Total epidermal cell walls of pea stems respond differently to auxin than does the outer epidermal wall alone

The effect of auxin on cell wall mass in the epidermis of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using epidermal peels, to determine whether epidermal peels reflect the behavior of the outer epidermal cell wall. In contrast to the outer epidermal wall itself, where auxin caused thinning in proportion to growth (M.S. Bret-Harte et al, 1991, Planta 185, 462–471), auxin promoted an increase in wall mass in epidermal peels from treated internode segments in the absence of exogenously supplied sugar. The percentage gain in mass was smaller than the percentage elongation, however, so mass per unit length decreased in peels from auxin-treated segments. Epidermal peels from auxin-treated segments gained more wall mass than control peels even when adhering internal tissue at the basal end of the peel was removed. Epidermal peels also had a gross composition different from that of the outer wall alone (M.S. Bret-Harte and L.D. Talbott, 1993, Planta 190, 369–378). These discrepancies can be explained by the observation that the outer wall makes up only 30% of the mass of the epidermal peel. It appears that the inner walls of the epidermis, and walls of the outer layer of cortical cells that remain attached to the epidermis during peeling, nearly maintain their thickness by biosynthesis while the outer wall loses mass as previously described (Bret-Harte et al. 1991). These results indicate that epidermal peels may not be a good system for examining the biochemical and physiological properties of the outer epidermal cell wall.I would like to thank Dr. Peter M. Ray, of Stanford University, for the use of experimental facilities, helpful discussions, and technical and editorial assistance, Dr. Winslow R. Briggs, of the Carnegie Institute of Washington, for helpful discussions and for the use of experimental facilities, Dr. Paul B. Green, of Stanford University, for financial support, and Dr. Wendy K. Silk, of the Department of Land, Air, and Water Resources, University of California, Davis, for financial support. This work was supported by a National Science Foundation Graduate Fellowship, National Science Foundation grant DCB8801493 to Paul B. Green, and the generosity of Wendy K. Silk in the final writing.     

Auxin stimulates both deposition and breakdown of material in the pea outer epidermal cell wall,as measured interferometrically

The effect of auxin on the mass per area in the outer epidermal walls of third internodes of Pisum sativum L. cv. Alaska grown in dim red light was investigated using interference microscopy, and rates of net deposition of wall material were calculated. Examination of these net rates under different growth conditions showed that there is no simple relationship between the deposition of mass and growth. Net deposition can be proportional to growth when sufficient substrate for wall synthesis is available, as in intact plants, and in segments treated with indole-3-acetic acid (IAA) plus glucose. Net deposition can cause thickening of the walls when growth is small, as in the case of segments kept without IAA in the presence or absence of glucose, or segments whose growth is inhibited with mannitol. When substrate is limited and growth is large, however, wall expansion can occur with no net deposition, or an actual net loss of wall material can even take place. Auxin appears to induce a breakdown in the walls of segments treated in the absence of glucose, although it promotes synthesis when glucose is present. It is likely that IAA always induces a breakdown of wall material, but that the breakdown is masked when substrate is available for synthesis. Our results indicate that pea epidermal cells have two different auxin-stimulated mechanisms, wall synthesis and wall breakdown, potentially available to loosen their outer epidermal walls to bring about cell enlargement, alternatives which could be employed to different extents depending on substrate conditions.Abbreviation IAA indole-3-acetic acid M.S. Bret-Harte would like to thank Drs. Peter M. Ray, Stanford University, Winslow R. Briggs, Carnegie Institute of Washington, Stanford, Calif. USA, and Wendy K. Silk, of the University of California Davis USA, for helpful discussions, Dr. Briggs and the Carnegie Institute of Washington for the use of experimental facilities, and Dr. Ray for editorial assistance. This work was supported by a National Science Foundation Graduate Fellowship to M.S.B.-H., a National Science Foundation Postdoctoral Fellowship to T.I.B., and National Science Foundation grant DCB8801493 to P.B.G.     

