Cap Formation during the Elongation of Lateral Roots of Vicia faba L.

Cell proliferation was examined in the cap initials of newly-emerged0·2 and 4·0 cm long lateral roots of Vicia fabafrom an investigation of the passage of labelled cells throughmitosis following a 1-h pulse with tritiated thymidine. Celldoubling time was found to increase in this group of initialcells as the secondary roots elongated, this increase beinga result of a gradual lengthening in the duration of the mitoticcycle of both the fast and slow cycling meristematic cells andof a decrease in the size of both the growth fraction and thoseproliferating cells with a short cycle time. The rate of cellproduction by the cap initials was maximal in the newly emergedsecondary roots and showed a gradual decline with subsequentlateral elongation. This change in the rate of cell proliferationin the cap initials has been shown to be related to the initiationof the quiescent centre following lateral root emergence fromthe tissues of the primary. The number of cells making up the cap of 0·2–4·0cm long secondary roots is known to be constant and thus therate at which cells are sloughed off of the cap into the soilmust be the same as the rate of cap cell formation in theseroots, all of the cap cells being replaced every 6–9 days.The rate of cap cell formation in 0·2–4·0cm long secondary roots was used to calculate that one 11-dayold Vicia plant will have contributed between 56 000 and 85000 cap cells to the rhizosphere from its root system sincegermination.     

Protein Synthesis in Root Meristems

Comparative rates of incorporation of sulphate and L-leucmeinto proteins have been determined from autoradiographs forthe cells of root apices. This has confirmed the existence inmost roots of a quiescent centre where the rate of synthesisis much lower than in the surrounding cells. A more detailedaccount of leucine incorporation in the roots of Zea has beenmade by counting silver grains in quadrats on the autoradiographs.The rate of incorporation per unit volume increases on eitherside of the quiescent centre and then falls. On the proximalside the lowest rate of incorporation is below that in the quiescentcentre and is reached at 1–2 mm. from the tip. Fartherfrom the tip the rate rises. The rate of incorporation per celldoes not alter much in the first a mm. of the meristem, butbeyond that the rate rises steeply to between 15- and 5o-foldat 3 mm. from the tip. The patterns of change in rates of incorporationare different for the different tissues of the root.     

Nuclear Chromatin Structure in Relation to Cell Differentiation and Cell Activation in the Cap and Quiescent Centre of Zea mays L.

Barlow, P. W. 1985. Nuclear chromatin structure in relationto cell differentiation and cell activation in the cap and quiescentcentre of Zea mays L —J. exp. Bot. 36: 1492–1503.Nuclear chromatin structure has been analysed by electron microscopyof thin sections of cells in four zones of the root cap—meristem,central, slime-secreting and outermost cells—and alsoin the quiescent centre of the root before and after decapping.The chromatin pattern has been related to the DNA and RNA syntheticactivity of the nuclei. During cap cell maturation there wasa progressive condensation of the chromatin and this was accompaniedby some reduction of RNA synthesis. The degree of condensationwas estimated from the area and number of pieces of electrondense chromatin which increased and decreased, respectively,during cap maturation. The volume fraction of condensed chromatinwas also estimated but, in the cap, was not found to be a goodindicator of nuclear activity. The outermost cells of the capshowed the greatest degree of chromatin condensation but werestill active in RNA synthesis. Microdensitometry of their nuclearDNA contents gave an indication of loss of DNA in some of thenuclei. Decapping activated DNA and RNA synthesis in the quiescentcentre and also stimulated a decondensation of chromatin: thenumber of condensed pieces of chromatin increased, and theirsize and volume fraction both decreased 4 h after decapping.The number of pores per unit length of nuclear envelope profilewas also estimated. In the cap this number increased duringcap maturation; in the activated quiescent centre the numberremained constant except for a small rise 4 h after decapping Key words: Zea mays, chromatin, root cap, quiescent centre     

Localization of Nucleic Acid Synthesis in Root Meristems

Adenine-8-C1¹⁴ was supplied to roots of Vicia faba and Alliumascalonicum and its incorporation into DNA was studied fromautoradiographs of hydrolysed sections. These roots have a quiescentcentre to the meristem where the cells do not synthesize DNAand probably, therefore, play no part in the construction ofthe root. The boundary between the quiescent centre and thecentral cap initials is clearly denned and this suggests thatthere is as little cell interchange between the histogens asthere is in roots with visibly discrete histogens.     

