The role of plant hormones in higher plant cellular differentiation. II. Experiments with the vascular cambium, and sclereid and tracheid differentiation in the pine, Pinus contorta

In sterile-cultured explants of stems of the pine Pinus contorta Dougl., fusiform cambial cells differentiated entirely into axial parenchyma cells when exogenous indol-3yl-acetic acid (IAA) was omitted. The normal appearance of the cambial zone was maintained when IAA was included in the medium. The IAA-maintained stability of cambial structure suggests physiological rather than epigenetic control over vascular cambium structure. IAA was essential for the occurrence of callus growth in stem explants. Callus growth was similar in appearance and extent in winter- and summer-explanted material. Tracheids differentiated in explants only when actively differentiating tracheids were already present at the moment of explanting, suggesting the absence of factors necessary for tracheid differentiation in over-wintering tissues. Sclereid differentiation, which normally does not occur in phloem or xylem development in P. contorta, occurred in callus derived from active cambial explants. The sclereids were identical to sclereids which differentiated in pith of intact stems. The possibility that sclereid and tracheid differentiation may be fundamentally similar types of gene expression is discussed. Growth of P. contorta trees in continuous darkness resulted in extensive compression-wood tracheid differentiation in the upright main stem. Normal-wood tracheids differentiated in similar trees grown in light. More tracheids differentiated in light than in darkness. This apparently is the first report of induction of compression-wood tracheid differentiation in the absence of hormone treatment or tilting of trees. Different types and numbers of tracheids differentiated at different position in two-year-old disbudded defoliated stem cuttings of P. contorta in response to apically supplied IAA. No evidence for new tracheid differentiation was seen in control cuttings; however, the results suggest that neither cambial cell division nor tracheid differentiation were actually initiated by IAA. Directed transport of additional regulatory factors toward areas of high IAA concentration is formulated as a hypothesis to explain these observations. Gibberellic acid, (S)-abscisic acid and IAA inhibited tracheid differentiation when individually supplied to basal ends of P. contorta cuttings predisposed to differentiate new tracheids. Experiments with single intact needles on Pinus cembroides var. monophylla cuttings confirmed a previous interpretation that the mature pine needle, rather than the short-shoot apical meristem at its base, promotes tracheid differentiation in the stem.     

Protoplasmic Changes in Cambial Cells Induced by a Tracheid-Differentiation Factor from Pine Needles

Direct differentiation of ‘dormant’ fusiform cambialcells (fees) into tracheids in stem cuttings of Pinus contortaDougl. was investigated by light and transmission electron microscopy.In cuttings from which all but one pair of needles had beenremoved, differentiation occurred close to the needle tracewithout prior cell division or expansion. The highly osmiophillic,granular cytoplasm of differentiating fees exhibited numerousmitochondria, dictyosomes and endoplasmic reticulum (ER) cisternaeand was enriched with vesicles and polyribosomes. The protoplasmdid not become highly vacuolated, as it normally does, untildifferentiating tracheids were approaching maturation. During differentiation, secondary walls were deposited in feesas ribs of annular and spiralled thickenings and also as borderedpit-likestructures devoid of margo and torus and lacking the typicallamellar structure of secondary-xylem tracheids. The fee plasmamembrane at sites of secondary-wall growth was in dynamic flux,dilated vesicles derived from both ER and dictyosomes apparentlyfusing with it at these locations. In addition, electron-denseblebs were invariably plasma membrane-associated and appearedto be exocytotic; these blebs became rare during autolysis andthey were never seen in non-differentiating cells. Depositionof oriented nascent cellulose microfibrils appeared to occurin the absence of associated cortical microtubules. As secondary-walldeposition neared completion, vacuoles derived from rough ERgrew and fused; concomitantly, the protoplasm disappeared resultingin fully autolysed tracheids. Auxin (indol-3-ylacetic acid, IAA) promoted the gel-like protoplasmof dormant fees to became highly vacuolated, and cell divisionand expansion followed; however, tracheid differentiation didnot occur. Polyribosomes, rough ER, and dictysomes were lessabundant than in differentiating fees and the cytoplasm of auxin-treatedfees was fine grained and less densely stained. The electron-denseexocytotic blebs found at the plasmalemma in differentiatingfees were not induced by IAA treatment. Fees of control cuttingsenlarged somewhat and became more vacuolated but otherwise remaineddormant. Key words: Cambium, exocytosis, Gymnospermae, ultrastructure, xylogenesis     