Cytochemical identification of arabinogalactan protein in the outer epidermal wall of maize coleoptiles

Artificial carbohydrate antigen (Yariv reagent), fluorescence-labeled -l-fucose-binding lectin, and -D-galactose-binding lectin were used to localize arabinogalactan protein in sections of maize (Zea mays L.) coleoptiles. All three probes bind to cell walls of vascular tissue and the outer epidermis. Intense staining is obtained at the outer and inner faces of the growth-controlling outer epidermal wall. At the inner face of this wall the auxin-inducible osmiophilic particles, hitherto observed only by electron microscope (Kutschera et al. 1987, Planta 170, 168–180), are strongly stained by all three probes and can therefore be identified as deposits of arabinogalactan protein. It is proposed that this proteoglycan acts as an epidermal wallloosening factor in auxin-mediated coleoptile growth.Abbreviation AGP arabinogalactan protein I thank Dr. R. Bergfeld for the electron micrograph of Fig. 13. This work was supported by the Deutsche Forschungsgemeinschaft.     

The role of the epidermis in auxin-induced and fusicoccin-induced elongation of Pisum sativum stem segments

The effects of peeling and wounding on the indole-3-acetic acid (IAA) and fusicoccin (FC) growth response of etiolated Pisum sativum L. cv. Alaska stem tissue were examined. Over a 5 h growth period, peeling was found to virtually eliminate the IAA response, but about 30% of the FC response remained. In contrast, unpeeled segments wounded with six vertical slits exhibited significant responses to both IAA and FC, indicating that peeling does not act by damaging the tissue. Microscopy showed that the epidermis was removed intact and that the underlying tissue was essentially undamaged. Neither the addition of 2% sucrose to the incubation medium nor the use of a range of IAA concentrations down to 10^-8 M restored IAA-induced growth in peeled segments, suggesting that lack of osmotic solutes and supra-optimal uptake of IAA were not important factors over this time period. It is concluded that, although the possibility remains that peeling merely allows leakage of hydrogen ions into the medium, it seems more likely that peeling off the epidermis removes the auxin responsive tissue.Abbreviations IAA indole-3-acetic acid - FC fusicoccin     

Changes in cell-wall polysaccharide composition of developingNitella internodes

Changes in the uronide, neutral-polysacharide, and cellulose composition of the cell wall ofNitella axillaris Braun were followed throughout development of the internodes and correlated with changes in growth rate. As the cells increased in length from 4 to 80 mm during development, the relative growth rate decreased. Cell wall thickness, as measured by wall density, increased in direct proportion to diameter, indicating that cell-wall stress did not change during elogation. Cell-wall analyses were adapted to allow determination of the composition of the wall of single cells. The total amounts of uronides, neutral sugars and cellulose all increased during development. However, as the growth rate decreased, the relative proportions of uronides and neutral sugars, expressed as percent of the dry weight of the wall, decreased, while the proportion of cellulose increased. The neutral sugars liberated upon hydrolysis ofNitella walls are qualitatively similar to those found in hydrolysates of higher plant cell walls: glucose, xylose, mannose, galactose, arabinose fucose and rhamnose. Only the percentage of galactose was found to increase in walls of mature cells, while the percentage of all other sugars decreased. The rate of apposition (g of wall material deposited per unit wall surface area per hour) of neutral polysaccharides decreased rapidly with decreasing growth rate during the early stages of development. The rate of apposition of uronides decreased more steadily throughout development, while that of cellulose, after an early decline, remained constant until dropping off at the end of the elongation period. These correlations between decreasing growth rate and decreasing rate of apposition of neutral sugars and uronides indicate that synthesis of these cell-wall components could be involved in the regulation of the rate of cell elongation inNitella.     

