Fungal decomposition of Abies needle and Betula leaf litter

The effect of litter type and incubation temperature on the ability of fungi to decompose leaf litter of subalpine trees was examined by a pure-culture test. Mass loss of Abies needle and Betula leaf litter and utilization patterns of lignin and carbohydrates were investigated under two temperature conditions (20 C and 10 C) and compared for 29 species in basidiomycetes, ascomycetes and zygomycetes. The decomposing ability was generally higher in basidiomycetes than in ascomycetes and zygomycetes. Mass loss (% original mass) of litter was higher in Betula than in Abies and higher at 20 C than at 10 C. The 29 fungi were divided into lignocellulose decomposers, cellulose decomposers and sugar fungi based on their substrate utilization in Abies and Betula litter. Mass loss of lignin and carbohydrates by lignocellulose and cellulose decomposers was higher in Betula than in Abies. Mass loss of carbohydrates was higher at 20 C than at 10 C, but the temperature did not influence mass loss of lignin, indicating lignin decomposition by fungi was less sensitive to temperature than carbohydrate decomposition. Lignin/carbohydrate loss ratio (L/C) of Collybia spp. that caused selective delignification was lower at 20 C than at 10 C. These results indicate that the decomposability of litter, lignin and carbohydrate was different between Abies and Betula and that temperature affected not only the rate at which fungi decompose litter but also the ability of fungi to use lignin and carbohydrates.     

Decomposition of organic chemical components in relation to nitrogen dynamics in leaf litter of 14 tree species in a cool temperate forest

Decomposition of lignin, holocellulose, polyphenols and soluble carbohydrates was investigated in relation to nitrogen (N) dynamics in leaf litter of 14 tree species. The influence of organic chemical components and N on litter mass loss rate was then evaluated for 14 litter types. The study was carried out over a 3-year period on upper and lower parts of a forest slope in a cool temperate forest in Japan. The decomposition processes were divided into early and late phases based on N immobilization and mobilization. Mass loss rate of whole litter and organic chemical components was similar for the upper and lower sites. Litter mass loss was faster in the immobilization phase than in the mobilization phase in each of 14 litter types, which was ascribed to the decreased mass loss of holocellulose, polyphenols and soluble carbohydrates in the mobilization phase as compared to the immobilization phase. Mass loss rate of lignin was not different between the phases. Litter mass loss rate in the immobilization and mobilization phases was negatively correlated to lignin content and positively correlated to contents of polyphenols and soluble carbohydrates at the start of these phases, but was not correlated to holocellulose and N contents in either phase.     

A C solid state nuclear magnetic resonance spectroscopic study of cork cell wall structure: the effect of suberin removal

Solid state ¹³C NMR measurements of cork, before and after suberin removal, showed that aliphatic suberin is spatially separated from carbohydrate and lignin and experiences higher motional freedom. Two types of chain methylenes, differing in chemical shift and in dynamic properties, were identified in aliphatic suberin. Experimental evidence indicated that the more motionally hindered methylenes are those situated nearer the linkages of aliphatic suberin to the cell wall. These linkages were shown to involve –CH₂O– groups, probably engaged in ester linkages to phenylpropane units and carbohydrate C6 carbons. Spectral intensity changes indicated that, during the first steps of alkaline desuberization, these linkages are broken and the shorter aliphatic suberin chains removed. Longer chains require hydrolysis of the ester linkages within the chains and are removed upon stronger alkaline treatment. T₁(C), T_1ρ(H) and T_1ρ(C) relaxation times have shown that the removal of suberin from cork leads to a motionally restricted and more compact environment, on the megahertz and mid-kilohertz timescales. The properties of cork suberin showed that suberin organization in cork is distinct from that in potato tissue.     

Factors regulating litter mass loss and lignin degradation in late decomposition stages

We studied late-stages decomposition of four types of coniferous needle and three types of deciduous leaf litter at two sites, one nutrient-poor boreal and one nutrient-rich temperate. The late stage was identified by that reached by litters at the onset of net loss of lignin mass, i.e. at about 1 year after the incubation when the highest amount of lignin had been detected; the study extended over the following 2 year period. Decomposition rates were significantly lower at the boreal than at the temperate site and did not differ between needle litter and leaf litter. In the boreal forest: (1) mass-loss was positively correlated with N and Mn release, (2) Mn concentration at the start of the late stage was positively correlated with lignin decay, (3) Ca concentration was negatively correlated to litter mass loss and lignin decay. In the temperate forest neither lignin, N, Mn, and Ca concentration at the start of the late stage, nor their dynamics were related to litter decomposition rates and lignin decay. In leaf litter mass-loss and lignin decay were positively correlated with N and Ca release and with Ca concentration. In needle litter mass-loss was positively correlated to Mn release and N concentration negatively with lignin decay. We concluded that Ca, N and Mn have different roles in controlling lignin decay depending on type of litter and site conditions.     