Function and dynamics of auxin and carbohydrates during earlywood/latewood transition in scots pine

In temperate regions the annual pattern of wood development is characterized by the formation of radially narrow and thick walled latewood cells. This takes place at the later part of the growing season when cambial cell division declines. To gain new insight into the regulation of this process, micro-analytical techniques were used to visualize the distribution of indole-3-acetic acid (IAA), soluble carbohydrates, and activities of sucrose (Suc)-metabolizing enzymes across the cambial region tissues in Scots pine (Pinus sylvestris). The total amount of IAA in the cambial region did not change with latewood initiation. But its radial distribution pattern was altered, resulting in an increased concentration in the cambial meristem and its recent derivatives. Thus, initiation of latewood formation and cessation of cambial cell division is not a consequence of decreased IAA concentrations in dividing and expanding cells. Rather, IAA most likely has a role in defining the altered developmental pattern associated with latewood formation. Carbohydrates and enzyme activities showed distinctive radial distribution patterns. Suc peaked in the phloem and decreased sharply to low levels across the cambial zone, whereas fructose and glucose reached their highest levels in the maturing tracheids. Suc synthase was the dominating Suc cleaving enzyme with a peak in the secondary wall-forming tracheids and in the phloem. Soluble acid invertase peaked in dividing and expanding cells. Suc-phosphate synthase had its highest activities in the phloem. Activities of cell wall bound invertase were low. The absence of major seasonal variations indicates that carbohydrate availability is not a trigger for latewood initiation. However, steep concentration gradients of the sugars suggest a role for sugar signaling in vascular development.     

The regulation of cambial division and secondary xylem differentiation in xanthium by auxins and gibberellin

Cambial division continued in decapitated Xanthium plants without concomitant xylem fiber differentiation. The application of indoleacetic acid to these plants did not affect the production of cambial derivatives or induce xylem fiber differentiation. When naphthaleneacetic acid was applied either to the second internode or to the stump of a lateral shoot, xylem fiber differentiation was induced in the newly formed cambial derivatives on the xylem side of the cambium in the stem. When naphthaleneacetic acid was applied unilaterally, xylem fiber differentiation was restricted to that side of the stem in the first internode and hypocotyl. Naphthaleneacetic acid also enhanced the production of cambial derivatives. Gibberellic acid enhanced cambial derivative production but did not affect the differentiation of xylem fibers. Similar numbers of cambial derivatives were produced in some naphthaleneacetic acid-treated plants in which xylem fiber differentiation was induced and in gibberellic acid-treated plants which did not differentiate xylem. When naphthaleneacetic acid was applied 72 hours after decapitation, the oldest of the cambial derivatives on the xylem side failed to develop into fibers although younger cells did. These results suggest that auxin has its direct effect on the induction of xylem differentiation rather than the induction of divisions prerequisite to differentiation.     

Short-term Effects of Some Chemicals on Cambial Activity

Aqueous solutions of indol-3yl-acetic acid (IAA), 1-naphthyl-aceticacid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), gibberellicacid (GA), 6-furfuryl-aminopurine (FAP), myo-inositol, and sucrosewere applied singly and in mixtures to the apical ends of disbuddedstem segments of willow. After 4 weeks all substances had hadsome effect on differentiation of xylem from cambial derivatives.The production of potential xylem cells, as well as their differentiationwere most markedly enhanced when IAA, GA, and FAP were appliedtogether, although the response was further augmented by additionof inositol or sucrose. The action of the substances when appliedas mixtures was often synergistic. This means that it is difficultto assess the role of different chemicals in xylem productionby extrapolation from experiments involving the applicationof single substances.     

Effects of temperature and sink activity on the transport of 14C-labelled indol-3yl-acetic acid in the intact pea plant (Pisum sativum L.)