Changes in microfibril arrangement on the inner surface of the epidermal cell walls in the epicotyl of Vigna angularis ohwi et ohashi during cell growth

Throughout the entire period of cell growth, the microfibrils on the inner surface of the outer tangential walls of the epidermal cells of Vigna angularis epicotyls are running parallel to one another and their orientation differs from cell to cell. Although transverse, oblique and longitudinal microfibrils can be observed irrespective of cell age, the frequency distribution of microfibril orientation changes with age. In young cells, transversely oriented microfibrils predominate. In cells of medium age, which are still undergoing elongation, transverse, oblique and longitudinal microfibrils are present in quite similar frequencies. In old, non-growing cells, longitudinally oriented microfibrils are predominent. A decrease in the relative frequency of transversely oriented microfibrils with cell age was also observed in the radial epidermal walls.     

Spatial distribution of growth rates and of epidermal cell lengths in the elongation zone during leaf development in Lolium perenne L.

Relative elemental growth rates (REGR) and lengths of epidermal cells along the elongation zone of Lolium perenne L. leaves were determined at four developmental stages ranging from shortly after emergence of the leaf tip to shortly before cessation of leaf growth. Plants were grown at constant light and temperature. At all developmental stages the length of epidermal cells in the elongation zone of both the blade and sheath increased from 12 m at the leaf base to about 550 m at the distal end of the elongation zone, whereas the length of epidermal cells within the joint region only increased from 12 to 40 m. Throughout the developmental stages elongation was confined to the basal 20 to 30 mm of the leaf with maximum REGR occurring near the center of the elongation zone. Leaf elongation rate (LER) and the spatial distributions of REGR and epidermal cell lengths were steady to a first approximation between emergence of the leaf tip and transition from blade to sheath growth. Elongation of epidermal cells in the sheath started immediately after the onset of elongation of the most proximal blade epidermal cells. During transition from blade to sheath growth the length of the blade and sheath portion of the elongation zone decreased and increased, respectively, with the total length of the elongation zone and the spatial distribution of REGR staying near constant, with exception of the joint region which elongated little during displacement through the elongation zone. Leaf elongation rate decreased rapidly during the phase when only the sheath was growing. This was associated with decreasing REGR and only a small decrease in the length of the elongation zone. Data on the spatial distributions of growth rates and of epidermal cell lengths during blade elongation were used to derive the temporal pattern of epidermal cell elongation. These data demonstrate that the elongation rate of an epidermal cell increased for days and that cessation of epidermal cell elongation was an abrupt event with cell elongation rate declining from maximum to zero within less than 10 h.Abbreviations LER leaf elongation rate - REGR relative elemental growth rates     

Distribution of pectins in cell walls of maize coleoptiles and evidence against their involvement in auxin-induced extension growth

Summary Aiming to elucidate the possible involvement of pectins in auxin-mediated elongation growth the distribution of pectins in cell walls of maize coleoptiles was investigated. Antibodies against defined epitopes of pectin were used: JIM 5 recognizing pectin with a low degree of esterification, JIM 7 recognizing highly esterified pectin and 2F4 recognizing a pectin epitope induced by Ca²⁺. JIM 5 weakly labeled the outer third of the outer epidermal wall and the center of filled cell corners in the parenchyma. A similar labeling pattern was obtained with 2F4. In contrast, JIM 7 densely labeled the whole outer epidermal wall except the innermost layer, the middle lamellae, and the inner edges of open cell corners in the parenchyma. Enzymatic de-esterification with pectin methylesterase increased the labeling by JIM 5 and 2F4 substantially. A further increase of the labeling density by JIM 5 and 2F4 and an extension of the labeling over the whole outer epidermal wall could be observed after chemical de-esterification with alkali. This indicates that both methyl- and other esters exist in maize outer epidermal walls. Thus, in the growth-controlling outer epidermal wall a clear zonation of pectin fractions was observed: the outermost layer (about one third to one half of wall thickness) contains unesterified pectin epitopes, presumably cross-linked by Ca²⁺ extract. Tracer experiments with³H-myo-inositol showed rapid accumulation of tracer in all extractable pectin fractions and in a fraction tightly bound to the cell wall. A stimulatory effect of IAA on tracer incorporation could not be detected in any fraction. Summarizing the data a model of the pectin distribution in the cell walls of maize coleoptiles was developed and its implications for the mechanism of auxin-induced wall loosening are discussed.Abbreviations CDTA trans-1,2-diaminocyclohexane-N,N,N,N-tetraacetic acid - CWP cell-wall pellet - IAA indole-3-acetic acid - LSE low-salt extract - TCA trichloroacetic acid; Tris tris-(hydroxy-methyl)aminoethane     

