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The formation of suberized and lignified barriers in the exodermis is suggested to be part of a suite of adaptations to flooded or waterlogged conditions, adjusting transport of solutes and gases in and out of roots. In this study, the composition of apoplasmic barriers in hypodermal cell walls and oxygen profiles in roots and the surrounding medium of four Amazon tree species that are subjected to long-term flooding at their habitat was analyzed. In hypodermal cell walls of the deciduous tree Crateva benthami, suberization is very weak and dominated by monoacids, 2-hydroxy acids, and -hydroxycarboxylic acids. This species does not show any morphological adaptations to flooding and overcomes the aquatic period in a dormant state. Hypodermal cells of Tabernaemontana juruana, a tree which is able to maintain its leaf system during the aquatic phase, are characterized by extensively suberized walls, incrusted mainly by the unsaturated C 18 -hydroxycarboxylic acid and the ␣,-dicarboxylic acid analogon, known as typical suberin markers. Two other evergreen species, Laetia corymbulosa and Salix martiana, contained 3-to 4-fold less aliphatic suberin in the exodermis, but more than 85% of the aromatic moiety of suberin are composed of para-hydroxybenzoic acid, suggesting a function of suberin in pathogen defense. No major differences in the lignin content among the species were observed. Determination of oxygen distribution in the roots and rhizosphere of the four species revealed that radial loss of oxygen can be effectively restricted by the formation of suberized barriers but not by lignification of exodermal cell walls.Suberin is a heterogeneous extracellular biopolymer closely attached to the inner primary cell wall (Schreiber et al., 1999). On the basis of chemical analysis of enzymatically isolated cell walls, the composition of suberin in the exodermis was shown to consist of long-chain aliphatic monomers esterified with aromatic compounds like ferulic and coumaric acids and cell wall carbohydrates (Zeier and Schreiber, 1997; Kolattukudy, 2001). Recently, glycerol has been identified as a new important structural element in the suberin macromolecule, which is supposed to cross-link the aliphatic and aromatic suberin domains (Moire et al., 1999; Pereira, 2000a, 2000b). The aliphatic monomers of suberin are synthesized via the fatty acid biosynthetic pathway, catalyzed by fatty acid elongases in the root cells (Domeregue et al., 1998;Schreiber et al., 2000). Hydroxylation is mediated by cytochrome P450-dependent enzymes, converting -hydroxyacids to either 1,-dicarboxylic acids or alcohols (Agrawal and Kolattukudy, 1978;Le Bouquin et al., 2001). The assembly of the aromatic moiety of suberin, in most cases cinnamic acid derivatives, proceeds via the general phenylpropanoid pathway with Phe ammonia-lyase as the central enzyme (Kolattukudy, 2001). Similar to suberin, lignin is a highly variable biopolymer synthesized in a complex pathway. The basic lignin molecule is derived from the oxidative polymerization of the ...

Vµrzeas are species-rich forest communities of the Central Amazon floodplains, inhabited by highly adapted tree species that can withstand long flooding periods. The leaf shedding behaviour and morphological traits that may contribute to adaptation to low oxygen levels were studied at the Ilha de Marchantaria on the lower Solimðes-Amazonas river, Brazil, and in greenhouse experiments with cuttings of six tree species typical of the Amazon floodplain. Comparison of deciduousness in situ revealed that four of the species under investigation, Salix martiana, Tabernaemontana juruana, Laetia corymbulosa and Pouteria glomerata, are able to maintain their leaf system during the aquatic period. Adventitious roots were formed by S. martiana and T. juruana, but anatomical characteristics differed between the species. Whereas S. martiana developed lysigenous aerenchyma in its roots during aerobic and anaerobic growth, only small intercellular spaces of schizogenous origin were formed in the root cortex of T. juruana. Similar to the latter species, such spaces were constitutively formed in the deciduous species Crateva benthami and Vitex cymosa. Suberin deposits were observed in tangential and radial cell walls of the hypodermis of roots from T. juruana, L. corymbulosa and P. glomerata. Suberin deposits were less pronounced in roots of S. martiana and absent in V. cymosa and C. benthami. The data show that different, almost contrasting, survival mechanisms have evolved in roots of plants with similar life forms in the same habitat. The results further suggest that the morphological traits of the investigated trees are causally linked with the in situ leaf shedding behaviour.

