Peroxidase Activity in the Leaf Elongation Zone of Tall Fescue : II. Spatial Distribution of Apoplastic Peroxidase Activity in Genotypes Differing in Length of the Elongation Zone

Previous work suggested that cell wall peroxidase activity increased as cells were displaced through the elongation zone in leaf blades of tall fescue (Festuca arundinacea Schreb.). In this study, two genotypes that differ in length of the elongation zone were used to examine the relationship between peroxidase activity in apoplastic fluid of intact leaf blade segments and the spatial distribution of leaf growth. Apoplastic fluid was extracted by vacuum infiltration and centrifugation, and peroxidase activity was assayed spectrophotometrically. Isoelectric focusing was used to characterize the isoforms of apoplastic peroxidase within the region of elongation and in the region of secondary cell wall deposition, which is distal to the elongation zone. A striking correlation was found in each genotype between both the location and timing of increase in apoplastic peroxidase activity and the onset of growth deceleration. Only cationic isoforms of apoplastic peroxidase could be identified in the elongation zone, whereas additional anionic isoforms appeared in the region of secondary cell wall deposition. We conclude that cessation of elongation growth in tall fescue leaf blades is likely to be related to the secretion of cationic isoforms of peroxidase into the cell wall.     

A transient increase in apoplastic peroxidase activity precedes decrease in elongation rate of B73 maize (Zea mays) leaf blades

Peroxidase (EC 1.11.1.7) activity from homogenized tissue or in apoplastic fluid was analyzed along the developmental gradient of expanding B73 maize ( Zea mays L.) leaf blades. Soluble plus ionically bound peroxidase activity from homogenized tissue was present in high levels at the leaf base, which includes the region of cell division, and decreased as tissue was displaced away from the base by growth. A different pattern of change in peroxidase activity was seen in apoplastic fluid extracted from segments of intact tissue, where an increase in peroxidase activity preceded a rapid decrease in leaf elongation rate. Similar patterns in peroxidase activity from homogenized and intact tissue have been found in leaf blades of tall fescue ( Festuca arundinacea Schreb.), suggesting a common phenomenon. At the location within the elongation zone where the increase in apoplastic peroxidase activity occurred, the activities of neutral and acidic (pl 4.6) peroxidase isoforms were also elevated in both the homogenate and in apoplastic fluid. The coincidence of these isoforms with the decline in leaf elongation rate suggests they may contribute to cessation of growth. At the distal end of the elongation zone, the activities of other acidic peroxidases (pI 5.6 and 5.7) increased in the homogenate and in apoplastic fluid, and remained elevated as tissue was displaced into the maturation region. The location of their appearance and their relatively high activity in the maturation region suggest the involvement of these isoforms in lignification.     

Gibberellic acid and dwarfism effects on the growth dynamics of B73 maize (Zea mays L.) leaf blades: a transient increase in apoplastic peroxidase activity precedes cessation of cell elongation

The relationship between apoplastic peroxidase (EC 1.11.1.7) activity and cessation of growth in maize (Zea mays L.) leaf blades was investigated by altering elongation zone length. Apoplastic peroxidase activity in the elongation and secondary cell wall deposition zones of elongating leaf blades of the maize inbred line B73 was used as a control and compared to leaves of the dwarf mutant D8-81127, a near-isogenic line of B73 unresponsive to gibberellins, and to leaves of B73 plants to which gibberellic acid (GA(3)) had been applied via root uptake. Elongation zone length was increased by treatment with GA(3) through an increase in cell number as well as increased final cell length. The shorter elongation zone of dwarf leaves occurred primarily through reduced final cell length. Although elongation zone length differed among dwarf, control, and GA(3)-treated leaf blades, in all three treatments a transient increase in apoplastic peroxidase activity preceded a reduction in the segmental elongation rate in leaves. A peroxidase isoenzyme with pI 7.0 occurred in the leaf elongation zone during growth deceleration in all three treatments, and its activity decreased as growth displaced tissue into the region of secondary cell wall deposition. Growth cessation for all treatments coincided with the first appearance of peroxidase isozymes with pIs of 5.6 and 5.7. Based on the activity of particular isozymes relative to growth and differentiation, the pI 7.0 isoenzyme is most likely to be involved in cessation of cell elongation, while isozymes with pIs 5.6 and 5.7 are likely to be active in lignification.     

