Longitudinal Water Movement in the Primary Root of Zea mays

The rates of transfer of tritiated water (THO) along lengthsof excised primary roots of Zea mays have been measured undera variety of conditions. The following values of ‘apparentdiffusion coefficients’ for THO in the root tissue havebeen evaluated: 1.5±0.1x10^-5 cm² sec^-1 in roots boiledfor 3 min before use,0.5±0.03x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaF,0.9±0.07x10^-5 cm² sec^-1 in rootspoisoned with 10^-2 M NaN₃,and 2.1±0.2x10^-5 cm² sec^-1in normal roots. The bathing medium in all cases was 1.0 mMKCl/0.1 mM CaCl₂ with the addition of the inhibitors where appropriate.Thefourfold increase in the rate of THO transfer in normal rootscompared with poisoned ones is attributed to the existence ofa long-distance convective flow in the first case, which isterminated by the addition of inhibitors. Since experimentsshow that this convective flow must occur both acropetally andbasipetally with equal velocity, it is thought to occur in thephloem.By assuming the ‘streaming transcellular strands’model for phloem transport, the rate of movement required togive the observed transfer has been computed as approximately4.5x10^-2 cm sec^-1 (160 cm h^-1).The earlier report of the existenceof a highly impermeable barrier surrounding the xylem vesselshas been further substantiated by the experiments reported here.     

Relation Between Calcium and Strontium Transport Rates as Determined Simultaneously in the Primary Root of Zea mays

Root segments of Zea mays 55 mm long, were exposed to nutrient containing ⁸⁵Sr and ⁴⁵Ca tracers. Translocation rather than uptake was measured, using a newly-designed glass compartmentation system and validated tracer analytic model. Ca transport from solutions containing between 0.25 and 5.0 mm Ca was only slightly affected by concentration, but translocation from 0.25 to 0.05 mm solutions was markedly reduced. Maximum transport of strontium from nutrient containing 0.05 mm Ca was twice that from 2.5 mm Ca, and also twice the maximum calcium transported. Thus, under the condition simulating calcium depletion, i.e., 0.05 mm Ca, greater amounts of strontium were transported. In these cases the solutions also contained stable strontium at concentrations between 0.25 and 5.0 mm. In simultaneous determinations, the ratio of Sr to Ca moved was exactly equal to the ratio of their concentrations in nutrient solution, and there was no evidence of discrimination. Dinitrophenol reduced transport of Sr and Ca to an equivalent extent, amounting to between 2 and 9% of non-treated control levels.     

RNA Synthesis in the Root Apex of Zea mays

The rate of RNA synthesis in eight regions of the root apexof Zea mays has been determined. Cells of the quiescent centrehave a lower level of RNA synthesis than any other region studied.The level of RNA synthesis appears to be a function of the positionof the cell in the apex. All the newly synthesized RNA detectedin chromatin and cytoplasm can be accounted for by synthesisin the nucleolus and subsequent transport to these compartments.Newly synthesized RNA appears in the three compartments at differentrates depending on the location of the cell.     

Root Cap Inhibitor Formation in Isolated Root Caps of Zea mays

Root caps were isolated and cultured aseptically on variousdefined media. Under appropriate culture conditions plus suitableillumination a substance able to produce a positive geotropicresponse (i.e. a downward bending of roots) was formed in isolatedroot caps. The presence of this substance, or root cap inhibitor,was assessed by substituting cultured caps in place of capsof dark-grown roots. Normally these roots if kept in continuousdarkness will not respond to gravity (i.e. no bending). Optimalroot cap inhibitor production occurs on a relatively simplemedium, lacking sucrose, but supplemented with 10^–9 MIAA. Protein synthesis is necessary for inhibitor productionand/or expression, whereas DNA synthesis is not.     

The Exchange of sodium Ions in the Root of Zea mays

The apparent cation-exchange capacity of maize roots has beenestimated in 100 mM NaCL and I mM NaCl by observing the effluxkinetics of isotopically labelled sodium from this tissue. Thisapproach indicated that the apparent capacity was 70 m.equiv/kgof tissue (fresh weight) and 2 m.equiv/kg of tissue (fresh weight)in 100 mM and I mM NaCl respectively; the pH of the medium was5.8 in each case.Similar sodium efflux experiments were performedon isolated stelar tissue from these roots and such experimentsgave an apparent exchange capacity of 86 m.equiv/kg of tissue(fresh weight) and 6 m.equiv/kg of tissue (fresh weight) in100 mM and I mM NaCl respectively, both at pH 5.8.The dependenceof the magnitude of the apparent exchange capacity upon theelectrolyte concentration of the external medium is discussed.     