The velocity and intensity of basipetal transport of ¹⁴C-labelled indol-3yl-acetic acid (IAA) applied to the apical bud of the intact pea plant were influenced by the temperature to which the stem was exposed and were not influenced by changes in the temperature of the root system when this was controlled independently between 5°C and 35°C. The velocity of transport increased steadily with temperature to a maximum in excess of 35°C and then fell sharply with further increase in temperature. The Q₁₀ for velocity, determined from Arrhenius plots, was low (ca. 1.3). Transport intensity increased to a maximum at about 25°C (Q₁₀=2.2) and then declined gradually with further increase in temperature. It is suggested that transport velocity and transport intensity are controlled independently.The characteristics of auxin transport through the stem were not affected by removal of the root system, or by the withdrawl of root aeration. Labelled IAA did not pass a region of the stem cooled to about 1.0°C, or through a narrow zone of stem tissue killed by heat treatment. In the latter case the heat treatment was shown not to interfere with the upward transport of water in the xylem. Labelled IAA continued to move into, and to accumulate in, the tissues immediately above a cooled or heat-killed region of the stem. It was concluded that the long-distance basipetal transport of auxin through the stem of the intact plant is driven by the transporting cells themselves and is independent of the activity of sinks for the transported auxin.The fronts of the observed tracer profiles in the stem were closely fitted by error function diffusion analogue curves. However, diffusion of IAA alone could not account for the observed characteristics of the transport and it is suggested that the curvilinear fronts of the profiles resulted from a diffusive mixing of exogenous IAA (or IAA-carrier complexes) with endogenous IAA already in the transport pathway.Abbreviations IAA indol-3yl-acetic acid - IAAsp indol-3yl-acetyl aspartic acid - CFM methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin) - TIBA 2,3,5-triiodobenzoic acid - ABA abscisic acid     

Tracheary Element Differentiation Induced in Isolated Cylinders of Lettuce Pith: a Bipolar Gradient Technique

To study the influence of morphogenetic gradients on vasculardifferentiation patterns, a new technique was developed whichallows different substances to be applied at opposite ends ofa tissue block. It yielded information on the mobility of particularmorphogens and on the dependence of callus formation and trachearyelement differentiation on their presence. Application of indol-3ylacetic acid (1AA) (10 mg l^–1), zeatin (0.1 mg l^–1)and sucrose (3 per cent, w/v) in various combinations to theends of cylindrical explants of lettuce pith (Lactuca sativaL.) showed that (a) callus formation was stimulated by IAA,whereas induction of tracheary elements required both IAA andzeatin; (b) callus was confined to a few millimetres at theends of the explants, and tracheary elements occurred mainlywithin the callus; (c) sucrose or its metabolic products diffusedthe 10 mm length of the explants, while IAA and zeatin wereeffective only close to the application site; and (d) some callusand tracheary elements formed when no sucrose was applied, butboth increased with sucrose application, though inhibition oftracheary elements formation occurred with high sucrose concentrations. differentiation, pith explant, tissue culture, xylogenesis, indol-3yl acetic acid, sucrose, zeatin, lettuce, Lactuca sativa     

Stem anatomy of Dolichos lablab Linn (Fabaceae): Origin of cambium and reverse orientation of vascular bundles

Secondary growth in the stem of Dolichos lablab is achieved by the formation of eccentric successive rings of vascular bundles. The stem is composed of parenchymatous ground tissue and xylem and phloem confined to portions of small cambial segments. However, development of new cambial segments can be observed from the obliterating ray parenchyma, the outermost phloem parenchyma and the secondary cortical parenchyma. Initially cambium develops as small segments, which latter become joined to form a complete cylinder of vascular cambium. Each cambial ring is functionally divided into two distinct regions. The one segment of cambium produces thick-walled lignified xylem derivatives in centripetal direction and phloem elements centrifugally. The other segment produces only thin-walled parenchyma on both xylem and phloem side. In mature stems, some of the axial parenchyma embedded deep inside the xylem acquires meristematic activity and leads to the formation of thick-walled xylem derivatives centrifugally and phloem elements centripetally. The secondary xylem comprises vessel elements, tracheids, fibres and axial parenchyma. Rays are uni-multiseriate in the region of cambium that produces xylem and phloem derivatives, while in some of the regions of cambium large multiseriate, compound, aggregate and polycentric rays can be noticed.     

