The Growth of the Primary Roots and Root Hairs of Sinapis alba and Lepidium sativus in Triton X-100

Seeds of Lepidium and Sinapis were germinated and grown for 3 days in different concentrations of Triton X-100 (0.001–0.1 % v/v). The elongation of the primary root was slightly stimulated by low concentrations. In concentrations above 0.01 %, Triton inhibited root growth and forked root hairs developed. The hairs elongated both at the apex and at the base, exhibited protoplasmic streaming and activity of particulate non-specific esterase. In contrast the growth of the hypocotyl of both Lepidium and Sinapis diminished steadily in increasing concentrations of Triton. Triton also affected the percentage germination of Lepidium, which increased or decreased according to the concentration used. The changes in root growth and germination and the appearance of branched root hairs in abundance coincide with a change in the detergent solution from monomer to aggregated molecules.     

The Cytosolic Ca2+ Concentration Gradient of Sinapis alba Root Hairs as Revealed by Ca2+-Selective Microelectrode Tests and Fura-Dextran Ratio Imaging

Using Ca2+-selective microelectrodes and fura 2-dextran ratio imaging, the cytosolic free [Ca2+] was measured in Sinapis alba root hair cells. Both methods yielded comparable results, i.e. values between 158 to 251 nM for the basal [Ca2+] of the cells and an elevated [Ca2+] of 446 to 707 nM in the tip region. The zone of elevated [Ca2+] reaches 40 to 60 [mu]m into the cell and is congruent with the region of inwardly directed Ca2+ net currents measured with an external Ca2+- selective vibrating electrode. The channel-blocker La3+ eliminates these currents, stops growth, and almost completely eliminates the cytosolic [Ca2+] gradient without affecting the basal level of the ion. Growth is also inhibited by pressure-injected dibromo-1,2-bis(o-aminophenoxy)ethane-N,N,N[prime],N[prime]-tetraacetic acid, which causes a decrease in the [Ca2+] in the tip in a concentration-dependent manner. Indole-3-acetic acid, used as a model stimulus, decreases cytosolic free [Ca2+] by 0.2 to 0.3 pCa units in the tip, but only by about 0.1 pCa unit in the shank. Nongrowing root hairs may or may not display a [Ca2+] gradient, but still reversibly respond to external stimuli such as La3+, Ca2+, or indole-3-acetic acid with changes in cytosolic free [Ca2+]. During short time periods, dicyclohexylcarbodiimide inhibition of the plasma membrane H+-ATPase, which stops growth, does not abolish the [Ca2+] gradient, nor does it change significantly the basal [Ca2+] level. We conclude that the cytosolic [Ca2+] gradient and an elevated [Ca2+] in the tip, as in other tip-growing cells, is essential for tip growth in root hairs; however, its presence does not indicate growth under all circumstances. We argue that with respect to Ca2+, tip growth regulation and responses to external signals may not interfere with each other. Finally, we suggest that the combination of the methods applied adds considerably to our understanding of the role of cytosolic free [Ca2+] in signal transduction and cellular growth.     

The Control of Root Initiation and Development in Detached Cotyledons of Sinapis alba L. and Raphanus sativus L.

The initiation and development of root primordia in detachedcotyledons of Sinapis alba (white mustard) and Raphanus sativus(radish) are studied, together with the inhibitory effects ofsucrose and mineral nutrients on these processes. Root primordium initiation on petioles of excised mustard cotyledonscultured in petridishes in water commenced after 3 days andwas completed after 5 days in culture, by which time a numberof the primordia had extended and emerged from the petiole.Both sucrose and mineral nutrient solution had an inhibitoryeffect which was most marked when the cotyledonswere culturedin the solution from the time of excision. The total numberof primordia initiated, their rate of development, and the finaltotal number of emerged roots were all reduced. The later thetime of transfer from water either to sucrose or to nutrient,the less marked the inhibition. Indeed, nutrient solution enhancedroot growth in mustard when cotyledons were transferred after5 days in water when root emergence had just commenced. The effects of sucrose and nutrients in relation to applicationbefore and after initial meristem formation has taken placeare discussed, together with the ways in which these two solutionsmay exert their effect on root initiation and development.     