Free Radicals in the Root System and Stem of Zea mays

The concentration of free radicals in different parts of theroot system and the stem of 130, 158 and 165 d old maize plantswere measured. The highest concentrations of free radicals werefound in the tissue of the primary root and in the adventitiousroots of the 1st and 2nd nodes. In the stem tissues, the highestconcentration of free radicals was found in the tissues of the14th to 15th internode. The concentration of free radicals inthe tissues of the nodal roots decreased with increasing nodenumber, whereas in the tissues of the overground stem it increasedwith internode number. This pattern may be correlated with themetabolic activity of tissues.     

Chimeras and the Origin of Lateral Root Primordia in Zea mays

The difficulty of determining the contribution made by the pericycleand endodermis of mother roots to lateral primordia in the Gramineaehas been solved by inducing polyploid chimeras at initiation. The endodermis forms a layer covering the primordium, but thislayer does not form the epidermis of the lateral. It does formthe root cap of the young primordium, but this is replaced ata variable stage of development by the quiescent centre donatinga new set of cap initials of pericyclic origin. Reasons forthe previous diversity of interpretations are presented. chimera, lateral root primordia, Zea mays     

Characterizing Pathways by Which Gravitropic Effectors Could Move from the Root Cap to the Root of Primary Roots of Zea mays

Plasmodesmata linking the root cap and root in primary rootsZea mays are restricted to approx. 400 protodermal cells borderingapprox. 110000 µm² of the calyptrogen of the root cap.This area is less than 10% of the cross-sectional area of theroot-tip at the cap junction. Therefore, gravitropic effectorsmoving from the root cap to the root can move symplasticallyonly through a relatively small area in the centre of the root.Decapped roots are non-responsive to gravity. However, decappedroots whose caps are replaced immediately after decapping arestrongly graviresponsive. Thus, gravicurvature occurs only whenthe root cap contacts the root, and symplastic continuity betweenthe cap and root is not required for gravicurvature. Completelyremoving mucilage from the root tip renders the root non-responsiveto gravity. Taken together, these data suggest that gravitropiceffectors move apoplastically through mucilage from the capto the root. Calyptrogen, open meristem, protoderm, root cap, root gravitropism, Zea mays     

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     

An Automata-theoretical Model of Meristem Development as Applied to the Primary Root of Zea mays L.

Observations were made of the sequence of division within thecellular packets (groups of cells of common descent) which comprisethe cell files that run the length of the central cortex ofthe primary root meristem ofZea mays. These sequences, and alsothe relative lengths of the cells within the packets recordedat various times during root growth, indicate that cell-filedevelopment can be expressed using one, or a limited number,of deterministic ‘bootstrap’ L-systems which assigndifferent lifespans to sister cells of successive cell generations.The outcome is a regular pattern of divisions from which daughtercells emerge usually with unequal, but definite, lengths. Inthe immediately post-germination stage of root growth, one divisionpathway is especially common in the cortex and generates sequencesof unequal daughters having a particular basi-apical orientation.Later in root growth, the cellular pattern in the cortex indicatesthat this pathway is replaced by another where unequal divisionsare not so marked, but which nevertheless continues to maintaina regular arrangement of differently sized cells. This latterpathway is characteristic of a zone close to the initial cellsof the cortex. It is present at all stages of root growth andspreads along the length of the cortex as the descendants ofthese initials proliferate. The development of the whole corticalcell file can be simulated from knowledge of the growth functionsof the bootstrap systems. The files so generated contain allthe observed cell patterns. The growth functions also predictthe sequence in which cells cease dividing near the proximalmargin of the meristem, but for this it is necessary to incorporatea counter for the number of divisions that will be accomplishedin the cell file. Cytological requirements for the propagationof unequal divisions, together with a consideration of the natureof the division counter, as well as the significance of theswitch in division pathways encountered during early root growth,are discussed in the context of this deterministic model ofcell division. Cell division; root meristem; L-systems; Zea mays     