Microtubule orientation in pea stem cells: a change in orientation follows the initiation of growth rate decline

The orientation of microtubules in cells of redlight-grown pea plants (Pisum sativum L.) was examined by means of immunofluorescence. Microtubules (MTs) in rapidly elongating, subepidermal cells commonly form multiple, parallel strands that run transverse to the cell's axis of elongation. By contrast, MTs in nonelongating subepidermal cells form steeply pitched helical arrays; MTs in non-elongating epidermal cells are oriented parallel to the axis of elongation. This change in orientation occurs during the time interval in which growth rate is declining. The transition is abrupt rather than gradual and occurs in both epidermal and subepidermal cells at the same time. Plants irradiated for 2 h with a growth-inhibiting fluence of blue light did not undergo the same transition, indicating that factors other than changing elongation rates must be responsible for triggering the reorganization of MT arrays.     

Indole-3-acetic acid levels after phytochrome-mediated changes in the stem elongation rate of dark- and light-grownPisum seedlings

The effect of red (R) and far-red (FR) light on stem elongation and indole-3-acetic acid (IAA) levels was examined in dwarf and tall Pisum sativum L. seedlings. Red light reduced the extension-growth rate of etiolated seedlings by 70–90% after 3 h, and this inhibition was reversible by FR. Inhibition occurred throughout the growing zone. After 3 h of R, the level of extractable IAA in whole stem sections from the growing zone of etiolated plants either increased or showed no change. By contrast, extractable IAA from epidermal peels consistently decreased 3 h after R treatments. Decreases of 40% were observed for epidermal peels from the top 1 cm of tall plants receiving 3 h R. Brief R treatments resulted in smaller decreases in epidermal IAA levels and these decreases were not as great when FR followed R. In lightgrown plants, end-of-day FR stimulated growth during the following dark period in a photoreversible manner. The uppermost 1 cm of expanding third internodes was most responsive to the FR. Extractable IAA from epidermal peels from the upper 1 cm of third internodes increased by 30% or more 5 h after FR. When R followed the FR the increases were smaller. Levels of IAA in whole stem sections did not change and were twofold greater than in dark-grown plants. In both dark- and light-grown tall plants, IAA levels were lower in epidermal peels than in whole stem segments. These results provide evidence that IAA is compartmentalized at the tissue level within the growing stem and that phytochrome regulation of stem elongation rates may be partly based on modulating the level of IAA within the epidermis.Abbreviations IAA indole-3-acetic acid - R red light - FR farred light We thank Yu-Xian Zhu for helping to develop methods for IAA analysis, James Reid for supplying the genetic lines of Pisum and Richard Cyr for the use of microscopy equipment. This work was supported by NSF grant DCB-8801880 and by Hatch funds from the College of Agriculture and Life Sciences at Cornell University. The gas chromatograph-mass spectrometer was funded by NSF grant DMB-8505974 and funds from the College of Agriculture and Life Sciences at Cornell University. A preliminary report of some of these experiments has appeared in Plant Growth Substances, 1991 (Behringer et al. 1992 b).     