Some quantitative effects of indoleacetic acid on the wood production and tracheid dimensions of Picea

The diameter and wall thickness of tracheids produced after indoleacetic acid treatment were not significantly different from those of the intact controls, for the first few weeks after treatment of disbudded shoots of Picea abies Karst. and Picea sitchensis (Bong.) Carr. However, lateral application of indoleacetic acid (IAA) to intact shoots increased both tracheid diameter and wall thickness; it is suggested that IAA acted synergistically with another endogenous growth regulator, which was also removed by disbudding. Increase in wall thickness after exogenous IAA was associated with increase in duration of the wall thickening phase of tracheid differentiation; this is discussed in relation to the seasonal change from early to latewood. Cambial dormancy was induced by disbudding during active wood production. Since this occurred with or without the presence of current leaves, it is concluded that in Picea continued cambial activity depends upon supply of auxin from the buds, and cannot be supplied from expanded leaves or from the internode itself. Neither indoleacetic acid nor gibberellic acid stimulated renewed cambial activity when applied after the cessation of wood production. With both disbudded and intact shoots, the effectiveness of exogenous IAA declined with time, probably due to decreasing penetration through callus developing at the wounded surface. It is suggested that this apparent change in IAA effectiveness may explain some discrepancies between the results of previous observers.Abbreviations IAA Indoleacetic acid - GA₃ Gibberellic acid     

Developmental changes in the secondary xylem ofAcer rubrum induced by gibberellic acid,various auxins and 2,3,5-tri-iodobenzoic acid

Summary The effect of GA on the development of the secondary xylem ofAcer rubrum was studied. GA was applied to seedlings (1) separately, (2) simultaneously with TIBA, and (3) simultaneously with TIBA and an auxin. GA alone is without effect on the development of the secondary xylem. In seedlings treated with TIBA-GA or TIBA-GA-auxin, the differentiation of the xylem elements is in general similar to that of seedlings treated with TIBA or TIBA-auxin respectively. GA however, does stimulate cambial activity in a localized region of the stem if the endogenous rate of cambial division is low.The following abbreviations will be used TIBA (2,3,5-tri-iodobenzoic acid) - IAA (indole-3-acetic acid) - GA (gibberellic acid) - NAA (naphthaleneacetic acid) - 2,4-D (2,4-dichlorophenoxyacetic acid) This material was included in a doctoral thesis submitted by P. R.Morey to the graduate school of Yale University, New Haven.     

Components of auxin transport in stem segments of Pisum sativum L.

1. The uptake of indol-3-yl acetic acid ([1-¹⁴C]IAA, 0–2.0 M) into light-grown pea stem segments was measured under various conditions to investigate the extent to which mechanisms of auxin transport in crown gall suspension culture cells (Rubery and Sheldrake, Planta 118, 101–121, 1974) are also found in a tissue capable of polar auxin transport. — 2. IAA uptake increased as the external pH was lowered. IAA uptake was less than that of benzoic acid (BA), naphthylacetic acid (NAA) or 2,4 dichlorophenoxyacetic acid (2,4D) under equivalent conditions. TIBA enhanced net IAA uptake through inhibition of efflux, and to a lesser extent, also increased uptake of NAA and 2,4D while it had no effect on BA uptake. — 3. Both DNP and, at higher concentrations, BA, reduced IAA uptake probably because of a reduction of cytoplasmic pH. However, low concentrations of both BA and DNP caused a slight enhancement of IAA net uptake, possibly through a reduction of carrier-mediated IAA efflux. In the presence of TIBA, the inhibitory effects of DNP and BA were more severe and there was no enhancement of uptake at low concentrations. — 4. Non-radioactive IAA (10 M) reduced uptake of labelled IAA but further increases in concentration up to 1.0 mM produced first an inhibition (0–10 min) of labelled IAA uptake, followed by a stimulation at later times. Non-radioactive 2,4 D decreased, but was not observed to stimulate, uptake of labelled IAA. In the presence of TIBA labelled IAA uptake was inhibited by non-radioactive IAA regardless of its concentration. — 5. Sulphydryl reagents PCMB and PCMBS promoted or inhibited IAA uptake depending, respectively, on whether they penetrated or were excluded from the cells. The penetrant PCMB also reduced the promotion of labelled IAA uptake by TIBA or by high concentrations of added non-labelled IAA. — 6. Our findings are interpreted as being consistent with the diffusive entry of unionised IAA into cells together with some carrier-mediated uptake. Auxin efflux from the cells also appears to have a carrier-mediated contribution, at least part of which is inhibited by TIBA, and which has a capacity at least as great as that of the uptake carrier. The data indicate that pea stem segments contain cells whose mechanisms of trans-membrane auxin transport fit the model of polar auxin transport proposed from experiments with crown gall suspension cells, although differences, particularly of carrier specificity, are apparent between the two systems.Abbreviations IAA indol-3-yl acetic acid - BA benzoic acid - NAA 1-naphthylacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - TIBA 2,3,5-triiodobenzoic acid - DNP 2,4-dinitrophenol - PCMB p-chloromercuribenzoic acid - PCMBS p-chloromercuribenzene sulphonic acid This work was performed in Cambridge during the tenure of a sabbatical leave by P.J.D. Supported by a grant for supplies from the American Philosophical Society to P.J.D.     