Effect of N deficiency on leaf growth and cell wall peroxidase activity in contrasting maize genotypes

Two maize genotypes differing in leaf elongation rate (high-LER and low-LER) were used for the investigation of the effects of nitrogen deficiency on leaf growth and development and activity of enzyme cell wall peroxidase in the leaf growth zone. Plants were grown in a growth cabinet in perlite as a substrate and watered with complete N-NO₃ solution (+N) and N-NO₃ deficient solution (–N). Comparison between the investigated genotypes showed that final leaf length in both N treatments was related with LER, but not with the duration of leaf elongation. Faster leaf elongation rate in high-LER compared with low-LER genotype, was associated with longer growth zone, a bigger number of cells in it, and higher cell flux rate, although cell elongation rate was similar in both genotypes. These lines of evidence indirectly indicated that leaves of the faster growing genotype were characterized by higher meristematic activity. Nitrogen deficiency reduced the flux of cells and cell elongation rate, length of cell division zone and the number of cells in whole zone, significantly for both genotypes, although duration of cell elongation was increased and final epidermal cell length was unchanged. These results showed that N deficiency reduced both cell division and cell elongation, which in turn resulted in decreased leaf length and prolonged time for leaf development. Nitrogen deficiency significantly increased both bulk and segmental cell wall peroxidase activity in the growth zone of both investigated genotypes, thus showing an interaction between leaf growth cessation and enzyme activity.     

Specific Leaf Weight in Zones of Cell Division, Elongation and Maturation in Tall Fescue Leaf Blades

Patterns of change in specific leaf weight (SLW), water-solublecarbohydrate (WSC) content and leaf width were used to delineatethe region of secondary cell wall accumulation, and determinethe rate of increase in structural material along a developingleaf blade of tall fescue (Festuca arundinacea Schreb.). Structuralspecific leaf weight (SSLW) was determined by subtracting WSCmass from dry weight to emphasize structural material. Becausemeristematic activity, cell elongation, and cellular maturationare arranged successively in the grass leaf, these patternsrepresent a developmental sequence through which each segmentof the leaf blade passes. Patterns were generally similar fortwo genotypes, one selected for high (HYT) and the other forlow (LYT) yield per tiller, for a single genotype grown at 17or 25 C, and for two field-grown populations which differedin leaf area expansion rate (LAER). In all three studies, the elongation zone of the developingleaf had 31 to 39 per cent WSC on a dry weight basis. The LYTgenotype had a higher SLW at all stages of development whengrown at 17 than at 25 C, due to greater WSC accumulation.At 20 C, the HYT genotype had a higher SLW all along the elongatingleaf blade than the LYT genotype. This difference was due toa difference in SSLW, while WSC content was similar. The LERwas 64 per cent higher in the high population than the low,but elongation zones were similar in WSC. In all cases, SSLWwas high in the meristematic region, lowest near the distalend of the cell elongation zone, then increased linearly astissue matured. Rate of increase in SSLW was 8.5 and 5.2 g m^–2d^–1 for the HYT and LYT genotypes, respectively, and 7.6and 6.7 g m^–2 d^–1 for the high and low LAER populations,respectively. Festuca arundinacea Schreb., tall fescue, specific leaf weight, leaf width, water-soluble carbohydrates, leaf elongation rate     

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     

Assessment of spatial distribution of growth in the elongation zone of grass leaf blades

Knowledge about the spatial distribution of growth is essential for understanding the leaf growth process. In grasses the elongation zone is located at the base of the leaf blade and is enclosed by sheaths of older leaves. Assessment of spatial growth distribution, therefore, necessitates use of a destructive method. We used a fine needle to make holes through bases of tillers at the location of the leaf elongation zone of tall fescue (Festuca arundinacea Schreb.), then measured the displacement of the holes after a 6 or 24 h interval. Needle holes caused a 22 to 41% decrease in daily leaf elongation so experiments were conducted to investigate if the spatial distribution of growth in the elongation zone was altered. Leaf elongation rate was reduced similarly when needle holes were made within or above the zone where cell elongation occurs. Distribution of elongation within the zone was the same when estimated by displacement of needle holes or ink marks placed on the epidermis of the elongation zone after surrounding tissue had been removed. Making holes at different locations within the elongation zone did not differentially affect the relative contribution of the damaged or undamaged parts to leaf elongation. These findings demonstrate that needle holes or ink marks in paired leaves can be used to estimate the relative distribution of growth in the elongation zone of undamaged tall fescue leaf blades.     