The Proportion of Cells that Divide in Root Meristems of Zea mays L.

The proportion of cells that divide in four regions of the rootmeristem of Zea mays has been determined by an analysis of itscells pulse-labelled with tritiated thymidine. In the quiescent centre less than half of the cells divide andthe fastest of these (less than half of them) have a mitoticcycle duration of about 40 h at 23 °C compared with a cell-doublingtime of 230 h for the region. In the cap initials 80–90per cent of the cells divide and about 80 per cent of thesedivide once in 10 h. In the stele about 80 per cent of cellsdivide near the quiescent centre and all divide at 200 µmfrom the quesecent centre. The fast cells divide every 14 hin both regions, but the cell-doubling time increases from 18to 25 h near the quiescent centre. The root cap is completely replaced by its initials every dayand 10 000 cells are sloughed off. The rest of the meristemadds about 170 000 cells to the root every day. These figures are discussed in relation to the role of the quiescentcentre and the control of cell division.     

Growth and Development of Roots of Grapevine (Vitis vinifera L.) in Relation to Water Uptake from Soil

Developmental patterns of lateral roots and their vascular differentiationwere investigated for Vitis vinifera L. cv. Shiraz to assessthe likely contribution of lateral roots to total water uptakeof plants subjected to different irrigation regimes. Correlationanalyses showed a significant positive correlation between mainroot diameter and the diameter of first order lateral rootsof well-watered plants, but in water-stressed plants the twowere not significantly correlated. The correlations betweendiameters of first order lateral roots and the diameters ofmain roots were greater than correlations between the lengthsof first order laterals and the diameters of main roots. Thesuberised surface area of well-watered main roots increasedfrom 4% of total surface area at 0·25 cm to 100% at 10cm from the tip, whereas that of stressed plants increased from15% at 0·25 cm to 100% at 5 cm from the tip. In all treatmentsthe highest linear density of first order laterals was about7 laterals cm^-1 of main root. More than 50% of first order lateralshad diameters less than 0·05 cm, and more than 90% ofthem had lengths less than 5 cm. Calculations of axial resistancesbased on xylem diameter measurements suggest that the axialresistances of root segments may not be uniform along rootsas is often assumed in models of water uptake. Water flow intothe main roots via the lateral root pathway is likely to bemuch smaller than that via the direct radial flow pathway asonly about 1% of surface area of main roots is directly occupiedby lateral roots, leaving the other 99% of main root surfacearea available for the direct radial flow pathway.Copyright1994, 1999 Academic Press Axial resistance, grapevine (Vitis vinifera L. cv. Shiraz) roots, root diameter, root length, xylem vessels     

Some Observations on Invertase Activity in Roots of Vicia faba L.

Invertase activity has been determined at intervals along primaryroots of Vicia faba as they elongated from 0·5 to 8 cm.Little activity was evident in 0·5–1·0 cmlong primaries but in those 2–8 cm in length the mainpeak of enzyme activity was associated with the region of cellelongation. Changes took place in the pattern of invertase activityalong the primary roots as they lengthened and these changeshave been correlated with fluctuations in both the rate of rootelongation and the supply of sucrose to the root from the cotyledons.The presence of a root cap did not increase the activity ofthis enzyme in the apical 1 mm of these roots. Invertase activity was higher in lateral root primordia thanin most parts of the primary root basal to the meristem, presumablybecause of the presence of sucrose in the adjacent cavity inthe cortex of the primary root. The peaks of invertase activityfound basal to the region of cell elongation in 3–8 cmlong primary roots probably resulted from the development ofroot pnmordia in these parts of the root.     

