, and the fibrous root system involved in acquisition of water and nutrients. Here, we analyze the effects of temperature (12 vs 25°C) on growth, carbohydrate accumulation and whole-plant morphology in two carrot cultivars. Our working hypothesis is that suboptimal temperature favors active formation of reserve structures, rather than passive accumulation of storage carbohydrates. In comparison with plants grown at 25°C, plants grown at 12°C had: (1) higher fibrous root:shoot ratio (13%) , (2) thicker (10–15%) and smaller (up to two- to three-fold) leaves, (3) lower leaf cuticular permeance (two- to four-fold), (4) higher taproot:shoot ratio (two-fold), (5) higher phloem:xylem ratios in taproot (two- to six-fold), (6) unchanged percentage dry matter content (%DMC) in leaves, petioles or fibrous roots and (7) higher %DMC in taproot (20%). However, %DMC of individual taproot tissues (phloem and xylem) was unaffected by temperatures and was consistently higher in the phloem (up to 30%). Therefore, the higher %DMC of whole taproots at 12°C was attributed solely to the increased development of phloem tissue. Carrot, therefore, shares many of the most conspicuous elements of temperate plant responses to low temperatures. Consistently with our hypothesis, however, carrots grown at suboptimal temperature promoted reserve structures, rather than the increase in carbohydrate concentration typical of most temperate annual species and woody perennials.     

Carbon allocation to shoots and roots in relation to nitrogen supply is mediated by cytokinins and sucrose: Opinion

In this paper we firstly show some general responses of biomass partitioning upon nitrogen deprivation. Secondly, these responses are explained in terms of allocation of carbon and nitrogen, photosynthesis and respiration, using a simulation model. Thirdly, we present a hypothesis for the regulation of biomass partitioning to shoots and roots.Shortly after nitrogen deprivation, the relative growth rate (RGR) of the roots generally increases and thereafter decreases, whereas that of the shoot decreases immediately. The increased RGR of the root and decreased RGR of the shoot shortly after a reduction in the nitrogen supply, cause the root weight ratio (root weight per unit plant weight) to increase rapidly.We showed previously that allocation of carbon and nitrogen to shoots and roots can satisfactorily be described as a function of the internal organic plant nitrogen concentration. Using these functions in a simulation model, we analyzed why the relative growth rate of the roots increases shortly after a reduction in nitrogen supply. The model predicts that upon nitrogen deprivation, the plant nitrogen concentration and the rate of photosynthesis per unit plant weight rapidly decrease, and the allocation of recently assimilated carbon and nitrogen to roots rapidly increases. Simulations show that the increased relative growth rate of the root upon nitrogen deprivation is explained by decreased use of carbon for root respiration, due to decreased carbon costs for nitrogen uptake. The stimulation of the relative growth rate of the root is further amplified by the increased allocation of carbon and nitrogen to roots. Using the simple relation between the plant nitrogen concentration and allocation, the model describes plant responses quite realistically.Based on information in the literature and on our own experiments we hypothesize that allocation of carbon is mediated by sucrose and cytokinins. We propose that nitrogen deprivation leads to a reduced cytokinin production, a decreased rate of cytokinin export from the roots to the shoot, and decreased cytokinin concentrations. A reduced cytokinin concentration in the shoot represses cell division in leaves, whereas a low cytokinin concentration in roots neutralizes the inhibitory effect of cytokinins on cell division. A reduced rate of cell division in the leaves leads to a reduced unloading of sucrose from the phloem into the expanding cells. Consequently, the sucrose concentration in the phloem nearby the expanding cells increases, leading to an increase in turgor pressure in the phloem nearby the leaf's division zone. In the roots, cell division continues and no accumulation of sugars occurs in dividing cells, leading to only marginal changes in osmotic potential and turgor pressure in the phloem nearby the root's cell division zone. These changes in turgor pressure in the phloem of roots and sink leaves affect the turgor pressure gradients between source leaf-sink leaf and source leaf-root in such a way that relatively more carbohydrates are exported to the roots. As a consequence RWR increases after nitrogen deprivation. This hypothesis also explains the strong relationship between allocation and the plant nitrogen status.     