Relation of cell wall peroxidase activity with growth in epicotyls of Cicer arietinum. Effects of calmodulin inhibitors

Sánchez, O.J., Pan, A., Nicolás, G. and Labrador, E. 1989. Relation of cell wall peroxidase activity with growth in epicotyls of Cicer arietinum. Effects of calmodulin inhibitors.
Peroxidases are bound ionically to cell walls in epicotyls of Cicer arietinum L. cv. Castellana. The cell wall peroxidase activity increases during the growth of epicotyls, being the lowest in 3-day-old epicotyls with high growth capacity. The cell wall phenolic compounds, postulated natural substrates of cell wall peroxidases, also increase during growth.
The calmodulin inhibitors chlorpromazine and trifluoperazine decrease the elongation rate of epicotyls of Cicer arietinum. These inhibitors also cause an increase in the cell wall peroxidase activity and in the level of phenolic compounds. A possible regulatory effect of calmodulin on peroxidase activity is postulated.     

Cell Wall and Protoplast Isoperoxidases of Corn Leaves in Relation to Cut Injury and Infection with Helminthosporium maydis

Young leaves of two corn (Zea mays) inbreds with normal and Texas male-sterile cytoplasm, which differed in their susceptibility to Helminthosporium maydis Nisikado and Miyake race T, showed no significant qualitative or quantitative differences in their isoperoxidase patterns. Of six cathodic and four anodic isoenzymes present in the soluble fraction, four and two, respectively, comprised the fraction ionically bound to the cell wall. Peroxidase fractions ionically and covalently bound to the wall constituted about 20% of the total peroxidase activity. No new isoperoxidases were detected in either inbred line in response to cutting, infection, or detachment only and exposure to darkness for 40 hours. Three isoperoxidases, all cathodic, mainly reacted to cutting injury as well as fungal infection. One of the isoperoxidases appeared responsible for the increase in the peroxidase activity of the soluble fraction while the other two were responsible for the increase in that of the fraction ionically bound to the walls. The relative increase in the latter fraction was greater for infected leaves than for mechanically injured ones. No significant differences were found between the two inbreds in their peroxidase reactions to cutting injury or infection. Thus, the corn leaf isoperoxidases were distinctive in their distribution in the cell and in their reaction to injury. Changes in their activity induced by infection may result from a nonspecific response to injury.     

Hydroxyproline and Peroxidases in Cell Walls of Pisum sativum: Regulation by Ethylene

Purified cell-wall preparations from the epicotyl of etiolatedPisum sativum contain covalently bound peroxidases and hydroxyproline-richproteins. Towards the end of cell elongation there is a largerise in these wall components and thereafter a continuing slowrise which is associated with increasing age of tissue. Ethyleneat concentrations of 0.1 ppm or more increases both peroxidaseactivity and hydroxyproline levels in the walls, the greatestresponse occurring in immature tissue including the apical hook.Growth of these tissues is highly sensitive to ethylene whichcauses an inhibition of elongation in extending cells and anenhanced lateral cell expansion. We suggest that the effectsof ethylene on wall-bound peroxidase and hydroxyproline areimplicated in the ethylene regulation of cell growth. The covalently bound wall peroxidase was found to be extremelystable and to contain unique isoenzymes which do not occur ineither the cytoplasm or in the peroxidase which is ionicallybound to walls. Ethylene increases peroxidase activity in boththe cytoplasmic and the ionically bound wall fractions, butthere is little or no increase in their hydroxyproline content.The possible relationships between covalently bound wall peroxidaseand hydroxyproline are discussed and we speculate that thisperoxidase may be involved in the hydroxylation of proline inthe walls.     

Can cell wall peroxidase activity explain the leaf growth response of Lolium temulentum L. during drought?