Non-uniform distribution and seasonal variation of endogenous indol-3yl-acetic acid in the cambial region of Pinus contorta Dougl.

Endogenous, free indol-3yl-acetic acid (IAA) levels were measured in the main stem in the 10-year-old cambial zone, in the adjoining differentiating xylem, and in the adjoining mature xylem of 15–20-year-old Pinus contorta Dougl. by single-ion-current monitoring, combined gas chromatography — mass spectrometry, on several dates from early spring to early winter. Microscopy was used to determine the state of cambial activity on each harvest date. The IAA levels were found to be nearly constant at 1 g g^-1 DW in the cambial zone from March to July, then to increase to near 2 g g^-1 DW during the remainder of the growth season. No clear correlation was evident between number of fusiform cells per radial file and IAA content in the cambial zone. By contrast, the IAA content in differentiating xylem was higher than that in the adjoining meristematic zone on all harvest dates and also exhibited marked seasonal variation, peaking near 16 g g^-1 DW in mid summer, and declining to 1 g g^-1 DW in autumn. In mature xylem, IAA levels were very low and showed negligible variation. The fresh weight to dry weight ratio of differentiating xylem was greater than that of the cambial zone, and greater in the cambial zone than in mature xylem.     

Level of endogenous indole-3-acetic acid in the stem of Pinus sylvestris in relation to the seasonal variation of cambial activity

Abstract. Gas chromatography – selected ion monitoring – mass spectrometry was used to measure the level of indole-3-acetic acid (IAA) in the cambial region at the top and bottom of the branchless portion of the main stem of three large Scots pine trees, at weekly intervals from 28 April to 13 July. During this period, the cambium reactivated from the dormant state and entered its 'grand' period of xylem and phloem production, which was monitored by microscopy. The total amount of IAA (ng cm⁻²) increased steadily from 28 April until late June, and thereafter remained constant. In contrast, the concentration of IAA (ng g⁻¹ fresh weight) was high at the start of cambial reactivation, declined when the number of differentiating tracheids began to increase, and then rose as the number of cells decreased. The timing and magnitude of the changes in xylem and phloem production and in IAA level were similar at the two sampling positions. It is concluded that the seasonal changes in cambial activity in the conifer stem cannot be ascribed simply to a fluctuation in the level of endogenous IAA in the cambial region.     

The Effect of Applied Growth Hormones on Cambial Division and the Differentiation of the Cambial Derivatives

When indole-3-acetic acid (IAA) is applied to woody shoots cambialdivision is stimulated and the cambial derivatives differentiateto produce xylem tissue. When gibberellic acid (GA) is applied,cambial division occurs but the resultant derivatives on thexylem side of the cambium remain undifferentiated. The relativelevels of applied IAA and GA are important in determining whethermainly xylem or phloem tissue is produced. High IAA/low GA concentrationsfavour xylem formation, whereas low IAA/high GA concentrationsfavour phloem production. The new phloem tissue produced asa result of hormone treatment is fully differentiated, containingsieve elements and sieve plates. IAA is important in promotingthe elongation of the cambial derivatives to produce xylem vesseland fibre elements, though in the case of xylem fibres appliedGA causes further elongation. IAA is an important factor indetermining vessel diameter in the ring-porous species Robiniapseudacacia, high levels of applied IAA giving wide springwood-typevessels and low levels giving narrow ‘summerwood’vessels.     