Roles of diverse fungi in larch needle-litter decomposition

Functional biodiversity of fungi in larch (Larix leptolepis) forests needle-litter decomposition was examined by a pure-culture test. Weight loss of larch-needle litter, utilization pattern of lignocellulose and chemical composition of remaining litter were investigated and compared for 31 isolates in 27 species of basidiomycetes and ascomycetes. Weight loss (% original weight) of litter ranged from -2.0% to 14.2%. Mean weight loss of litter caused by the basidiomycetes was not significantly different from that caused by the ascomycetes. Basidiomycetes caused loss of lignin and carbohydrates in variable proportions, while ascomycetes exclusively attacked carbohydrates without delignification. The content of lignin and nitrogen in remaining litter was not significantly correlated when both basidiomycetes and ascomycetes were included. However, the correlation coefficient was significant when the relationship was examined separately for basidiomycetes, indicating that the degree of selective delignification determined the final nitrogen content in litter. Possible effects of fungal colonization on needle-litter decomposition in larch forests are discussed.     

Sources of plant-derived carbon and stability of organic matter in soil: implications for global change

Alterations in forest productivity and changes in the relative proportion of above‐ and belowground biomass may have nonlinear effects on soil organic matter (SOM) storage. To study the influence of plant litter inputs on SOM accumulation, the Detritus Input Removal and Transfer (DIRT) Experiment continuously alters above‐ and belowground plant inputs to soil by a combination of trenching, screening, and litter addition. Here, we used biogeochemical indicators [i.e., cupric oxide extractable lignin‐derived phenols and suberin/cutin‐derived substituted fatty acids (SFA)] to identify the dominant sources of plant biopolymers in SOM and various measures [i.e., soil density fractionation, laboratory incubation, and radiocarbon‐based mean residence time (MRT)] to assess the stability of SOM in two contrasting forests within the DIRT Experiment: an aggrading deciduous forest and an old‐growth coniferous forest. In the deciduous forest, removal of both above‐ and belowground inputs increased the total amount of SFA over threefold compared with the control, and shifted the SFA signature towards a root‐dominated source. Concurrently, light fraction MRT increased by 101 years and C mineralization during incubation decreased compared with the control. Together, these data suggest that root‐derived aliphatic compounds are a source of SOM with greater relative stability than leaf inputs at this site. In the coniferous forest, roots were an important source of soil lignin‐derived phenols but needle‐derived, rather than root‐derived, aliphatic compounds were preferentially preserved in soil. Fresh wood additions elevated the amount of soil C recovered as light fraction material but also elevated mineralization during incubation compared with other DIRT treatments, suggesting that not all of the added soil C is directly stabilized. Aboveground needle litter additions, which are more N‐rich than wood debris, resulted in accelerated mineralization of previously stored soil carbon. In summary, our work demonstrates that the dominant plant sources of SOM differed substantially between forest types. Furthermore, inputs to and losses from soil C pools likely will not be altered uniformly by changes in litter input rates.     

Humification processes of needle litters on forest floors in Japanese cedar (Cryptomeria japonica) and Hinoki cypress (Chamaecyparis obtusa) plantations in Japan

We quantitatively clarified the early humification processes on Japanese cedar and Hinoki cypress forest floors by using a litterbag experiment and the solid-state ¹³C CPMAS NMR technique. There was no significant effect on litter mass loss during early humification between both coniferous litters regardless of the shape of their needles. Carbon composition in both litters showed similar trends during early humification. A/O-A as a humification index was low, around 0.6, in both litters throughout the experiment period although 60% of litter mass was lost. Coniferous litter incubated for 3 years might not be well-humified and would be susceptible to physical fragmentation. Carbon mass loss rates in conifers were in the following order: O-alkyl > aliphatic > aromatic > carbonyl carbons, differing with hardwoods. Conifers had concomitantly higher and lower mass loss rates of aliphatic and aromatic carbons than hardwoods. Soil organic carbon (SOC) accumulated in topsoil for conifers had relatively high and low contents of aliphatic and aromatic carbons than that for hardwood. These compositional differences of SOC among forests could be caused by the high and low supply rates of aliphatic and aromatic carbons from litter to topsoil. Consequently, initial litter nature and humification processes can affect the compositional qualities of SOC accumulated in soil.     