In two experiments, the effect has been investigated of a mildand a more prolonged drought on the spatial distribution ofgrowth, epidermal cell lengths and cell wall peroxidase activitiesin the leaf elongation zone of the grass species Lolium temulentumL. In both experiments drought reduced the size of the elongationzone and local rates of elongation within it. Abrupt increasesin cell wall-associated peroxidase activity occurred at or closeto the position where elongation ceased in the leaf elongationzones of well-watered and mildly drought-stressed plants. Moreprolonged drought caused a 200–300% increase in the cellwallassociated peroxidase activity in the elongation zone only.The significant increase in the elongation zone cell wall peroxidaseactivity and its spatial variation provides evidence of a potentiallycausal role for cell wall-associated peroxidase in restrictingcell expansion during drought. Key words: Cell wall peroxidase, leaf expansion, drought     

Secondary cell wall deposition causes radial growth of fibre cells in the maturation zone of elongating tall fescue leaf blades

A gradient of development consisting of successive zones of cell division, cell elongation and cell maturation occurs along the longitudinal axis of elongating leaf blades of tall fescue (Festuca arundinacea Schreb.), a C3 grass. An increase in specific leaf weight (SLW; dry weight per unit leaf area) in the maturation region has been hypothesized to result from deposition of secondary cell walls in structural tissues. Our objective was to measure the transverse cell wall area (CWA) associated with the increase in SLW, which occurs following the cessation of leaf blade elongation at about 25 mm distal to the ligule. Digital image analysis of transverse sections at 5, 15, 45, 75 and 105 mm distal to the ligule was used to determine cell number, cell area and protoplast area of structural tissues, namely fibre bundles, mestome sheaths and xylem vessel elements, along the developmental gradient. Cell diameter, protoplast diameter and area, and cell wall thickness and area of fibre bundle cells were calculated from these data. CWA of structural tissues increased in sections up to 75 mm distal to the ligule, confirming the role of cell wall deposition in the increase in SLW (r2 = 0.924; P < or = 0.01). However, protoplast diameter of fibre cells did not decrease significantly as CWA increased, although mean thickness of fibre cell walls increased by 95 % between 15 and 105 mm distal to the ligule. Therefore, secondary cell wall deposition in fibre bundles of tall fescue leaf blades resulted in continued radial expansion of fibre cells rather than in a decrease in protoplast diameter.     

Soluble and Bound Apoplastic Activity for Peroxidase, beta-d-Glucosidase, Malate Dehydrogenase, and Nonspecific Arylesterase, in Barley (Hordeum vulgare L.) and Oat (Avena sativa L.) Primary Leaves

An intercellular washing solution containing about 1% of the soluble protein, 0.3% or less of the glucose-6-phosphate dehydrogenase activity, but up to 20% of the peroxidase and β-d-glucosidase activity of barley (Hordeum vulgare L.) or oat (Avena sativa L.) primary leaves was obtained by vacuum infiltrating peeled leaves with pH 6.9 buffered 200 millimolar NaCl. After this wash, segments were homogenized in buffer, centrifuged, and the supernatant was assayed for soluble cytoplasmic enzymes. The pellet was washed and resuspended in 1 molar NaCl to solubilize enzymes strongly ionically bound to the cell wall. The final pellet was assayed for enzyme activity covalently bound in the cell wall. Apoplastic (intercellular washing solution, ionically bound, and covalently bound) fractions contained up to 76% of the β-d-glucosidase activity, 36% of the peroxidase activity, 11% of the nonspecific arylesterase activity, 4% of the malate dehydrogenase activity, but less than 2% of the glucose-6-phosphate dehydrogenase activity of peeled leaf segments. The partitioning and salt-solubility of the enzymes between the apoplast and symplast differed considerably between these two species. Intercellular washing fluid prepared by centrifuging unpeeled leaves had higher activity for glucose-6-phosphate dehydrogenase, less soluble protein, and less peroxidase activity per leaf than intercellular washing solution obtained by our peeling-infiltration-washing technique. The results are discussed in relation to the roles of these enzymes in phenolic metabolism in the cell wall.     