Counteractive biomass allocation responses to drought and damage in the perennial herb Convolvulus demissus

Herbivory and water shortage are key ecological factors affecting plant performance. While plant compensatory responses to herbivory include reallocation of biomass from below‐ground to above‐ground structures, plant responses to reduced soil moisture involve increased biomass allocation to roots and a reduction in the number and size of leaves. In a greenhouse study we evaluated the effects of experimental drought and leaf damage on biomass allocation in Convolvulus demissus (Convolvulaceae), a perennial herb distributed in central Chile, where it experiences summer drought typical of Mediterranean ecosystems and defoliation by leaf beetles and livestock. The number of leaves and internode length were unaffected by the experimental treatments. The rest of plant traits showed interaction of effects. We detected that drought counteracted some plant responses to damage. Thus, only in the control watering environment was it observed that damaged plants produced more stems, even after correcting for main stem length (index of architecture). In the cases of shoot : root ratio, relative shoot biomass and relative root biomass we found that the damage treatment counteracted plant responses to drought. Thus, while undamaged plants under water shortage showed a significant increase in root relative biomass and a significant reduction in both shoot : root ratio and relative shoot biomass, none of these responses to drought was observed in damaged plants. Total plant biomass increased in response to simulated herbivory, apparently due to greater shoot size, and in response to drought, presumably due to greater root size. However, damaged plants under experimental drought had the same total biomass as control plants. Overall, our results showed counteractive biomass allocation responses to drought and damage in C. demissus. Further research must address the fitness consequences under field conditions of the patterns found. This would be of particular importance because both current and expected climatic trends for central Chile indicate increased aridity.     

Reversal of the Direction of Photosynthate Allocation in Urtica dioica L. Plants by Increasing Cytokinin Import into the Shoot

The natural cytokinin import from the root into the shoot of Urtica dioica plants was enhanced by supplying zeatin riboside (ZR) solutions of various concentrations to a portion less than 10 % of the root system after removal of their tips. After 6 h ZR pretreatment of the plants, ¹⁴CO₂ was supplied for 3 h to a mature (source) leaf or to an expanding leaf and the ¹⁴C-distribution in the whole plant was determined after a subsequent dark period of 14 h. ZR substantially increased ¹⁴C fixation by the expanding leaves and also enhanced export of carbon and transport to the shoot apex. The effect of the hormone treatment was, however, more pronounced when the ¹⁴CO₂ was supplied to a mature leaf. In the control plants these leaves exported carbon only to the roots: When the amount of the natural daily ZR input from the roots to the shoot was enhanced by 20%, the bulk of the ¹⁴C exported from a mature leaf moved to the shoot apex and only a minor portion of ¹⁴C was still detected in the root fraction. A several-fold increase of the natural daily ZR input into the shoot resulted in a flow of ¹⁴C only to the growing parts of the shoot. The results suggest control of the sink strength of the shoot apex by ZR in Urtica diocia.     

Low-temperature pretreatment of the root system of Brassica rapa L. plants: effects on the xylem sap exudation and on the nitrate absorption rate

Brassica rapa plants were exposed for a 52 h period (as pretreatment) to a differential temperature (DT) between roots (5°C) and shoots (20°C), while control plants were maintained with both shoot and roots at 20°C (warm grown = WG). Measured at 20°C, volume flow of xylem exudate from roots of DT plants was enhanced compared with that from WG plants, while transpiration flows were similar in pretreated and control plants. Both transpiration and exudation flows were dependent upon shoot/root ratio. Differences in the volume flow of exudate were principally related to increases in root hydraulic conductance. Anion fluxes (notably nitrate) into xylem exudate of DT plants were significantly greater than those into exudate of WG plants. This enhancement of nitrate flow from the pretreated roots was associated with a two-fold increase in nitrate uptake rate. The relationship of the cold-induced change in nitrate uptake capacity with shoot/root ratio is discussed in terms of control of nitrate absorption by shoot sink strength.     