Adaptation to prolonged flooding was investigated using cuttings of two tree species from the Central Amazon white-water floodplain (V�rzea). Morphological features and oxygen distribution patterns were correlated with metabolic changes under hypoxia, such as alterations in alcohol dehydrogenase (ADH) activity and adenylate energy charge (AEC) of root cells. Salix martiana (Leyb.) was able to react to hypoxic growth conditions with formation of adventitious roots rich in lysigenous aerenchyma, which facilitates root aeration by longitudinal oxygen transport and rhizosphere oxidation by radial oxygen loss (ROL). The oxygen concentration on the surface of adventitious roots of S. martiana reached 2-3 mg O2 L–1. The low resistance to gas exchange in Salix roots was reflected by low ADH activities, which ranged between 0.03-0.1 μmol NADH mg –1 min–1, and AEC values of 0.8-1 under hypoxic conditions. Adventitious roots were also formed by Tabernaemontana juruana ([Markgr.] Schumann ex. J.F. Macbride) during growth under low-oxygen conditions, although at a later stage. The gas-space continuum in roots of T. juruana was less pronounced, resulting in a 10-fold lower oxygen concentration in the root cortex under oxygen stress compared with adventitious roots of Salix. The lower oxygen content was reflected in 6-fold higher ADH activities and decreased AEC values. ROL occurred only at the non-suberized root tip, suggesting that the suberized hypodermis functions as a barrier against gas exchange between the root and the rhizosphere. These findings indicate that different strategies of adaptation to low oxygen levels are realized in the two species under investigation that occur naturally in the same ecosystem but inhabit different elevation sites.

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Tree species from the Central Amazon inundation areas are subjected to extreme flooding, with trees being partially submerged for up to 10 months. The rapidly advancing floodwater table at the onset of the aquatic phase interrupts the inward diffusion of oxygen from the atmosphere to submerged plant parts. Salix martiana (Leyb.) (Salicaceae) and Tabernaemontana juruana ((Markgr.) Schumann ex J. F. Macbride) (Apocynaceae), tree species typical of Amazon floodplains, respond to low oxygen concentrations by forming adventitious roots capable of longitudinal oxygen transport. Cuttings of these tree species were subjected to simulated flooding and the oxygen concentration of the root cortex was temporally monitored by oxygen microelectrodes that penetrated the roots. Changes in the floodwater table made it possible to localize precisely the entry points of atmospheric oxygen. Under experimental conditions, mathematical description of the transport kinetics revealed that longitudinal transport of O2 in both species was mainly attributable to diffusion. Based on the finding that diffusion was inhibited by a small increase in the floodwater table, we conclude that internal oxygen transport during a rising water table is only attainable when adventitious roots are continuously and rapidly developed, as is the case in S. martiana. In T. juruana, slow growth of adventitious roots and low root porosity suggest that other adaptations are required to overcome long flooding periods.

Trees inhabiting central Amazon floodplain forests are subjected to an annual flood-pulse lasting up to 10 months, leading to both oxygen shortage and accumulation of high levels of reduced iron. To understand the mechanisms underlying the adaptation to these conditions, cuttings from three tree species typical of várzea inundation forests ( Salix martiana, Tabernaemontana juruana, and Laetia corymbulosa), were cultivated either aerobically or anaerobically under different iron regimes in greenhouse experiments. Although all species are considered to be non-deciduous, Laetia corymbulosa lost and formed new leaves continuously during the experimental period. Although relative growth rates (RGRs) of all species declined in response to hypoxic conditions, no marked changes in RGRs were apparent among different iron concentrations in the growth medium, ranging from 50 to 500 µM, supplied in ferrous form as FeSO 4. Whereas roots exhibited color changes due to the formation of iron precipitates, no visual symptoms of iron toxicity were observed in the leaves. Iron concentration increased in all organs of all species with increasing iron concentrations in the medium, except for leaves of S. martiana and T. juruana, suggesting an effective restriction of iron influx into the leaf symplast. Although the leaf iron concentration was at the upper limit of the critical range at high external iron levels, it is suggested that internal active transport rather than intracellular detoxification mechanisms contribute to the tolerance to supra-optimal iron levels. Anatomical traits such as suberization of peripheral cell walls and the formation of aerenchyma appear to be of minor importance for Fe tolerance