Control of Seed Germination by Abscisic Acid: I. Time Course of Action in Sinapis alba L

The germination process of mustard seeds (Sinapis alba L.) has been characterized by the time courses of water uptake, rupturing of the seed coat (12 hours after sowing), onset of axis growth (18 hours after sowing), and the point of no return, where the seeds lose the ability to survive redesiccation (12 to 24 hours after sowing, depending on embryo part). Abscisic acid (ABA) reversibly arrests embryo development at the brink of radicle growth initiation, inhibiting the water uptake which accompanies embryo growth. Seeds which have been kept dormant by ABA for several days will, after removal of the hormone, rapidly take up water and continue the germination process. Seeds which have been preincubated in water lose the sensitivity to be arrested by ABA after about 12 hours after sowing. This escape from ABA-mediated dormancy is not due to an inactivation of the hormone but to a loss of competence to respond to ABA during the course of germination. The sensitivity to ABA can be restored in these seeds by redrying. It is concluded that a primary action of ABA in inhibiting seed germination is the control of water uptake of the embryo tissues rather than the control of DNA, RNA, or protein syntheses.     

Sinalbins A and B, phytoalexins from Sinapis alba: elicitation, isolation, and synthesis

The chemical structure and synthesis of sinalbin A is described. This cruciferous phytoalexin is produced by white mustard (Sinapis alba) after treatment with biotic and abiotic elicitors. In addition, a related metabolite, named sinalbin B, is present in extracts from elicited plants, but not in those from non-elicited controls. Sinalbin B, which was also synthesized, appears to be both a phytoalexin and a biosynthetic precursor of sinalbin A.     

Hypocotyl growth in Sinapis alba L: the roles of light quality and quantity

Abstract. A comparison is made of the relative effectiveness of light quality and light quantity on the elongation growth of Sinapis alba hypocotyls. The results show that hypocotyl extension rate in plants which have not previously been exposed to light is controlled primarily by the prevailing photon fluence rate when the phytochrome photostationary state lies between ∼0.033 and ∼0.81. Below ∼0.033, changes in photostationary state also have a marked effect on extension rate. Elongation growth in light-adapted plants is controlled by both photon fluence rate and the spectral quality of the incident radiation at all photoequilibria. Photosynthesis can modify these responses but is not essential as a prior condition for a green plant to respond to changes in light quality and quantity.     

Ammonium toxicity: description of the syndrome in Sinapis alba and the search for its causation

In many plant species, prolonged application of ammonium (NH₄⁺) as a source of nitrogen results in physiological and morphological disorders (‘ammonium toxicity’). In the mustard (Sinapis alba L.) seedling we have previously observed particularly severe symptoms of ammonium toxicity in the absence of external nitrate (NO₃^-) or with increasing NH₄⁺/NO₃^- ratios. In the present investigation we have studied the symptoms of this ‘toxicity’in more depth, i.e. at the morphological, plastidic, enzyme and mRNA levels, in an effort to elucidate the causation of the syndrome. It could be confirmed that the syndrome is specific for ammonium and is not caused by a surplus of nitrogen. The syndrome is caused neither by pH changes in the medium nor by non-specific osmotic effects. Furthermore, the syndrome is not causally related to the fact that nitrate reductase (NR; EC 1.6.6.1.) is induced by ammonium. Development of the syndrome requires neither photosynthesis nor intact plastids. Nevertheless, the plastids are severely affected by ammonium application as is anthocyanin synthesis. Enzymes are differently affected. Among the plastidic enzymes, levels of ribulose-1,5-bisphosphate carboxylase (RuBPCase; EC 4.1.1.39) and NADP-dependent glyceraldehyde-3-phosphate dehydrogenase (NADP-GPD; EC 1.2.1.13) are strongly reduced and abundance of translatable mRNA of the small subunit of RuBPCase is decreased, whereas nitrite reductase (NIR; EC 1.7.7.1) is not affected. Among extraplastidic enzymes, the level of chalcone synthase (CHS; EC 2.3.1.74) is strongly reduced, the NAD-dependent glyceraldehyde-3-phosphate dehydrogenase (NADGPD; EC 1.2.1.12) level is unaffected, whereas the isocitrate lyase (ICL; EC 4.1.3.1) level is strongly promoted. The fat → carbohydrate transformation seems to be impaired by ammonium: fat degradation is reduced, starch accumulation is strongly inhibited and the levels of glucose and fructose are decreased. It appears from the present data and from results obtained in a companion study (U. Hecht and H. Mohr, in preparation) that the ammonium toxicity syndrome is detectable as soon as ammonium accumulation occurs in the plant. However, the actual mechanism through which the excess ammonium affects metabolism remains unclear at present.     