Relationships between cambial activity,cell differentiation and the localization of starch in storage tissues around the cambium in locally heated stems of<Emphasis Type="Italic"> Abies sachalinensis</Emphasis> (Schmidt) Masters

A study was made, in a cool-temperate zone, of the extent of cell division in the cambium, the extent of differentiation of cambial derivatives, and the localization of storage starch around the cambium in locally heated (22–26°C) stems of the evergreen conifer Abies sachalinensis (Schmidt) Masters during cambial dormancy and immediately after natural reactivation of the cambium. In locally heated regions of stems during cambial dormancy, heating induced localized reactivation of the cambium. However, the cells in the heated and reactivated cambium stopped dividing soon after only a few cells had been generated. In addition, no differentiation of the xylem and the disappearance of starch from storage tissues around the cambium were observed. In regions of stem that had been locally heated after natural reactivation of the cambium, cell division continued in the cambium and earlywood tracheids with a large radial diameter and secondary walls were formed, with abundant starch in the storage tissues around the cambium. Our results suggest that the extent of both cell division in the cambium and cell differentiation depends on the amount of starch in storage tissues around the cambium in the locally heated stems of an evergreen conifer growing in a cool-temperate zone.     

Indole-3-acetic acid and 2-(indol-3-ylmethyl)indol-3-yl acetic acid in the thermophilic archaebacterium Sulfolobus acidocaldarius.

Indole-3-acetic acid (IAA) and 2-(indol-3-ylmethyl)indol-3-yl acetic acid were identified in lipid extracts of Sulfolobus acidocaldarius; they occurred at concentrations of 0.57 and 0.59 mumol/g (dry weight), respectively. The amount of IAA found in these cells is more than a thousand times greater than that found in a typical extract of a plant in which IAA serves as a plant growth hormone. Neither of these compounds was detected in the other archaebacteria that were analyzed; these included Sulfolobus sulfataricus, Halobacterium salinarium, and several strains of methanogenic bacteria. This is the first report of the natural occurrence of 2-(indol-3-ylmethyl)indol-3-yl acetic acid.     

Localized cooling of stems induces latewood formation and cambial dormancy during seasons of active cambium in conifers

Background and Aims In temperate regions, trees undergo annual cycles of cambial growth, with periods of cambial activity and dormancy. Environmental factors might regulate the cambial growth, as well as the development of cambial derivatives. We investigated the effects of low temperature by localized cooling on cambial activity and latewood formation in two conifers, Chamaecyparis obtusa and Cryptomeria japonica.Methods A plastic rubber tube that contained cooled water was wrapped around a 30-cm-wide portion of the main stem of Chamaecyparis obtusa and Cryptomeria japonica trees during seasons of active cambium. Small blocks were collected from both cooled and non-cooled control portions of the stems for sequential observations of cambial activity and for anatomical measurements of cell morphology by light microscopy and image analysis.Key Results The effect of localized cooling was first observed on differentiating tracheids. Tracheids narrow in diameter and with significantly decreased cambial activity were evident 5 weeks after the start of cooling in these stems. Eight weeks after the start of cooling, tracheids with clearly diminished diameters and thickened cell walls were observed in these stems. Thus, localized low temperature induced narrow diameters and obvious thickening of secondary cell walls of tracheids, which were identified as latewood tracheids. Two months after the cessation of cooling, a false annual ring was observed and cambium became active again and produced new tracheids. In Cryptomeria japonica, cambial activity ceased earlier in locally cooled portions of stems than in non-cooled stems, indicating that the cambium had entered dormancy sooner in the cooled stems.Conclusions Artificial cooling of stems induced latewood formation and cessation of cambial activity, indicating that cambium and its derivatives can respond directly to changes in temperature. A decrease in the temperature of the stem is a critical factor in the control of cambial activity and xylem differentiation in trees.     