树木根系碳分配格局及其影响因子

根系作为树木提供养分和水分的“源”和消耗C的“汇”,在陆地生态系统C平衡研究中具有重要的理论意义。尽管20多年来的研究已经认识到根系消耗净初级生产力占总净初级生产力较大的比例,但是,根系（尤其是细根）消耗C的机理以及C分配的去向一直没有研究清楚。主要原因是细根消耗光合产物的生理生态过程相当复杂,准确估计各个组分消耗的C具有很大的不确定性,常常受树种和环境空间和时间异质性、以及研究方法的限制。综述了分配到地下的C主要去向,即细根生产和周转、呼吸及养分吸收与同化、分泌有机物、土壤植食动物,及有关林木地下碳分配机理的几种假说,分析了地下碳分配估计中存在的不确定性。目的是在全球变化C循环研究中对生态系统地下部分根系消耗的C以及分配格局引起重视。     

Carbon Partitioning in Split Root Systems of Barley: Relation to Metabolism

We tested four hypotheses for the control of partitioning ofphotoassimilated C-11 between the two halves of split root systemsof young barley plants. Our data supported the hypothesis thatphloem is unloaded without the use of metabolic energy, sinceseveral metabolic inhibitors applied to one half of a splitroot system reduced respiratory oxygen uptake without alteringimport of C-11. The hypothesis that rate of import C-11 is directlyrelated to metabolic activity in the root was rejected, since(a) certain inhibitors reduced respiration but not import and(b) exogenous sucrose reduced import into the root half to whichit was supplied. Our data were consistent with the hypothesisthat import is related to the total ability (metabolism plusstorage) of the sink to use sucrose. Treatments that would haveled to greatly decreased use of sucrose (iodoacetate inhibition)decreased import before those which would have led to a smallerdecrease in sucrose use (FCCP inhibition). These data, and thereduction in import to a root half supplied exogenously withsucrose, supported the hypothesis that the size of soluble sugarpools within the roots is, in the short-term, inversely proportionalto rate of import, the soluble sugar pools thus acting as amediator between rate of sucrose use and supply from the phloem.     

Contribution of adventitious vs initial roots to growth and physiology of black spruce seedlings

Black spruce (Picea mariana [Mill.] BSP) is a boreal tree species characterized by the formation of an adventitious root system. Unlike initial roots from seed germination, adventitious roots gradually appear above the root collar, until they constitute most of mature black spruce root system. Little is known about the physiological role they play and their influence on tree growth relative to initial roots. We hypothesized that adventitious roots present an advantage over initial roots in acquiring water and nutrients. To test this hypothesis, the absorptive capacities of the two root systems were explored in a controlled environment during one growing season. Black spruce seedlings were placed in a double‐pot system allowing irrigation (25 and 100% water container capacity) and fertilization (with or without fertilizer) inputs independent to initial and adventitious roots. After 14 weeks, growth parameters (height, diameter, biomass), physiology (net photosynthetic rate, stomatal conductance, shoot water potential) and nutrient content (N, P, K, Ca and Mg foliar content) were compared. Most measured parameters showed no difference for the same treatment on adventitious or initial roots, except for root biomass. Indeed, fertilized black spruce seedlings invested heavily in adventitious root production, twice as much as initial roots. This was also the case when adventitious roots alone were irrigated, while seedlings with adventitious roots subjected to low irrigation produced initial root biomass equivalent to that of adventitious roots. We conclude that black spruce seedlings perform equally well through adventitious and initial roots, but if resources are abundant, they strongly promote development of adventitious roots.     

The role of adenine in the synthesis of cytokinins in tomato plants and in cell-free root extracts

The incorporation of labelled adenine into cytokinin-like compounds was investigated in intact tomato plants, decapitated tomato roots and cell-free root extracts. In all three cases evidence was found for the incorporation of adenine into endogenous cytokinins. In intact plants and decapitated root systems no evidence was found for the incorporation into cytokinin nucleotides. Cytokinin bases and nucleosides were however, labelled. In the cell-free root extract there was some evidence for the incorporation of labelled adenine into cytokinin nucleotides. This suggests that the biosynthetic process may be strictly compartmentalized. The present results provide no evidence for the relative importance of cytokinin nucleotides in the biosynthetic process. What is clear is that the rate of adenine incorporation into cytokinins is extremely low and that only a small proportion of the cytokinins which became labelled were exported to the shoot via the root exudate.The financial support of the CSIR/Israel Collaborative Programme is gratefully acknowledged.     