Respiration-Linked Proton Transport, Changes in External pH, and Membrane Energization in Cells of Escherichia coli

The kinetics of respiration-dependent proton efflux and membrane energization have been studied in intact cells of logarithmic-phase Escherichia coli. Parallel measurements of the rate and extent of proton efflux into the external medium (half-time, about 10 s; ratio of H(+) to O, about 0.5) and the oxidation of E. coli cytochrome b (half-time, 相似文献   

Inward-Rectifying K+ Channels in Root Hairs of Wheat (A Mechanism for Aluminum-Sensitive Low-Affinity K+ Uptake and Membrane Potential Control)

K+ is the most abundant cation in cells of higher plants, and it plays vital roles in plant growth and development. Extensive studies on the kinetics of K+ uptake in roots have shown that K+ uptake is mediated by at least two transport mechanisms, one with a high and one with a low affinity for K+. However, the precise molecular mechanisms of K+ uptake from soils into root epidermal cells remain unknown. In the present study we have pursued the biophysical identification and characterization of mechanisms of K+ uptake into single root hairs of wheat (Triticum aestivum L.), since root hairs constitute an important site of nutrient uptake from the soil. These patch-clamp studies showed activation of a large inward current carried by K+ ions into root hairs at membrane potentials more negative than -75 mV. This K+ influx current was mediated by hyperpolarization-activated K+-selective ion channels, with a selectivity sequence for monovalent cations of K+ > Rb+ [almost equal to] NH4+ >> Na+ [almost equal to] Li+ > Cs+. Kinetic analysis of K+ channel currents yielded an apparent K+ equilibrium dissociation constant (Km) of [almost equal to]8.8 mM, which closely correlates to the major component of low-affinity K+ uptake. These channels did not inactivate during prolonged stimulation and would thus enable long-term K+ uptake driven by the plasma membrane proton-extruding pump. Aluminum, which is known to inhibit cation uptake at the root epidermis, blocked these inward-rectifying K+ channels with half-maximal current inhibition at [almost equal to]8 [mu]M free Al3+. Aluminum block of K+ channels at these Al3+ concentrations correlates closely to Al3+ phytotoxicity. It is concluded that inward-rectifying K+ channels in root hairs can function as both a physiologically important mechanism for low-affinity K+ uptake and as regulators of membrane potential. The identification of this mechanism is a major step toward a detailed molecular characterization of the multiple components involved in K+ uptake, transport, and membrane potential control in root epidermal cells.     

Activation and Proton Transport Mechanism in Influenza A M2 Channel

Molecular dynamics trajectories 2 μs in length have been generated for the pH-activated, tetrameric M2 proton channel of the influenza A virus in all protonation states of the pH sensor located at the His³⁷ tetrad. All simulated structures are in very good agreement with high-resolution structures. Changes in the channel caused by progressive protonation of His³⁷ provide insight into the mechanism of proton transport. The channel is closed at both His³⁷ and Trp⁴¹ sites in the singly and doubly protonated states, but it opens at Trp⁴¹ upon further protonation. Anions access the charged His³⁷ and by doing so stabilize the protonated states of the channel. The narrow opening at the His³⁷ site, further blocked by anions, is inconsistent with the water-wire mechanism of proton transport. Instead, conformational interconversions of His³⁷ correlated with hydrogen bonding to water molecules indicate that these residues shuttle protons in high-protonation states. Hydrogen bonds between charged and uncharged histidines are rare. The valve at Val²⁷ remains on average quite narrow in all protonation states but fluctuates sufficiently to support water and proton transport. A proton transport mechanism in which the channel, depending on pH, opens at either the histidine or valine gate is only partially supported by the simulations.     