A Correlation between Structure and Function in the Root of Zea mays

The exudation rates of fluid and potassium ions from isolatedmaize roots were determined before and after excision of certainlengths of root tip. The results of this study suggest thatexcised maize roots possess the ability to absorb potassium(and presumably chloride) ions and concomitant amounts of waterover a considerable distance (10 cm) from the tip. Moreover,the observed power of absorption of ions and water into thetranslocatory pathway decreases in passing from the tip towardsthe base of the root. Both light and electron microscope techniques were used to examinethe anatomy of primary roots similar to those used in the physiologicalexperiments. The principal observation was that the xylem vesselsnear the root tip contain membrane-bounded cytoplasm with organelles.The number of mature xylem vessels, i.e. without cytoplasm,progressively increased in transverse sections cut from 1 to10 cm from the root tip; above 10 cm from the root tip all ofthe xylem vessels were found to be completely mature. It isevident that prima facie a connexion exists between this singleaspect of root anatomy and fluid exudation from excised roots. The uptake of tritiated water by roots and its transport intoexudates was examined. These data were analysed on the assumptionthat the exchange of external labelled water with the exudatewas achieved by the fluid exudation itself; this analysis indicatedthat an operational volume, similar to that of the total xylemvolume within the root, must become labelled during the formationof the exudate.     

Root Growth Inhibitors from Root Cap and Root Meristem of Zea mays L.

A micro-assay based on the growth inhibition of root segmentsof the seminal roots of Zea mays has been used to investigatethe root-growth-inhibiting substances in root caps and meristemsrespectively of the roots of Zea mays. This micro-assay is sensitiveto 50 pg of IAA or less. Paper chromatography of the acid fractionof methanolic extracts shows the presence of one main inhibitorin root caps and a different main inhibitor in root meristems.Neither is IAA, whose presence in meristems is sometimes indicatedby small inhibitions (or stimulations) at the characteristicRf of IAA. A Commelina leaf-epidermis assay shows the presenceof one stomata-closing ABA-like substance in root caps and onein meristems, one corresponding in Rf to the main root-growthinhibitor from the root cap. The implications of these findingsfor the geotropic responses of roots is briefly discussed.     

The Ultrastructure of the Regenerating Root Cap of Zea mays L

Removal of the cap from the primary root of Zea mays activatescell division in the quiescent centre. It is the descendentsof these cells that eventually regenerate a new cap—aprocess that is complete in about 4 days at 23 °C. The ultrastructureof the cells of the regenerating cap was examined at daily intervals.During the first day after decapping the dictyosomes in theexposed outer layer of cells change from a relatively quiescentstate to one where they are secreting a polysaccharide slimewhich accumulates between the plasmalemma and the outer cellwall. Amyloplasts grow in size and appear to divide, and theendoplasmic reticulum proliferates. Many different cytoplasmicfeatures that are normally characteristic of cells in distinctlocations within the undisturbed cap occur, at first, all togetherwithin the few cells that are the source of the new regeneratingtissue. Regeneration of a normal structure in the new cap isachieved by progressive changes in the structures of the cellorganelles, apparently in response to the position that thecells containing them occupy within the growing cellular ensembleat the root apex. Zea mays, regeneration, root cap, ultrastructure     

Hypoxic Induction of Anoxia Tolerance in Root Tips of Zea mays

When root tips of fully aerobic, intact maize (Zea mays L.) seedlings are made anaerobic, viability normally is only 24 hours or less at 25°C. We find that viability can be extended to at least 96 hours if seedlings are given a hypoxic pretreatment for 18 hours by sparging the solution with 4% O₂ in nitrogen (v/v) before anoxia. Fully aerobic root tips (sparged with 40% O₂) had very low alcohol dehydrogenase (ADH) activity (per gram root fresh weight), and the level remained low under anoxia. In hypoxically pretreated roots, however, high levels of ADH activity were induced, and activity rose further during the initial 24 hours of anoxia, and then remained high at about 20 times that of controls in 40% O₂. ADH activity in roots in solution sparged with air (21% O₂) was about three times that in 40% O₂. Improved viability of hypoxically pretreated root tips was associated with maintenance of a high energy metabolism (ATP concentration, total adenylates, and adenylate energy charge). Roots that were not pretreated lost 94% of the total adenylates and ATP at 24 hours of anoxia. The relation between induced ADH activity, energy metabolism, and improved anoxia-tolerance in acclimated maize root tips is discussed.     