Ethylene and auxin-induced cell growth in relation to auxin transport and metabolism and ethylene production in the semi-aquatic plant,Regnellidium diphyllum

Cell elongation in the rachis of the semiaquatic fern Regnellidium diphyllum is induced by the addition of ethylene or indoleacetic acid (IAA). Experiments with whole plants or rachis segments have shown that ethylene-induced growth requires the presence of auxin. Ethylene does not cause a modification in either endogenous auxin levels or in the extent of auxin metabolism but auxin transport is reduced. Rates of ethylene production in Regnellidium are not altered by either mechanical excitation or by the addition of auxin. A two-hormone control of cell expansion is proposed in which an initial, auxin-dependent growth event pre-conditions the cells to a further subsequent (or synchronous) ethylene-dependent growth event.Abbreviation IAA indole-3yl-acetic acid     

Composition of the cell wall of Chlorella fusca

Isolated cell walls of mature Chlorella fusca consisted of about 80% carbohydrate, 7% protein, and 13% unidentified material. Mannose and glucose were present in a ratio of about 2.7:1 and accounted for most of the carbohydrate. Minor components were glucuronic acid, rhamnose, and traces of other sugars; galactose was absent. After treatment with 2 M trifluoroacetic acid or with 80% acetic acid/HNO₃ (10/1, v/v), a residue with a mannose/glucose ratio of 0.3:1 was obtained, probably representing a structural polysaccharide. An X-ray diffraction diagram of the walls showed one diffuse reflection at 0.44 nm and no reflections characteristic of cellulose. Walls from young cells contained about 51% carbohydrate, 12% protein, and 37% unidentified material. Mannose and glucose were also the main sugars; their absolute amounts per wall increased 6–7 fold during cell growth. Walls isolated with omission of a dodecylsulphate/mercaptoethanol/urea extraction step had a higher protein content and, with young walls, a significantly higher glucose and fucose content. These data and other published cell wall analyses show a wide variability in cell wall composition of the members of the genus Chlorella.Abbreviations GLC gas liquid chromatography - TFA trifluoroacetic acid     

The roles of cell-wall acidification and proton-pump stimulation in auxin-induced growth: studies using monensin

The carboxylic ionophore, monensin, rapidly induced cell-wall acidification and a decrease in cytosolic pH when added to maize coleoptiles at low external pH and Na⁺ concentration. Elongation growth at rates equivalent to those obtained with indole-3-acetic acid was induced for about 1 h. Stimulation of the outwardly directed proton pump apparently occurred, since under the same conditions monensin induced membrane hyperpolarization of maize root rhizodermis cells. When the external pH was high (>8) and Na⁺ present, monensin treatment caused only minimal changes in membrane potential and cytosolic pH. Although the ionophore transported protons out of the cell, resulting in cell-wall acidification, no elongation growth occurred. However, under identical conditions, indole-3-acetic acid dit induce growth. The data indicates that stimulation of the outwardly directed electrogenic proton pump rather than the subsequent acidification of the cell wall is vital for the induction of elongation growth.Abbreviations CFA₂ 6-carboxyfluorescein diacetate - FA₂ fluorescein diacetate - Hepes 4-(2-hydroxyethyl-1-piperazinepropanesulfonic acid - IAA indole-3-acetic acid - Mes 2-(N-morpholino) ethanesulfonic acid - Tris 2-amino-2-(hydroxymethyl)-1,3-propanediol     

Regulation of polysaccharide breakdown during auxin-induced cell wall loosening

Auxin induces cell elongation by increasing the extensibility of the cell wall. Biochemical modifications of wall constituents lead to such changes in the mechanical properties of the cell wall (wall loosening). The results obtained in the studies using antibodies and lectins as specific probes indicate that the breakdown of xyloglucans in dicotyledons and (1→3), (1→4)-β-glucans in Poaceae is involved in auxin-induced wall loosening. In dicotyledons, xyloglucans are degraded by the direct hydrolysis with an endoglucanase to oligosaccharides and by the two-step reaction via a product with intermediate size. (1→3), (1→4)-β-Glucan breakdown in Poaceae coleoptiles is mediated by the two-step reaction with endo-and exoglucanases. Although auxin inducesde novo synthesis of some hydrolases involved in breakdown of these polysaccharides, the breakdown activity is also regulated by the wall environment such as pH, by the mobility of hydrolases through wall networks, by the interaction of hydrolases with wall polysaccharide complex, and by the presence and the concentrations of different types of regulatory molecules. Recipient of the Botanical Society Award of Young Scientists, 1992.     

