No abstract

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 ...

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.

No abstract