Activation and Proton Transport Mechanism in Influenza A M2 Channel

Molecular dynamics trajectories 2 μs in length have been generated for the pH-activated, tetrameric M2 proton channel of the influenza A virus in all protonation states of the pH sensor located at the His³⁷ tetrad. All simulated structures are in very good agreement with high-resolution structures. Changes in the channel caused by progressive protonation of His³⁷ provide insight into the mechanism of proton transport. The channel is closed at both His³⁷ and Trp⁴¹ sites in the singly and doubly protonated states, but it opens at Trp⁴¹ upon further protonation. Anions access the charged His³⁷ and by doing so stabilize the protonated states of the channel. The narrow opening at the His³⁷ site, further blocked by anions, is inconsistent with the water-wire mechanism of proton transport. Instead, conformational interconversions of His³⁷ correlated with hydrogen bonding to water molecules indicate that these residues shuttle protons in high-protonation states. Hydrogen bonds between charged and uncharged histidines are rare. The valve at Val²⁷ remains on average quite narrow in all protonation states but fluctuates sufficiently to support water and proton transport. A proton transport mechanism in which the channel, depending on pH, opens at either the histidine or valine gate is only partially supported by the simulations.     

Mucilage on the Root Surface and Root Hairs of Sorghum: Heterogeneity in Structure, Manner of Production and Site of Accumulation

Small drops of a mucilaginous character near the tip of roothairs were seen by light microscopy in several species of Sorghumand the Sorghum hybrid Vidan. Electron microscopy revealed thatthe drops are formed from at least two distinct substances,both apparently secreted from the endoplasmic reticulum. In addition, a patchy, fibrillar mucilaginous layer, also withat least two components, was found on the cell wall of the roothairs and on the outer wall of ordinary root epidermal cells.Golgi bodies as well as mitochondria take part in its production.As a rule, the mucilaginous patches are colonized by bacteria. Sorghum, root hairs, mucilage     

Propidium Iodide Competes with Ca2+ to Label Pectin in Pollen Tubes and Arabidopsis Root Hairs