The kinetics of bidirectional growth of stem sections from etiolated pea seedlings in response to acid,auxin and fusicoccin

Growth in length and diameter of abraded stem sections from etiolated pea (Pisum sativum L.) seedlings was monitored continuously using a double laser optical level auxanometer system. Acidic solutions (pH 4.0–4.5) induced rapid elongation accompanied by lateral shrinkage (up to 8% of the initial diameter). The shrinkage phase lasted for 30–45 min. Pretreatment with permeant solutes (KCl, NaCl, sucrose or glucose) prevented lateral shrinkage, while pretreatment with the impermeant solute, polyethylene glycol, did not block lateral contraction in response to acid. A slight turgor step-up given during the shrinkage phase inhibited lateral shrinkage and increased the elongation rate. Visual observation confirmed that shrinkage occurred and that the same region of the stem that contracted in diameter also elongated. It is proposed that lateral shrinkage results from a decrease in turgor pressure during acid-stimulated elongation. Elongation induced by auxin and fusicoccin (FC) was also accompanied by a decrease in the diameter; this decrease could be prevented by pretreatment with KCl or glucose. Thus, the early phase of auxin and FC action is acid-like. However, the shrinkage is of shorter duration (14–20 min) and it is less drastic (ca. 2%). In addition, FC caused lateral expansion after a 20-min lag period in stems pretreated with KCl. The results are consistent with an acid-growth mechanism during the early phase (first 20–40 min) of the responses to both auxin and FC. It is suggested that enhanced osmoregulation subsequently inhibits further lateral shrinkage and helps to maintain steady-state growth. FC, unlike auxin, may alter the anisotropic character of the wall.Abbreviations FC fusicoccin - IAA indole-3-acetic acid - LOLA laser optical levar auxanometer - PEG polyethyleneglycol 600     

Ionic currents flow throughMicrasterias andClosterium cells during expansion of the primary cell wall

The results of studies of Micrasterias rotata (Grev.) Ralfs, M. thomasiana Archer (biradiate and uniradiate forms) and Closterium sp. using one- and two-dimensional vibrating probes show that transcellular ionic currents are detectable only around cells undergoing expansion of the primary cell wall (half-cell); current enters local regions of expansion and exits over both the rigid surface of the secondary wall and regions of the primary wall where hardening of the wall prevents further expansion. Current densities remain at steady levels until expansion stops with maturation of the primary wall, whereupon currents are no longer detectable. The temporal and spatial correlation between the currents and regions of wall expansion is particularly evident because morphogenesis of the half-cell is a determinate process. Measurements of inward currents ranged from 0.1 to 5.4 A · cm^–2, and outward currents ranged from-0.05 to -1.5 A · cm^–2 measured at 18 from the cell surface. The results of ion substitution and channel-blocker studies indicate that the currents may be carried at least in part by Ca²⁺, Cl^–, H⁺ and K⁺ ions. The possible role of a Ca²⁺ influx during tip growth in desmids is discussed.This work was conducted at the National Vibrating Probe Facility, Marine Biological Laboratory, Woods Hole, Mass., USA. Dr. Lionel F. Jaffe, Director of the Facility, and Dr. Jeremy D. PickettHeaps, University of Colorado, Boulder, USA, provided valuable guidance and support, and gave unstinting encouragement during these studies. Dr. Franklin M. Harold provided support for the writing of this paper during C.L.T.'s postdoctoral year at the National Jewish Center for Immunology and Respiratory Research, Denver. Mr. Alan Shipley and Mr. Steve Dixon provided talented technical assistance. C.L.T. is grateful for support received from a National Institutes of Health Pre-doctoral Training Grant in the Department of Molecular, Cellular and Developmental Biology, University of Colorado. The work was supported by N.I.H. grants 5 P41 RR01395 and 3 P41 RR01395-02S1 (to L.F.J.), National Science Foundation grants No. BSR 82 14199 and PCM 83 09331 (to J.P.-H.), and No. DCB 86 18694 (to F.M.H.).     