Changes in cucumber hypocotyl cell wall dynamics caused by Azospirillum brasilense inoculation

We previously reported that Azospirillum brasilense induced a more elastic cell wall and a higher apoplastic water fraction in both wheat coleoptile and flag leaf. These biophysical characteristics could permit increased growth. Knowledge of the biochemical effects the bacteria could elicit in plant cell walls and how these responses change plant physiology is still scarce. The objective of this work was to analyze whether A. brasilense Sp245 inoculation affected elongation and extensibility of growing cucumber (Cucumis sativus) hypocotyls and ionically bound cell wall peroxidase activities. Hypocotyl tip and basal segments were excised from A. brasilense Sp245-inoculated cucumber seedlings growing in darkness under hydroponic conditions. Elongation, cell wall extensibility, cell wall peroxidase activities against ferulic acid and guaiacol and NADH oxidase activities were analyzed. Azospirillum-inoculated cucumber seedlings grew bigger than non-inoculated ones. Dynamic cell wall differences were detected between inoculated and non-inoculated hypocotyls. They included greater acid-induced cell wall extension and in vivo elongation when incubated in distilled water. Although there was no difference between treatments in either region of the hypocotyl NADH oxidase and ferulic acid peroxidase activities were lower in both regions in inoculated seedlings. These lesser activities could be delaying the stiffening of cell wall in inoculated seedlings. These results showed that the cell wall is a target for A. brasilense growth promotion.     

Growth rates and assimilate partitioning in the elongation zone of tall fescue leaf blades at high and low irradiance

Tall fescue (Festuca arundinacea Schreb.) leaf blades elongated 33% faster at continuous low than at continuous high irradiance (60 versus 300 micromoles per second per square meter photosynthetic photon flux density) when temperature of the leaf elongation zone was held constant at 21°C. Increased rate of elongation was associated with a near proportional increase in length of the elongation zone (+38%). In contrast, growth in width and thickness was decreased at low irradiance, resulting in only a 12% increase in leaf area production and 5% less total growth-associated water deposition than at high irradiance. At low irradiance dry matter (DM) import into the elongation zone was 28% less, and 55% less DM was used per unit leaf area produced. DM use in synthesis of structural components (i.e. DM less water-soluble carbohydrates) was only 13% less at low irradiance, whereas water-soluble carbohydrates (WSC) deposition was 43% less. The lower rate of WSC deposition at low irradiance was associated with a higher net rate of monosaccharide deposition (+39%), whereas net deposition rates for sucrose (−27%) and fructan (−56%) were less than at high irradiance. Still, at low irradiance, net fructan accumulation accounted for 64% of WSC deposition, i.e. 25% of DM import, demonstrating the high sink strength of the leaf elongation zone.     

Physiological and biochemical changes associated with cotton fibre development. II. Auxin oxidising system

Changes in IAA oxidase, and in cytoplasmic and ionically wall-bound peroxidase activities were studied in the developing fibres of three cotton cultivars ( Gossypium hirsutum L. cv. Gujarat-67, cv. Khandwa-2 and G. herbaceum L. cv. Digvijay), designated as long, medium and short staple cultivars, respectively. In all the three cultivars IAA oxidase activity was low during the fibre elongation phase, while the activity increased significantly during the secondary thickening phase. The increase in IAA oxidase activity in the three cultivars showed close correspondence with their respective total period of elongation. No relationship between cytoplasmic peroxidase activity and fibre development was discernible. The ionically bound wall peroxidase activity, however, recorded low levels during the elongation phase and higher levels during the secondary thickening phase. The role of wall peroxidase in cessation of elongation growth is discussed.     

NaCl induced changes in ionically bound peroxidase activity in roots of rice seedlings

The changes in ionically bound peroxidase activity in roots of NaCl-stressed rice seedlings and their correlation with root growth were investigated. Increasing concentrations of NaCl from 50 to 150 mM progressively decreases root growth. The reduction of root growth by NaCl is closely correlated with the increase in ionically bound peroxidase activity. Since proline and ammonium accumulations are associated with root growth inhibition caused by NaCl, we determined the effects of proline or NH4Cl on root growth and ionically bound peroxidase activity in roots. External application of proline or NH4Cl markedly inhibited root growth and increased ionically bound peroxidase activity in roots of rice seedlings in the absence of NaCl. An increase in ionically bound peroxidase activity in roots preceded inhibition of root growth caused by NaCl, NH4Cl or proline. Mannitol inhibited root growth, but decreased rather than increased ionically bound peroxidase activity at the concentration iso-osmotic with NaCl. The inhibition of root growth and the increase in ionically bound peroxidase activity in roots by NaClis reversible and is associated with ionic rather than osmotic component. This revised version was published online in June 2006 with corrections to the Cover Date.     