We have used propidium iodide (PI) to investigate the dynamic properties of the primary cell wall at the apex of Arabidopsis (Arabidopsis thaliana) root hairs and pollen tubes and in lily (Lilium formosanum) pollen tubes. Our results show that in root hairs, as in pollen tubes, oscillatory peaks in PI fluorescence precede growth rate oscillations. Pectin forms the primary component of the cell wall at the tip of both root hairs and pollen tubes. Given the electronic structure of PI, we investigated whether PI binds to pectins in a manner analogous to Ca²⁺ binding. We first show that Ca²⁺ is able to abrogate PI growth inhibition in a dose-dependent manner. PI fluorescence itself also relies directly on the amount of Ca²⁺ in the growth solution. Exogenous pectin methyl esterase treatment of pollen tubes, which demethoxylates pectins, freeing more Ca²⁺-binding sites, leads to a dramatic increase in PI fluorescence. Treatment with pectinase leads to a corresponding decrease in fluorescence. These results are consistent with the hypothesis that PI binds to demethoxylated pectins. Unlike other pectin stains, PI at low yet useful concentration is vital and specifically does not alter the tip-focused Ca²⁺ gradient or growth oscillations. These data suggest that pectin secretion at the apex of tip-growing plant cells plays a critical role in regulating growth, and PI represents an excellent tool for examining the role of pectin and of Ca²⁺ in tip growth.The apical wall of tip-growing cells participates directly in the process of growth regulation (McKenna et al., 2009; Winship et al., 2010), yet few methods permit monitoring the wall properties of living cells. Despite this, several recent studies have enhanced our understanding of the apical cell wall. Chemical analyses of isolated pollen tube wall material have revealed a complex mixture of pectic polysaccharides with regions comprising long sequences of polygalacturonic acid. Important patterns of pectin methoxylation have been detected using immunocytochemical approaches, but these are limited to fixed cells (Dardelle et al., 2010). In a recent study, Parre and Geitmann (2005) used microindentation to show significant correlations between wall strength and growth rate. None of these techniques allow for easy investigation of the cell wall during growth.In an earlier study, we found that propidium iodide (PI) vitally stains pollen tubes of lily (Lilium formosanum) and tobacco (Nicotiana tabacum) and in particular reveals with great clarity the thickened apical cell wall (Fig. 1; McKenna et al., 2009). In addition, the apical PI fluorescence oscillates and in lily pollen tubes correlates tightly with oscillations in wall thickness measured by differential interference contrast (DIC) optics. Finally, these studies indicated that the PI fluorescence predicted cell growth rates with high confidence, suggesting that PI binding may provide useful information about the physical and chemical properties of the cell wall.Open in a separate windowFigure 1.PI fluorescence and growth rate oscillate in lily pollen tubes (A and B), Arabidopsis root hairs (C–E), and Arabidopsis pollen tubes (F and G). A, The top panel shows a DIC image of a lily pollen tube, and the bottom panel shows PI fluorescence of the same tube. The PI fluorescence is pseudocolored, with white representing high signal and blue representing low signal. Bar = 10 μm. B, Growth rate (blue) and PI fluorescence (red) are plotted on a line graph. Both oscillate with the same period but with different phases. C, DIC image (top panel) and PI fluorescence image (bottom panel) of an Arabidopsis root hair. Bar = 10 μm. D, Two PI fluorescence images of the same root hair focused on the apex representing peak (top) and trough (bottom) PI signals. Bar = 5 μm. E, A line graph showing the growth rate (blue) and peak PI fluorescence at the apex (red) for the same root hair shown in C and D. F, The top panel shows a DIC image of an Arabidopsis pollen tube, and the bottom panel shows PI fluorescence of the same tube. The PI fluorescence is pseudocolored, with white representing high signal and blue representing low signal. Bar = 5 μm. G, Growth rate (blue) and PI fluorescence (red) are plotted on a line graph. Both oscillate with the same period but with different phases. The growth rate between individual 3-s frames was smaller than the pixel size for our optics in both Arabidopsis cell types; to remove the noise this generated, a four-image (pollen) or five-image (root hair) running average is shown. A.U., Arbitrary units.PI is commonly used to visualize plant cell walls by wide-field fluorescence and confocal microscopy (Fiers et al., 2005; Tian et al., 2006; Estevez et al., 2008) and to select viable cells during cell sorting (Deitch et al., 1982; Jones and Senft, 1985). A positively charged phenanthridine derivative, the propidium ion stains cell walls but does not pass through the intact cell membranes of living cells. It readily diffuses into dead cells and forms highly fluorescent complexes by intercalation between base pairs of double-stranded nucleic acids, thus acting as an excellent indicator for cell vitality (Hudson et al., 1969). Binding to cell walls presumably occurs by a different mechanism, since it is not accompanied by the dramatic increase in fluorescence and shift in absorption and emission maxima observed when PI binds to nucleic acids. The mechanism of PI binding needs further exploration, as does the potential for broader use in other tip-growing plant cells.In this report, we test two hypotheses: first, that PI stains other tip-growing cells with pectin-containing cell walls; and second, that PI and Ca²⁺ bind to the same sites in these walls. This binding would occur through the interaction of partial positive charges caused by localized deficits in π-orbital electrons associated with three of the four nitrogen atoms of PI (Luedtke et al., 2005) coordinating with negatively charged carboxyl and hydroxyl groups on homogalacturonans (HGs), as has been suggested in Oedogonium bharuchae (Estevez et al., 2008).Our findings indicate that both hypotheses are satisfied. Notably, oscillatory changes in apical PI fluorescence occur and are observed to anticipate oscillations in growth rate in Arabidopsis (Arabidopsis thaliana) root hairs and Arabidopsis pollen tubes. In addition, competition studies indicate that PI and Ca²⁺ bind to the same sites in cell walls. Supporting these studies, we demonstrate that pectin methyl esterase (PME) creates more sites for PI binding, presumably by demethoxylating HGs as they are secreted, and that pectinase reduces PI fluorescence dramatically. However, unlike other pectin-binding dyes, PI does not block Ca²⁺ channels at the concentration used in live cell studies, nor does it alter oscillatory growth characteristics. Our findings provide evidence that PI may be employed as a quantitative measure of Ca²⁺-binding sites and thus may have use as an indicator of the degree of cross-linking of HGs and of cell wall extensibility.     

Control by phytochrome of urate oxidase and allantoinase activities during peroxisome development in the cotyledons of mustard (Sinapis alba L.) seedlings