Effect of galactose on auxin-induced cell elongation in oat coleoptile segments in mannitol solutions

Oat coleoptile segments were treated with or without 10 mM galactose in the presence or absence of 10 μM IAA and various concentrations of mannitol (pre-incubation). Auxin-induced growth was inhibited by galactose. Segments were then transferred to buffer solutions containing or not containing 10 mM galactose (post-incubation). Expansion growth due to rapid water absorption was observed. The expansion growth during the post-incubation was inhibited by galactose when galactose was applied during the post-incubation period or all through the pre- and post-incubation but was not affected by galactose when it was applied only during the pre-incubation. This result indicates that the galactose effect on the expansion growth is due to its inhibitory action during the post-incubation period. Galactose has been reported to be a specific inhibitor for cell wall synthesis. Thus, it is suggested that the expansion growth during post-incubation requires cell wall synthesis and is not just the process of passive water absorption. The primary action of auxin does not seem to require new synthesis of polysaccharides.     

Light-enhanced perception of gravity in stems of intact pea seedlings

Dark-grown, 6-d-old pea seedlings (Pisum sativum L. cv. Alaska) do not respond gravitropically to brief (approx. 3 min) horizontal presentations, but seedlings given a pulse of red light (R) 16–24 h earlier respond to such stimuli by vigorous curvature of the epicotyl. With continuous horizontal stimulation (approx. 100 min), the kinetics and extent of the gravitropic response are almost identical in irradiated and dark-control plants. Prior R thus increases graviperception without altering the rate-limiting steps underlying the generation of curvature. This effect of R on graviperception develops slowly; seedlings studied only a few hours after R show differences in the kinetics of the gravitropic response, but not in presentation time. Neither the kinetics nor the extent of gravitropic curvature should be used as criteria for establishing changes in primary processes in gravitropism.     

Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T _1/2=1 to 27 s were found. The volumetric clastic modulus () of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10^-6 cm s^-1 bar^-1. For cortical cells P=5 to 11 bar, T _1/2=0.3 to 1 s, Lp=0.4 to 9·10^-5 cm s^-1 bar^-1 and =6 to 215 bar. The T _1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T _1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T _1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not , but the reason for the change in cell permeability to water is not known.In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T _1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423–429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h^-1 and 5% h^-1, respectively. Thus, although the T _1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.Abbreviations P turgor pressure - T _1/2 half-time of water exchange of individual cell - Lp hydraulic conductivity - volumetric elastic modulus - t _1/2 average half-time of water exchange of tissue     

Cell-wall synthesis and elongation growth in hypocotyls of Helianthus annuus L.

The relationship between growth and increase in cell-wall material (wall synthesis) was investigated in hypocotyls of sunflower seedlings (Helianthus annuus L.) that were either grown in the dark or irradiated with continuous white light (WL). The peripheral three to four cell layers comprised 30–50% of the entire wall material of the hypocotyl. The increase in wall material during growth in the dark and WL, respectively, was larger in the inner tissues than in the peripheral cell layers. The wall mass per length decreased continuously, indicating that wall thinning occurs during growth of the hypocotyl. When dark-grown seedlings were transfered to WL, a 70% inhibition of growth was observed, but the increase in wall mass was unaffected. Likewise, the composition of the cell walls (cellulose, hemicellulose, pectic substances) was not affected by WL irradiation. Upon transfer of dark-grown seedlings into WL a drastic increase in wall thickness and a concomitant decrease in cell-wall plasticity was measured. The results indicate that cell-wall synthesis and cell elongation are independent processes and that, as a result, WL irradiation of etiolated hypocotyls leads to a thickening and mechanical stiffening of the cell walls.