Inhibition of Auxin Movement from the Shoot into the Root Inhibits Lateral Root Development in Arabidopsis

In roots two distinct polar movements of auxin have been reported that may control different developmental and growth events. To test the hypothesis that auxin derived from the shoot and transported toward the root controls lateral root development, the two polarities of auxin transport were uncoupled in Arabidopsis. Local application of the auxin-transport inhibitor naphthylphthalamic acid (NPA) at the root-shoot junction decreased the number and density of lateral roots and reduced the free indoleacetic acid (IAA) levels in the root and [³H]IAA transport into the root. Application of NPA to the basal half of or at several positions along the root only reduced lateral root density in regions that were in contact with NPA or in regions apical to the site of application. Lateral root development was restored by application of IAA apical to NPA application. Lateral root development in Arabidopsis roots was also inhibited by excision of the shoot or dark growth and this inhibition was reversible by IAA. Together, these results are consistent with auxin transport from the shoot into the root controlling lateral root development.     

Effects of phosphorus availability and vesicular–arbuscular mycorrhizas on the carbon budget of common bean (Phaseolus vulgaris)

Low phosphorus availability is often a primary constraint to plant productivity in native soils. Here we test the hypothesis that root carbon costs are a primary limitation to plant growth in low P soils by assessing the effect of P availability and mycorrhizal infection on whole plant C budgets in common bean ( Phaseolus vulgaris L.). Plants were grown in solid-phase-buffered silica sand providing a constant supply of low (1 μ m ) or moderate (10 μ m ) P. Carbon budgets were determined weekly during the vegetative growth phase. Mycorrhizal infection in low-P plants increased the root specific P absorption rate, but a concurrent increase in root respiration consumed the increased net C gain resulting from greater P uptake. The energy content of mycorrhizal and non-mycorrhizal roots was similar. We propose that the increase in root respiration in mycorrhizal roots was mainly due to increased maintenance and growth respiration of the fungal tissue. Plants grown with low P availability expended a significantly larger fraction of their total daily C budget on below-ground respiration at days 21, 28 and 35 after planting (29–40%) compared with plants grown with moderate P supply (18–25%). Relatively greater below-ground respiration in low P plants was mainly a result of their increased root:shoot ratio, although specific assimilation rate was reduced significantly at days 21 and 28 after planting. Specific root respiration was reduced over time by low P availability, by up to 40%. This reduction in specific root respiration was due to a reduction in ion uptake respiration and growth respiration, whereas maintenance respiration was increased in low-P plants. Our results support the hypothesis that root C costs are a primary limitation to plant growth in low-P soils.     

Growth and chemical defense in willow seedlings: trade-offs are transient

Many studies have failed to detect costs of defense and some have even found a positive correlation between growth and the concentrations of chemical defenses. These studies contradict the theoretical assumption that anti-herbivore defenses are costly—produced at the expense of growth and/or reproduction. Costs, however, may be transient and therefore difficult to detect. Here we tested the hypothesis that costs of defense would be pronounced early in development when root growth is prioritized (high percent root allocation), but not later in development. To test this hypothesis, we grew F₂ hybrid willow seedlings from five different families, and harvested cohorts of even-aged seedlings after 6, 7, 8 and 9 weeks of growth. Seedlings were divided into root and shoot tissue and shoots were analyzed for phenolics (condensed tannins and phenolic glycosides). We found evidence for transient costs of defense. The concentrations of phenolics were negatively correlated with total biomass, shoot biomass, and the proportion of biomass allocated to roots in week 6. After week 6, however, the concentrations of phenolics were positively correlated with shoot biomass and total biomass, while phenolics were uncorrelated with the proportion of biomass allocated to roots. These results, the first ever, to our knowledge, with woody plants, suggest that costs of defense were transient; specifically, costs were found in early development, when root establishment was a priority. Our findings suggest that studies should focus more on trade-offs early in plant development.     