The peroxisomal enzyme, urate oxidase (EC 1.7.3.3), and the next enzyme of the urate pathway, allantoinase (EC 3.5.2.5), demonstrate a lightmediated rise of activity in the cotyledons of mustard (Sinapis alba L.). The capacity of the peroxisomes for urate breakdown, marked by the time course of urate oxidase, develops distinctly later than the two other peroxisome functions (fatty acid breakdown, glyoxysomal function; glycolate breakdown, leaf peroxisomal function). The light effect on urate oxidase and allantoinase is mediated through the phytochrome system in all three seedling organs (cotyledons, hypocotyl, radicle), as revealed by induction/reversion experiments with red/far-red light pulses and continuous irradiation with far-red light (high irradiance reaction of phytochrome). Both enzyme activities can be induced by phytochrome in the seedling cotyledons only during a sensitive period of about 48 h prior to the actual light-mediated rise of activity, making it necessary to assume the existence of a long-lived intermediate (transmitter) in the signal response chain connecting enzyme formation to the phytochrome system. Detailed kinetic investigation, designed to test whether urate oxidase and allantoinase are controlled by phytochrome via the same signal response chain (coordinate induction), revealed large differences between the two enzymes: (i) a different onset of the loss of reversibility of a red light induction by a far-red light pulse (=onset of transmitter formation=coupling point; 48 h/24 h after sowing for urate oxidase/allantoinase); (ii) a different onset of the response (=onset of competence for transmitter= starting point; 72 h/48 h); (iii) full loss of reversibility (=completion of transmitter formation) is reached at different times (independence point, 90 h/52 h). These differences show that phytochrome controls urate oxidase and allantoinase via separate signal response chains. While urate oxidase can be localized in the peroxisomal fraction isolated from crude organelle extracts of the cotyledons by density gradient centrifugation, most of the allantoinase activity found in the peroxisomal fraction did not appear to be an integral part of the peroxisome but originated presumably from adhering membrane fragments.Abbreviations AL allantoinase, EC 3.5.2.5 - CAT catalase, EC 1.11.1.6 - GO glycolate oxidase, EC 1.1.3.1 - ICL isocitrate lyase, EC 4.1.3.1 - UO urate oxidase, EC 1.7.3.3. P_r - P_fr red and far-red absorbing forms of phytochrome On the occasion of his 80th birthday we dedicate this paper to Prof. Dr. phil., Dr. mult. h.c. Kurt Mothes, pioneer in research on metabolism of urates     

UV-B Inhibition of Phytochrome-Mediated Anthocyanin Formation in Sinapis alba L. Cotyledons : Action Spectrum and the Role of Photoreactivation

An action spectrum was measured for ultraviolet (UV) radiation-induced damage to (inhibition of) phytochrome-induced anthocyanin formation in cotyledons of 40-hour-old Sinapis alba L. seedlings. The action spectrum showed maximum effectiveness in the 260 to 280 nanometer waveband with little effect above 295 nanometers. The damaging effect of UV could be photorepaired by subsequent exposure to sunlight or to long wavelength (360 nanometers) UV radiation. Because this form of damage is subject to photorepair (photoreactivation), it is probably due to the formation of pyrimidine dimers, and the results suggest that it would not be ecologically relevant even if there was an increase in solar UV due to a decrease in stratospheric ozone levels of about 30%. If a dark period of more than 1 hour is interspersed between the phytochrome induction and the UV irradiation, the inhibition of the phytochrome induction gradually decreases with increasing dark period.     

The Effects of Light Intensity and Sucrose on Root Formation, Photosynthetic Ability, and Senescence in Detached Cotyledons of Sinapis alba L. and Raphanus sativus L.

The ability of detached cotyledons cultured in the light toassimilate ¹⁴CO₂, was reduced by the presence of sucrose inthe culture medium. This was due, at least in part, to an increasedrate of chlorophyll loss and yellowing of the blade. When cotyledondiscs were used, the inhibition of ¹⁴Carbon fixation by sucrosewas even more marked than in entire cotyledons. This could bedue to a higher level of penetration of the sucrose into discsor to the absence of the petiole which normally accumulatesphotosynthetic products. Sucrose culture also inhibited root production in cotyledonscultured in the light but promoted root formation in dark-grownor DCMU-treated cotyledons. The DCMU-inhibition of ¹⁴Carbonfixation by the blades was alleviated to some extent by sucroseculture. The sucrose effect on rooting was not permanent inthat transfer into water from sucrose led to root formationalthough this was delayed and present in a lower proportionof cotyledons than the controls. Thus, although a carbohydrate source either from photosynthesisor as applied sucrose, is essential for root production to takeplace, the combination of culture in the light with the presenceof sucrose in the medium may lead to an accumulation of carbohydrateto a level which directly or indirectly increases blade yellowingand inhibits root production.     

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