In Vitro Transformation of Root meristem to Shoot and Plantlets in Vanilla planifolia

A study is reported of histogenesis and organogenesis duringthe processes leading up to plantlet formation in tip culturesof aerial roots of Vanilla planifolia. Young root tips excisedfrom aerial roots, less than 15 cm long, when cultured in liquidMS medium containing IAA and KN showed gravitropic responseuntil cap lysis began. With the collapse of the distal halfof the cap, the cells of the quiescent centre divided forminga hemispherical mass of cells. Further localized divisions onthe periphery of the hemisphere resulted in a number of meristemoidseach of which differentiated into a shoot meristem with leafprimordia. Procambium differentiated first beneath the apicalmeristem after two to three leaf primordia had formed and thenat the base of the leaves. After a few leaves have been formeda root meristem differentiated in close lateral proximity tothe basal end of the shoot procambium. Formation of a plateof vasculature at the nodal region of the first formed leaf,procambialization of the root and the bridging up of the shootand root vasculature with the nodal plate are described. Vanilla planifolia, root tip, in vitro, quiescent centre, meristemoid, plantlet     

The Response of the Primary Root Meristem of Zea mays L. to Various Periods of Cold

Primary roots of maize (Zea mays L.) grown in nutrient solutionat 5?C elongate at about 1% of the rate found at 20?C. The apicalmeristem becomes shorter and shows little proliferative activityat 5?C, but following transfer to 20?C mitoses increase in frequencyand the meristem regrows to its original length. Both the amountby which the meristem shortens and the time for its completeregrowth are related to the period spent at 5?C. The shorteningof the meristem suggests that at the lower temperature the balancewhich normally exists between cell production and differentiationis altered, the latter continuing at a relatively faster ratethan the former. A new, steady-state balance between the twoprocesses is re-established during the recovery period. Themeristem recovers as a result not only of its own mitotic activitybut also through stimulation of cell division in the quiescentcentre. The degree to which the quiescent centre is activated,as judged by its mitotic index and the number of nuclei labelledby feeding with tritiated thymidine, increases as the durationof the preceding cold treatment increases. The close relationshipbetween proliferative activity in the quiescent centre and theminimum length of the meristem following the cold treatmentsuggests that there is communication between these two zoneswhich co-ordinates their respective rates of cell productionand helps to maintain a normal meristem structure. The resultsemphasize the importance of the quiescent centre as a reservoirof cells that can re-establish a meristem rendered non-functionalthrough the impact of unfavourable environmental conditions. Key words: Meristem, quiescent centre, root, temperature, Zea mays     

Proliferating and Quiescent Cells in the Apical Meristem of Elongating Lateral Roots of Vicia faba L.

Whole root systems of Vicla faba were continuously exposed to³H-TdR for periods of up to 72h, following which LI was determinedin the cap initials, in the quiescent centre or in that partof the meristem in which a quiescent centre will develop, andin the stele and in the cortex-epidermis at intervals alongthe apical 800 µm basal to the junction between the capinitials and the rest of the meristem, in newly-emerged (NE),0.2 and 4.0 cm long lateral roots, after each exposure period.Cell doubling time (Td), mean cycling time (Tc) and the sizeof the growth fraction (GF) were then calculated for each partof the meristem investigated in each batch of roots, from thecurves recording increase in labelling index (LI) with increasein duration of the period of exposure to ³H-TdR and from therate of increase in LI over the initial l-12h labelling period.Since it is extremely difficult to eliminate all sources oferror in calculating GF from the values obtained for LI in continuouslabelling experiments, it is emphasized that the values of GFreported in the present paper may not be totally accurate. Thisis also true of the results obtained for Tc as Tc was derivedfrom the product of the corresponding values for Td and GF. Cell doubling time and mean cycling time were both longer inthe cells forming the quiescent centre in the 0.2 and 40 cmlong roots than in any other part of the apical meristem examined.The size of the GF was found to decrease basally along the steleand the cortex-epidermis from the most apical to the most basalsegment examined in the NE, while Td increased in duration.Similar changes took place along the stele of the 0.2 cm longlaterals, but not in the cortex-epidermis of these roots or,to any great extent, in any of the tissues examined in the 4.0cm secondary roots. No consistent trend was apparent in theduration of Tc basally along either tissue examined in theseroots. It was concluded from these results, and from supportingdata in the literature, that, as the laterals elongated fromNE to 4.0 cm, the apical meristem increased in length.     