Corn Leaf Isoperoxidase Reaction to Mechanical Injury and Infection with Helminthosporium maydis: Effects of Cycloheximide

Mechanical injury or infection with Helminthosporium maydis race T or O enhanced peroxidase activity in leaves of two corn inbreds which differ in their susceptibility to the fungal race T. Increases in activity were found in the soluble fraction extracted from tissues with 20 mm phosphate buffer (pH 6), and in the ionically bound fraction extracted from wall debris with 0.6 to 1 m NaCl; the covalently bound wall peroxidase fraction was unaffected. Mechanical injury and infection with either race enhanced the same distinctive cathodic isoforms present in the soluble fraction or in both the soluble and ionically bound fractions.     

Cell wall proteome in the maize primary root elongation zone. I. Extraction and identification of water-soluble and lightly ionically bound proteins

Cell wall proteins (CWPs) play important roles in various processes, including cell elongation. However, relatively little is known about the composition of CWPs in growing regions. We are using a proteomics approach to gain a comprehensive understanding of the identity of CWPs in the maize (Zea mays) primary root elongation zone. As the first step, we examined the effectiveness of a vacuum infiltration-centrifugation technique for extracting water-soluble and loosely ionically bound (fraction 1) CWPs from the root elongation zone. The purity of the CWP extract was evaluated by comparing with total soluble proteins extracted from homogenized tissue. Several lines of evidence indicated that the vacuum infiltration-centrifugation technique effectively enriched for CWPs. Protein identification revealed that 84% of the CWPs were different from the total soluble proteins. About 40% of the fraction 1 CWPs had traditional signal peptides and 33% were predicted to be nonclassical secretory proteins, whereas only 3% and 11%, respectively, of the total soluble proteins were in these categories. Many of the CWPs have previously been shown to be involved in cell wall metabolism and cell elongation. In addition, maize has type II cell walls, and several of the CWPs identified in this study have not been identified in previous cell wall proteomics studies that have focused only on type I walls. These proteins include endo-1,3;1,4-beta-D-glucanase and alpha-L-arabinofuranosidase, which act on the major polysaccharides only or mainly present in type II cell walls.     

Axial changes in apoplast properties in the elongation zone of maize mesocotyl

Changes in extensibility of cell walls and composition of apoplastic solution along the elongation zone were studied in mesocotyls of etiolated seedlings of maize (Zea mays L.). It was found that plastic and elastic extensibility of cell walls was much greater in the cells with a high rate of elongation. Basipetal decrease in hydrogen peroxide concentration in the apoplast (from 5.1 to 2.0 ??M) was detected. We determined the activity of cell wall enzymes participating in H₂O₂ metabolism and found that in basal direction, potential ability of these enzymes to decompose H₂O₂ rises stronger than the ability to produce it. We found a basipetal decrease in polyamine oxidase activity, an increase in oxalate oxidase activity, and a rise in the ratio between peroxidase and NADH-oxidase activities of peroxidases. IAA (10^?6 M) promoted elongation of mesocotyl segments, induced a steady elevation of H₂O₂ content in the apoplast, an increase in NADH-oxidase activity of peroxidases, and a transient decrease in oxalate oxidase activity. Treatment with ABA (10^?4 M) suppressed elongation of mesocotyl segments, induced a transient elevation of H₂O₂ content in the apoplast, and a decrease in oxalate oxidase activity. It was shown that the main metabolites of apoplastic solution are glucose (20?C30 mM), fructose (6?C7 mM), malic acid (3 mM), and amino acids, namely, Asp, Glu, Asn, Gln, Ala, Val, Ser, Thr, and Phe. In basal direction, we observed a decrease in the content of glucose (from 30 to 20 mM), inositol (from 0.24 to 0.08 mM), and total amino acids (from 5.5 to 3.3 mM), whereas concentration of orthophosphate (3 mM) and malate (3 mM) did not change significantly. A relationship between the detected changes in the apoplast composition and basipetal decrease in the elongation rate of mesocotyl cells is discussed.