An integrated spectroscopic and wet chemical approach to investigate grass litter decomposition chemistry

The chemical transformations that occur during litter decomposition are key processes for soil organic matter formation and terrestrial biogeochemistry; yet we still lack complete understanding of these chemical processes. Thus, we monitored the chemical composition of Andropogon gerardii (big bluestem grass) litter residue over a 36 month decomposition experiment in a prairie ecosystem using: traditional wet chemical fractionation based upon digestibility, solid state ¹³C nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared (FTIR) spectroscopy. The goals of this study were to (1) determine the chemical changes occurring during A. gerardii litter decomposition, and (2) compare the information obtained from each method to assess agreement. Overall, we observed a 97 % mass loss of the original litter, through a two-stage decomposition process. In the first stage, within 12 months, non-structural, cellulose and hemicellulose fractions not encrusted in lignin were preferentially and rapidly lost, while the acid unhydrolyzable residue (AUR) and microbial components increased. During the second stage, 12–36 months, all wet chemical fraction masses decreased equivalently and slowly with time, and the AUR and the lignin-encrusted cellulose fractions decomposition rates were comparable to each other. Method comparisons revealed that wet chemical fractionation did not accurately follow the initial litter structures, particularly lignin, likely because of chemical transformations and accumulation of microbial biomass. FTIR and NMR were able to determine bulk structural characteristics, and aid in elucidating chemical transformations but lacked the ability to measure absolute quantities of structural groups. As a result, we warn from the sole use of wet chemical methods, and strongly encourage coupling them with spectroscopic methods. Our results overall support the traditional chemical model of selective preservation of lignin, but shows that this is limited to the early stages of decomposition, while lignin is not selectively preserved at subsequent stages. Our study also provides important evidence regarding the impact of chemically different litter structures on decomposition rates and pathways.     

Comparison of litter decomposing ability among diverse fungi in a cool temperate deciduous forest in Japan

The litter decomposing ability of 79 fungal isolates (41 genera, 60 species) was assessed with the pure culture decomposition test. The isolates were collected qualitatively in a cool temperate deciduous forest in Japan during a 21-mo period. Loss of original weight of sterilized litter ranged from 0.1% to 57.6%. Six isolates in the Basidiomycota caused high weight losses ranging from 15.1% to 57.6%. Fourteen isolates in Xylaria and Geniculosporium (the Xylariaceae and its anamorph) also caused high weight losses ranging from 4.0% to 14.4%. Other isolates in the Ascomycota and associated anamorphs and in the Zygomycota caused low weight losses on mean. Six fungi in the Basidiomycota, and all in the Xylariaceae showed a bleaching activity of the litter and caused lignin and carbohydrate decomposition. Mean lignin/weight loss ratios (L/W) and lignin/carbohydrate loss ratios (L/C), were 0.9 and 0.7 for the Basidiomycota and 0.7 and 0.4 for the Xylariaceae, respectively. Significant differences were found in L/W and L/C between the two groups when the result of Xylaria sp. that showed marked delignification was excluded. These differences in lignin and carbohydrate utilization patterns are discussed in relation to the structural and the chemical properties of the decomposed litter and to the implications for organic chemical changes during litter decomposition processes.     

Chemical composition and ultrastructure of broad bean (<Emphasis Type="Italic">Vicia faba</Emphasis> L.) nodule endodermis in comparison to the root endodermis