The Quiescent Centre and Rates of Mitosis in the Root Meristem of Allium sativum

The root apices of Allium sativum have been examined by continuous-and pulse-labelling with tritiated thymidine and by colchicinetreatment to measure the time parameters of the mitotic cyclein various parts of the meristem. There is a quiescent centre of 30–50 cells whose averagerate of mitosis is low because the G₁ period is extended toabout 140 h compared with about 4 h in the othe regions of themeristem. The stele just above the quiescent centre and at 200microns above it and the cap initials just below the quiescentcentre are very similar in their mitotic cycles, the total lengthsof which are about 30 h of which nearly half is taken up byDNA synthesis. Allium thus differs from Zea in having root capinitials whose mitotic cycle is not telescoped by the eliminationof the G₁ phase. These facts are discussed in relation to theradiosensitivity of the meristem.     

Lateral root elongation inVicia faba L.

Summary Mitotic index (MI) was determined in the cap, epidermis, cortex and stele at successive intervals along the apical mm of lateral roots of various specific lengths. This parameter was found to decrease basally along roots of each length examined and to decline as the lateral root elongated. Moreover, MI was different in the various tissues investigated. These results have been discussed with respect to changes in the other parameters of cell proliferation and to root growth by elongation. It is suggested on the basis of the data obtained, and other results in the literature, that epidermal initial cells could be differentiated from those of the cortex and stele before the quiescent centre appeared in these roots, but the initial cells of the latter two tissues were indistinguishable until after the quiescent centre began to form.     

Biomass and distribution of fine and coarse roots from blue oak (Quercus douglasii) trees in the northern Sierra Nevada foothills of California

This research adds to the limited data on coarse and fine root biomass for blue oak (Quercus douglasii Hook and Arn.), a California deciduous oak species found extensively throughout the interior foothills surrounding the Central Valley. Root systems of six blue oak trees were analyzed using three methods — backhoe excavation, quantitative pits, and soil cores. Coarse root biomass ranged from 7 to 177 kg per tree. Rooting depth for the main root system ranged from 0.5 to 1.5 m, with an average of 70% of excavated root biomass located above 0.5 m. Of the total biomass in excavated central root systems, primary roots (including burls) accounted for 56% and large lateral roots (> 20 mm diameter) accounted for 36%. Data from cores indicated that most biomass outside of the root crown was located in fine roots and that fine root biomass decreased with depth. At surface depths (0–20 cm), small-fine (< 0.5 mm diameter) roots accounted for 71%, large-fine (0.5–2.0 mm) for 25%, and coarse (> 2 mm) for 4% of total root biomass collected with cores. Mean fine root biomass density in the top 50 cm was 0.43 kg m⁻³. Fine root biomass did not change with increasing distance from the trees (up to approximately 5 m). Thus, fine roots were not concentrated under the tree canopies. Our results emphasize the importance of the smallest size class of roots (<0.5 mm), which had both higher N concentration and, in the area outside the central root system, greater biomass than large fine (0.5–2.0 mm) or coarse (> 2.0 mm) roots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Root Aeration and Respiration in Young Mangrove Plants (Avicennia marina (Forsk.) Vierh.)