The effect of photoperiod and temperature on growth and frost resistance of winter rye root systems

Root growth, development and frost resistance were examined in winter rye ( Secale cereale L. cv. Puma) plants grown under 6 combinations of temperature and photoperiod (20/16°C or 5/3°C, day/night; 8, 16- or 24-h days). Overall root system growth is influenced by the interaction of temperature and photoperiod. Maximum shoot growth occurs at a 24-h photoperiod in 20°C plants and at a 16-h photoperiod in 5°C plants, and is correlated in both treatments with a high root:shoot ratio. Frost resistance of rye roots is affected by short photoperiods in 2 ways. First, short photoperiod and low temperature delay production of new adventitious roots so that newly developing roots are not exposed to freezing temperatures. Second, short photoperiod alone can induce several degrees of frost tolerance in existing roots during the lag phase of growth. Low temperature alone does not decrease the rate of dry weight accumulation in rye root systems, but cold temperature does retard developmental processes within the roots. Rye roots grown at 5°C develop first order lateral roots, differentiate metaxylem vessels and suberize endodermal cell walls more slowly than roots grown at 20°C.     

Effect of phosphorus supply on the formation and function of proteoid roots of white lupin (Lupinus albus L.)

We investigated (1) the effect of constant and altered inorganic phosphate (P_i) supply (1–100 mmol m^–3) on proteoid root production by white lupin ( Lupinus albus L.); and (2) the variation in citrate efflux, enzyme activity and phosphate uptake along the proteoid root axis in solution culture. Proteoid root formation was greatest at P_i solution concentrations of 1–10 mmol m^–3 and was suppressed at 25 mmol m^–3 P_i and higher. Except at 1 mmol m^–3 P_i, the formation of proteoid roots did not affect plant dry matter yields or shoot to root dry matter ratios, indicating that proteoid roots can form under conditions of adequate P supply and not at the expense of dry matter production. Plants with over 50% of the root system as proteoid roots had tissue P concentrations considered adequate for maximum growth, providing additional evidence that proteoid roots can form on P-sufficient plants. There was an inverse relationship between the P_i concentration in the youngest mature leaf and proteoid root formation. Citrate efflux and the activities of enzymes associated with citric acid synthesis (phosphoenolpyruvate carboxylase and malate dehydrogenase) varied along the proteoid root axis, being greatest in young proteoid rootlets of the 1–3 cm region from the root tip. Citrate release from the 0–1 and 5–9 cm regions of the proteoid root was only 7% (per unit root length) of that from the 1–3 cm segment. Electrical potential and ³²P_i uptake measurements showed that P_i uptake was more uniform along the proteoid root than citrate efflux.     

Controls of biomass partitioning between roots and shoots: Atmospheric CO2 enrichment and the acquisition and allocation of carbon and nitrogen in wild radish

Summary The effects of CO₂ enrichment on plant growth, carbon and nitrogen acquisition and resource allocation were investigated in order to examine several hypotheses about the mechanisms that govern dry matter partitioning between shoots and roots. Wild radish plants (Raphanus sativus × raphanistrum) were grown for 25 d under three different atmospheric CO₂ concentrations (200 ppm, 330 ppm and 600 ppm) with a stable hydroponic 150 mol 1^–1 nitrate supply. Radish biomass accumulation, photosynthetic rate, water use efficiency, nitrogen per unit leaf area, and starch and soluble sugar levels in leaves increased with increasing atmospheric CO₂ concentration, whereas specific leaf area and nitrogen concentration of leaves significantly decreased. Despite substantial changes in radish growth, resource acquisition and resource partitioning, the rate at which leaves accumulated starch over the course of the light period and the partitioning of biomass between roots and shoots were not affected by CO₂ treatment. This phenomenon was consistent with the hypothesis that root/shoot partitioning is related to the daily rate of starch accumulation by leaves during the photoperiod, but is inconsistent with hypotheses suggesting that root/shoot partitioning is controlled by some aspect of plant C/N balance.