Ultrastructure and development of apoplastic barriers within indeterminate root nodules formed by Vicia faba L. were examined by light and electron microscopy. The nodule outer cortex is separated from the inner cortex by a heavily suberized nodule endodermis, which matures in submeristematic regions and possesses suberin lamellae. Unsuberized passage cells are present near vascular strands, which are surrounded by a vascular endodermis attached on the inner side of the nodule endodermal cell walls. The vascular endodermis appears immediately below the meristematic apex in developmental state I (Casparian bands), gradually develops suberin lamellae, and attains developmental state II at the base of the nodule. For chemical analysis apoplastic barrier tissues were dissected after enzymatic digestion of non-impregnated tissues. Root epidermal and endodermal cell walls as well as nodule outer cortex could be isolated as pure fractions; nodule endodermal cell walls could not be separated from vascular endodermal cell walls and enclosed xylem vessels. Gas chromatography-flame ionization detection and gas chromatography-mass spectrometry were applied for quantitative and qualitative analysis of suberin and lignin in isolated cell walls of these tissues. The suberin content of isolated endodermal cell walls of nodules was approximately twice that of the root endodermal cell walls. The suberin content of the nodule outer cortex and root epidermal cell walls was less than one-tenth of that of the nodule endodermal cell wall. Substantial amounts of lignin could only be found in the nodule endodermal cell wall fraction. Organic solvent extracts of the isolated tissues revealed long-chain aliphatic acids, steroids, and triterpenoid structures of the lupeol type. Surprisingly, extract from the outer cortex consisted of 89% triterpenoids whereas extracts from all other cell wall isolates contained not more than 16% total triterpenoids. The results of ultrastructural and chemical composition are in good correspondence and underline the important role of the examined tissues as apoplastic barriers.     

13C NMR study of pine needle decomposition

The quality of substrates in plantation forest litter, and their chemistry, can influence decomposition and N cycling. We studied the decomposition of Pinus radiata D. Don needles suspended on branches in windrows, for 3 yr after clear-cutting, using improved solid-state ¹³C NMR and chemical analysis. The NMR spectra suggested that the concentration of condensed tannins was 12–22%, and showed they were chemically altered during the period 4–12 months after clear-cutting. The spectra showed no evidence for further chemical modification of the tannins during the second or third years. Data for P. radiata needle decomposition in New Zealand indicated rapid loss of mass in the first 3 months, and condensed tannins did not appear to prevent mineralization of C or N. The tannin and lignin concentrations increased with decomposition of the needles, which was consistent with the early mineralization of readily available C compounds.     

The origin of soil organic matter controls its composition and bioreactivity across a mesic boreal forest latitudinal gradient

Warmer climates have been associated with reduced bioreactivity of soil organic matter (SOM) typically attributed to increased diagenesis; the combined biological and physiochemical transformation of SOM. In addition, cross‐site studies have indicated that ecosystem regime shifts, associated with long‐term climate warming, can affect SOM properties through changes in vegetation and plant litter production thereby altering the composition of soil inputs. The relative importance of these two controls, diagenesis and inputs, on SOM properties as ecosystems experience climate warming, however, remains poorly understood. To address this issue we characterized the elemental, chemical (nuclear magnetic resonance spectroscopy and total hydrolysable amino acids analysis), and isotopic composition of plant litter and SOM across a well‐constrained mesic boreal forest latitudinal transect in Atlantic Canada. Results across forest sites within each of three climate regions indicated that (1) climate history and diagenesis affect distinct parameters of SOM chemistry, (2) increases in SOM bioreactivity with latitude were associated with elevated proportions of carbohydrates relative to plant waxes and lignin, and (3) despite the common forest type across regions, differences in SOM chemistry by climate region were associated with chemically distinct litter inputs and not different degrees of diagenesis. The observed climate effects on vascular plant litter chemistry, however, explained only part of the regional differences in SOM chemistry, most notably the higher protein content of SOM from warmer regions. Greater proportions of lignin and aliphatic compounds and smaller proportions of carbohydrates in warmer sites' soils were explained by the higher proportion of vascular plant relative to moss litter in the warmer relative to cooler forests. These results indicate that climate change induced decreases in the proportion of moss inputs not only impacts SOM chemistry but also increases the resistance of SOM to decomposition, thus significantly altering SOM cycling in these boreal forest soils.     

Chemical composition of litter affects the growth and enzyme production by the saprotrophic basidiomycete Hypholoma fasciculare

Chemical composition of litter has previously been reported to affect in situ decomposition. To identify its effects on a single species level, the saprotrophic basidiomycete Hypholoma fasciculare was grown on 11 types of litter with variable chemical composition (N content of 3.4–28.9 mg g⁻¹), and the mass loss of litter and lignin, production of extracellular enzymes and fungal biomass were followed. After 12 weeks, mass loss ranged from 16 % to 34 %. During early decomposition stages, litter mass loss, fungal biomass production (estimated by ergosterol content) as well as fungal substrate use efficiency all increased with increasing initial N content of the litter. The initial litter decomposition rate was significantly positively correlated with the activities of arylsulfatase, cellobiohydrolase, endoxylanase and phosphatase. Contrary to expectations, the lignin content did not affect litter mass loss, when covariation with N content was accounted for. The ratio of lignin loss to total mass loss depended on the litter type and did not reflect the activities of ligninolytic enzymes.     