Endogenous indole-3-acetic acid and ethylene evolution in tilted<Emphasis Type="Italic"> Metasequoia glyptostroboides</Emphasis> stems in relation to compression-wood formation

Eight-year-old Metasequoia glyptostroboides seedlings were tilted at a 45° angle to induce compression-wood formation on the lower side of the stems. After 2 weeks of treatment, half of the seedlings were sampled and the remaining half were tilted to the opposite orientation to exchange the upper and lower sides and were kept for 2 more weeks until sampled. Cambium-emitted ethylene was analyzed by gas chromatography with flame-ionization detection. Endogenous indole-3-acetic acid (IAA) was measured by gas chromatography-mass spectrometry. Tracheid production and compression-wood formation were determined by light microscopy. Anatomical studies showed that tracheid production was promoted and compression-wood tracheids always developed on the gravitationally lower side of tilted stems in both the original tilting and the subsequent reverse-tilting periods. These were accompanied by an increase in IAA content in and an accelerated ethylene-evolution rate from the cambial region of the same side.     

Storage Organ Development in Radish (Raphanus sativus L.). 2. Effects of Growth Promoters on Cambial Activity in Cultured Roots, Decapitated Seedlings and Intact Plants

The radish varieties Cherry Belle and Long White Icicle wereused to investigate the role of the shoot and the effects ofsynthetic growth promoters in controlling cambial activity inthe seedling axis. Development was compared in excised roots, roots with hypocotylsattached and intact seedlings cultured aseptically on a nutrientmedium. No cambial divisions were seen in isolated radicleswhich had been cultured for ten days following excision butretention of hypocotyl tissue or the entire shoot resulted incambial activity and the production of secondary vascular tissues.Enriching the culture medium by raising the sucrose conantrationto 8% and including 10^–5 M indol-3yl acetic acid (IAA)5 x 10^–6 M 6-benzylaminopurine (BA) and 5 x 10^–4Minositol enhanced root thickening, increasing stele and xylemdiameters in roots cultured both with and without attached shoottissues. The effects of shoot tissues and enrichment of themedium were additive. The effects of auxin, cytokinin and gibberellin (gibberellicacid, GA²) were also studied on daxpitated seedlings. BA wasmuch more effective in inducing cell divisions in the hypocotylthan either IAA or GA supplied separately but a mixture of IAA+GAalso produced clearly defined arcs of cambial tissue. Littlesecondary tissue had been produced after seven days' treatment,and stelar enlargement was due to the development of a cambialzone and cell expansion in the primary tissues. Only minor differencesin response were observed between the two varieties. No stimulation of storage organ development occurred when auxin,cytokinin or inositol was inwrporated into the inorganic culturesolution in which plants of Cherry Belle were grown. Rnphanus sarivus, radish, storage organ, cambial activity, growth promoters, indol-3-ylacetic acid, 6-benzylaminopurine, gibberellic acid     

Effect of indole-3-acetic acid (IAA) and sucrose on vessel size and density in isolated stem segments of oak (Quercus robur)

The effects of several concentrations of indole-3-acetic acid (IAA) and sucrose on xylogenic cambial activity and secondary xylem differentiation were investigated in isolated stem segments of Quercus robur L. supplied with liquid medium in aseptic conditions. After 5 weeks of culture auxin controlled cambial cell division and the number and size of vessel elements even without sugar in the medium. Sucrose modified these IAA effects, although little cambial activity occurred without auxin. The xylem increment correlated with changes of auxin concentration with the optimum at 28.5 μ M IAA. The formation of wide vessels was correlated with the optimal concentration of auxin. The frequency of vessel differentiation increased with auxin concentration. High concentrations of sucrose (0.24 M and 0.96 M ) reduced both the number of vessels and their diameter. The frequency of vessel formation was inhibited more than the vessel size by changes of sugar concentration. The vessels formed under low concentrations of IAA were circular in transverse section. With increase in IAA concentration the shape of the vessel cross-section changed to oval with the largest dimension in the radial direction.