The roots of young plants of Avicennia marina (Forsk.) Vierh.grown under simulated tidal conditions were harvested so asto obtain the entire root system. The roots were subdividedand weighed and subsamples taken for manometric determinationof respiration rates at different temperatures. The supply capacityof the above-ground portion of the root system was determinedand the results compared in terms of supply and demand. Theoxygen consumption rate of the roots at 15°C was found tobe 1·69±0·07 µmol kg^–1 s^–1for cable roots and 3·27±0·12 µmolkg^–1 s^–1 for fine roots. The Q₁₀ for respirationwas 2·55 for oxygen consumption in both fine and cableroots, and for carbon dioxide production was 2·66 forfine roots and 3·04 for cable roots. The respiratoryquotient varied with temperature but was less than unity. Concentrationdifferences of between 1·8 mol m^–3 and 3·4mol m^–3 between the inside of root and the air were sufficientto permit aeration of the root system by diffusion alone, andthe aerenchyma contained sufficient oxygen to maintain aerobicconditions while the roots were covered with water. The effectof tide and seasonal temperature change on gas exchange, togetherwith the possibility of some form of carbon dioxide fixationwithin the root, are examined and the implications of theseeffects on growth and development are discussed. Key words: Mangrove, root aeration, respiration, aerenchyma     

Growth and Development of Young Roots of Barley (Hordeum vulgare L.) Genotypes

Barley (Hordeum vulgare L.) genotypes from countries with aMediterranean climate grown in temperature-controlled glasshousein nutrient solution to determine whether the co-ordinationof root branching and growth found by other workers appliedto a wider of up to 14 genotypes. There was substantial variationin the number of seminal axes produced by the genotypes, rangingfrom about seven for Hoshimasari and Swanneck to about fourfor Gerbel 'B'. The number of nodal axes was linearly relatedto the number of leaves and typically between one and two mainstemleaves were required before nodal axes appeared. There weresmall genotypic differences in the number of axes produced perleaf with values ranging from 1·5 to 2·3. Theproduction and growth of lateral roots were coordinated so thatthe mean length of laterals generally increased with time. Landraces(Arabic abiad and Arabic aswad) produced more lateral rootswith a faster rate extension compared with other genotypes.The length and number of primary and secondary lateral rootswere related linearly, but no genotypic differences in thisrelation were evident. Length of primary lateral roots increasedmore rapidly than that of secondary lateral roots throughoutthe three to five leaf stage. The ratio of root weight to totalplant weight decreased with time but there were only small differenceswithin this range of genotypes.Copyright 1995, 1999 AcademicPress Barley, seminal axes, nodal axes, primary lateral roots, relative extension rates, relative multiplication rates     

Control of soil physical properties and response ofBrassica rapa L. seedling roots

A system was designed, constructed, tested, and used to growBrassica rapa L. seedling roots which were exposed to O₂ concentrations from 0 to 0.21 mol mol⁻¹, water potentials from 0 to −80 kPa, temperatures from 10 to 34°C, and mechanical impedance from 0 to 20.8 kPa. The experimental design was a central composite rotatable design with seven replications of the center point. Measurements were taken of taproot length, taproot diameter at the point of initiation of root hairs (diameter 1), and one cm above the first measurement (diameter 2), and total length and number of first-order laterals. Temperature had the greatest effect on seedling root growth, with linear and quadratic temperature effects significant for all root measurements except taproot diameter 2 which just had a significant linear effect. Water potential had a significant linear effect on lateral length and number of laterals and a significant quadratic effect on taproot diameter 1. Mechanical impedance had a significant effect only on taproot diameter 2. Oxygen was not significant for any root measurement. The mechanical impedance by water potential interaction was significant for taproot length and taproot diameter 1. A temperature optimum was found for taproot length, taproot diameter 1, lateral length, and lateral number, at 26.0, 42.5, 26.5, and 26.4°C, respectively. Taproot diameter 1 had a water potential optimum at −36.5 kPa, whereas taproot diameter 2 had a mechanical impedance optimum at 12.5 kPa. A growth cell designed for this study allows independent control of soil strength, water potential, oxygen concentration, and temperature. Thus, the cell provides the capability which was demonstrated forBrassica rapa L. to grow seedling roots under complete control of the soil physical properties.     