Enzyme activities of fungi associated with Picea abies needles

Decomposition of Picea abies needles and production of extracellular enzymes involved in decomposition of lignin, cellulose, hemicelluloses and other organic compounds were studied in fungal strains of interior needle colonizers isolated from needles in different stages of decomposition (attached to trees, and early and late decomposition stages in the litter horizon). In total, 12 strains of ascomycetes (members of Helotiales, Hypocreales, Dothideales, Diaporthales and Eurotiales) and four basidiomycetes (Polyporales, Agaricales and Russulales) were tested. Significant decomposition of needles was recorded for all fungal isolates. All isolates produced cellobiohydrolase, β-glucosidase, β-xylosidase, N-acetylglucosaminidase, α-glucosidase, phosphatase and arylsulfatase and most fungi also produced endocellulase, endoxylanase and laccase in needle litter. In addition, other hemicellulases were produced by all strains. Mn-peroxidase was only produced by two basidiomycetes. Although enzyme activities varied, fungi associated with needles on fallen trees exhibited enzyme production comparable with later litter colonizers, and there was no significant difference in enzyme production between ascomycete and basidiomycete strains.     

华山松根与针叶凯氏带的比较研究

应用冰冻切片、酶解分离、荧光显微技术和傅里叶红外光谱分析(FTIR)等手段,对华山松初生根和针叶内皮层凯氏带进行了分离、显微结构特征和化学成分的比较。研究结果表明：针叶凯氏带的“网格”结构比较整齐,大小较一致,排列也较规则,同时在“网格”的纵向壁上具有明显的初生纹孔场。而初生根凯氏带网状结构的大小、排列均不规则,在其“网格”的纵向壁上的初生纹孔场不明显。根据FTIR的检测结果显示：初生根凯氏带中木栓质和木质素的含量均高于针叶,而纤维素的含量则明显低于针叶;两者细胞壁蛋白的含量基本相同。本文的研究结果为深入探讨植物地下部分和地上部分凯氏带的生理功能提供新的佐证。     

Decomposition and Fungi of Needle Litter from Slow- and Fast-growing Norway Spruce (Picea abies) Clones

The fungal species involved in the decomposition of needle litter and their response to intraspecific genetic variation of trees are poorly known. First, we compared the needle decomposition and fungal decomposers underneath eight different Norway spruce clones in situ. This experiment revealed 60-70% loss of needle mass in two years. Although spruce clones differed considerably in growth (twofold height difference) and their needles differed in chemical composition, no significant difference was found for loss of needle mass under the spruce clones. Furthermore, the spruce clones did not affect the community structure of the fungal decomposers. Fungi inhabiting needle litter were identified by extracting ribosomal RNA (rRNA) and sequencing complementary DNA (cDNA) of internal trascribed spacer 1 (ITS1) region. The most frequent identifications were Lophodermium, Pezizales, Mycena, and Marasmius, suggesting that endophytic fungi were involved in the decomposition process. Second, we evaluated the potential of endophytes to decompose needle material in a microcosm experiment in which all other fungi than endophytes were excluded. Within 2 years, the endophytes had decomposed 35-45% of the needle mass. Sequences of Mollisia, Lophodermium, Lachnum, and Phialocephala were most frequently found in rRNA and rDNA extracted from the needles at the end of the microcosm experiment. The dominant needle endophyte in fresh, green needles was Lophodermium piceae, and this species was also found frequently in the needle material after 2 years of decay both in the field and laboratory experiments. Moreover, the relative abundance of Lophodermium-derived denaturing gradient gel electrophoresis (DGGE) bands correlated positively with the decomposition in the microcosm experiment. Hence, our results suggest a significant role of endophytic fungi, and particularly L. piceae, in the process of needle decomposition in boreal forests.     