Experimental Modification of Cell Division Patterns in the Root Meristem of Zea mays

In the meristem of the young primary root of maize seedlingsthe first transverse division in the cortex 250 µm fromthe root apex results in two daughter cells of distinctly unequalsize. This division could be rendered equal by raising the seedlingsin up to 7.5% methanol. The pattern of the subsequent two orthree transverse divisions in the cortex, as revealed by thearrangement of the newly divided cells in the resultant cellularpackets, was acropetal in the methanol-treated roots but basipetalin the control roots. The sequence of division within a cellularpacket tended to follow the distribution of cell sizes - largercells divided earlier than smaller cells. A temporary arrestof cell division by exposing roots to cold (5 °C) conditionshad no effect on the sequence of divisions that followed whenthe roots were allowed to recover at 20 °C. The resultssuggest that the normally asymmetric position of the cell wallformed at cytokinesis is subject to active regulation and thatmethanol interferes with this process. The cytoplasm of certaincells in the root meristem was also found to be unequally distributed,as judged by Azure B staining, between the two ends of the cell.Cytoplasmic asymmetry was not directly correlated with inequalityof division, although it too was affected by methanol. Cell polarity, root meristem, unequal division, Zea mays     

Exit from the Mitotic Cycle in Root Meristems of Zea mays L.

The choice between two modes of exit from the mitotic cycleat the margins of meristems has been made easier by surveyingthe range of the numbers of cell contacts between contiguousfiles in root apices of Zea mays L. The range shows that somecells must go out of cycle while others remain in cycle forat least three further generations. The view that cycling endsby a fall in the proliferative fraction is supported by theexistence of pulse-labelled telophases in the proximal regionof the menstem. These are most likely due to acceleration ofthe mitotic cycle which has to be contrasted with decelerationof the overall rate of cell proliferation. The work is discussedin relation to patterns of cycling in the different tissuesof the apex. mitotic cycle, cell size, meristem, proliferative fraction, Zea mays L, maize     

Root Growth, Secondary Root Formation and Root Gravitropism in Carotenoid-deficient Seedlings of Zea mays L.

The effect of ABA on root growth, secondary-root formation androot gravitropism in seedlings of Zea mays was investigatedby using Fluridone-treated seedlings and a viviparous mutant,both of which lack carotenoids and ABA. Primary roots of seedlingsgrown in the presence of Fluridone grew significantly slowerthan those of control (i.e. untreated) roots. Elongation ofFluridone-treated roots was inhibited significantly by the exogenousapplication of 1 mM ABA. Exogenous application of 1 µMand 1 nM ABA had either no effect or only a slight stimulatoryeffect on root elongation, depending on the method of application.The absence of ABA in Fluridone-treated plants was not an importantfactor in secondary-root formation in seedlings less than 9–10d old. However, ABA may suppress secondary-root formation inolder seedlings, since 11-d-old control seedlings had significantlyfewer secondary roots than Fluridone-treated seedlings. Rootsof Fluridone-treated and control seedlings were graviresponsive.Similar data were obtained for vp-9 mutants of Z. mays, whichare phenotypically identical to Fluridone-treated seedlings.These results indicate that ABA is necessary for neither secondary-rootformation nor for positive gravitropism by primary roots. Zea mays, gravitropism, carotenoid-deficient, Fluridone, root growth, vp-9 mutant     

Localization of Acid Hydrolase Activity in Zea mays L. Root Tips

Naphthol AS-BI phosphatase, esterase, aryl sulphatase, glucuronidase,and ß-glycerophosphatase have been studied in frozensections of maize root tips. In general these enzymes showedhighest activities at the root surface and at particulate sitesin the cytoplasm although the indigogenic method for esteraseshowed no particulate activity and the naphthol AS-D acetatereaction gave no pronounced surface activity. With electronmicroscopy highest activity for ß-glycerophosphatasewas observed in the cell walls and associated with the vacuoles.The significance of these observations are discussed in relationto the function of surface hydrolytic activity and to the presenceof lysosome-like bodies in higher plant cells.     

Root Graviresponsiveness and Columella Cell Structure in Carotenoid-deficient Seedlings of Zea mays

Root graviresponsiveness in normal and carotenoid-deficientmutant seedlings of Zea mays was not significantly different.Columella cells in roots of mutant seedlings were characterizedby fewer, smaller, and a reduced relative volume of plastidsas compared to columella cells of normal seedlings. Plastidsin columella cells of mutant seedlings possessed reduced amountsof starch. Although approximately 10 per cent of the columellacells in mutant seedlings lacked starch, their plastids werelocated at the bottom of the cell. These results suggest that(i) carotenoids are not necessary for root gravitropism, (ii)graviresponsiveness is not necessarily proportional to the size,number, or relative volume of plastids in columella cells, and(iii) sedimentation of plastids in columella cells may not resultdirectly from their increased density due to starch content.Plastids in columella cells of normal and mutant seedlings wereassociated with bands of microtubule-like structures, suggestingthat these structures may be involved in ‘positioning’plastids in the cell. Zea mays, graviperception, graviresponsiveness, carotenoids, vp-9 mutant, columella cell, roots