Root growth as influenced by aggregate size

Summary This study examined the effects of aggregate size on root impedance and developed an equation to describe the root pressure necessary to avoid deflection around an aggregate. This critical root pressure was predicted to increase with increasing aggregate size, decreasing root diameter, and decreasing deflection angle. In growth chamber experiments, maize (Zea mays L.) seedlings were grown in A horizon material of Groseclose silt loam (Clayey, mixed, mesic, Typic Hapludult). The soil had been moist sieved into different aggregate sizes (0–1, 1–2, 2–3, and 3–6 mm diameter). The larger aggregates did constitute a slight root impedance as roots were deflected around them. Diameters of roots grown in 3–6 mm aggregates increased significantly, whereas root lengths were not always signficantly decreased. The smaller aggregates did not impede root growth and were readily displaced by roots. Large aggregates were more of an impedance to lateral roots than to main axes.     

The Development of Lateral Root Primordia in Vicia faba L. and Their Response to Colchicine

Lateral root primordia in i are first initiated 2–3 daysfollowing the onset of germination, after which they take 5.17–6.35days to complete their development and emerge as lateral roots.Variation in the amount of time elapsing between primordiuminitiation and emergence as a lateral is probably a reflectionof the cell number attained by any one primordium at the timeof emergence. The number of primordia produced per cm of primaryroot growth (5.35–6.65) was not affected by variationin the rate of root elongation, although the number of primordiaproduced each day increased with increase in the rate of rootgrowth. In colchicine-treated roots, the amount of time between primordiuminitiation in the C-tumour and the subsequent emergence of alateral (5.43–6.43 days) was similar to the value obtainedin control roots. Primordia which were present at the time ofcolchicine treatment responded to treatment in a number of differentways, depending on the stage of development reached. Primordiain the first 2.66 days of their development die following treatment;those between 2.66 and 3.69 days old have their developmentinhibited but stay alive; primordia which have been developingfor 3.69–4.91 days following initiation grow out as straightlaterals, while those between 4.91 and 5.77 days old form C-tumoursand emerge as inhibited laterals.     

High pH Causes Disintegration of the Root Surface in Lupinus angustifolius L.

The effect of high pH on the morphology and anatomy of the rootsof lupin (Lupinus angustifolius L. cv. Yandee) and pea (Pisumsativum L. cv. Dundale) was examined in buffered solution. Themorphology and anatomy of lupin roots were markedly altered,and root growth was reduced by increasing solution pH from 5·2to 7·5, whereas pea roots were unaffected. In lupin roots,pH 7·5 caused disintegration of the root surface andimpaired root hair formation. Lupin roots grown at pH 7·5also had decreased cell lengths but increased cell diameterin both the epidermis and the cortex in comparison to rootsgrown at pH 5·2. High pH reduced cell volume greatlyin the epidermis, to a lesser extent in the outer cortex andnot at all in the inner cortex. It appears that in lupins, theprimary detrimental effects of growth at pH 7·5 is reducedlongitudinal growth of cells near the root surface with a consequentreduction in elongation of the cells in inner cortex.Copyright1993, 1999 Academic Press Lupinus angustifolius L., Pisum sativum L., high pH, root morphology, root anatomy     

Root growth of potato crops on a marine-clay soil

Summary In 1982 and 1983 root samples were taken by auger from potato crops grown on marine clay in the Flevo-Polder. The roots increased their penetration depth throughout the periods of measurement, and ultimately reached depths between 80 cm and 100 cm below the hills. Between 50 and 60 days after emergence, decay of roots commenced, starting in the upper horizons. In the hill mean root length densities varied between 1 and 2 cm cm⁻³. Below the hills root density rarely exceeded 1 cm cm⁻³. The random variation in root density was equivalent to a coefficient of variation of 50%. There were significant effects of the position of sampling (relative to the centre of the plant) on root density; densities were usually lowest beneath the furrow. Depending on season and sampling date, total root length varied between 3.4 and 7.1 km m⁻², and root dry mass varied between 33 and 77 g m⁻². Representative figures for specific root length were 100–120 m g⁻¹ dry weight. About 90% of the root diameters were smaller than 0.44 mm; the most frequent class (35%) were roots with diameters between 0.12 and 0.20 mm.