Chemical analysis and immunolocalisation of lignin and suberin in endodermal and hypodermal/rhizodermal cell walls of developing maize (Zea mays L.) primary roots

The composition of suberin and lignin in endodermal cell walls (ECWs) and in rhizodermal/hypodermal cell walls (RHCWs) of developing primary maize (Zea mays L.) roots was analysed after depolymerisation of enzymatically isolated cell wall material. Absolute suberin amounts related to root length significantly increased from primary ECWs (Casparian strips) to secondary ECWs (suberin lamella). During further maturation of the endodermis, reaching the final tertiary developmental state characterised by the deposition of lignified secondary cell walls (u-shaped cell wall deposits), suberin amounts remained constant. Absolute amounts of lignin related to root length constantly increased throughout the change from primary to tertiary ECWs. The suberin of Casparian strips contained high amounts of carboxylic and 2-hydroxy acids, and differed substantially from the suberin of secondary and tertiary ECWs, which was dominated by high contents of ω-hydroxycarboxylic and 1,ω-dicarboxylic acids. Furthermore, the chain-length distribution of suberin monomers in primary ECWs ranged from C₁₆ to C₂₄, whereas in secondary and tertiary ECWs a shift towards higher chain lengths (C₁₆ to C₂₈) was observed. The lignin composition of Casparian strips (primary ECWs) showed a high syringyl content and was similar to lignin in secondary cell walls of the tertiary ECWs, whereas lignin in secondary ECWs contained higher amounts of p-hydroxyphenyl units. The suberin and lignin compositions of RHCWs rarely changed with increasing root age. However, compared to the suberin in ECWs, where C₁₆ and C₁₈ were the most prominent chain lengths, the suberin of RHCWs was dominated by the higher chain lengths (C₂₄ and C₂₆). The composition of RHCW lignin was similar to that of secondary-ECW lignin. Using lignin-specific antibodies, lignin epitopes were indeed found to be located in the Casparian strip. Surprisingly, the mature suberin layers of tertiary ECWs contained comparable amounts of lignin-like epitopes. Received: 19 August 1998 / Accepted: 3 February 1999     

Chemical Composition of Hypodermal and Endodermal Cell Walls and Xylem Vessels Isolated from Clivia miniata (Identification of the Biopolymers Lignin and Suberin)

The occurrence of the biopolymers lignin and suberin was investigated with hypodermal (HCW) and endodermal cell walls (ECW) and xylem vessels (XV) isolated from Clivia miniata Reg. roots. Both biopolymers were detected in HCW and ECW, whereas in XV, typical aliphatic suberin monomers were missing and only representative lignin monomers such as guaiacyl (G) and syringyl (S) units could be detected. The absolute amounts of lignin were about one order of magnitude higher compared with suberin in both HCW and ECW. The ratios of the two aromatic lignin units (G/S) decreased from 39 in XV and 10 in HCW to about 1 in ECW, indicating significant differences in lignin structure and function between the three investigated samples. Additionally, compared with the detectable lignin-derived aromatic units G and S, significantly higher amounts of esterified p-coumaric acid-derived aromatic monomers were obtained with HCW, but not with ECW. This is interpreted as a functional adaption of HCW toward pathogen defense at the root/soil interface. The final aim of this study was to provide a thorough chemical characterization of the composition of HCW, ECW, and XV, which in turn will form the basis for a better understanding of the relevant barriers toward the passive, radial, and apoplastic diffusion of solutes from the soil across the root cortex into the root cylinder.     

Review article. Apoplastic barriers in roots: chemical composition of endodermal and hypodermal cell walls

Based on the characterization of the chemical composition of endodermal and hypodermal cell walls isolated from seven monocotyledonous and three dicotyledonous plant species, a model of the composition of apoplastic barriers in roots is proposed. Depending on the species, endodermal and hypodermal cell walls of roots contained varying amounts of the biopolymers suberin, lignin, cell wall proteins, and carbohydrates. Although analysis of the chemical composition of these apoplastic barriers of roots is now possible, it is pointed out that conclusions from these data concerning the functional properties of these cell walls can not easily be drawn. However, in analogy to suberized periderms it is argued that the suberin should play a role in establishing an apoplastic transport barrier in roots, albeit not a perfect barrier. Furthermore, due to the combined occurrence of suberin, lignin and cell wall proteins it is argued that endodermal and hypodermal cell walls also have an important function as barriers towards pathogens. Finally, it is pointed out that additional experimental approaches combining the investigation of transport properties and of the chemical composition of apoplastic transport barriers in roots are necessary before the function of endodermal and hypodermal cell walls in roots can be fully understood.