Central Amazon floodplain: oscillation of a pulsing system
The Central Amazon floodplains represent one of the largest inundation area in the world. Dominated by forest societies, the floodplains of Central Amazonia consist of a complex system of inundated river valleys, shallow lakes, and channels along the Solimões-Amazonas river. The annual swelling of the rivers leads to lateral overflow of the adjacent forests, posing a multiplicity of constraints to the trees that inhabit this area. Near Manaus, Brasil, the floodplains are subjected to a monomodal floodpulse with an average amplitude of ten meters during a period of up to ten months. Tree species of the inundated forests have evolved different strategies to encounter these environmental conditions which become visible in the leaf shedding behavior during the aquatic phase. Some species show a rapid turnover of the photosynthetic active area by continuous production of new leaves, other species defoliate completely or do not show any change in leaf production during the year at all. The target of our research is to decipher the different stratagems of tree species naturally occurring in the white-water inundation areas (várzea) to the severe growth conditions with special emphasis on adaptive processes of roots. The study is part of the research which is carried out jointly by the Tropical Ecology Group of the Max-Planck-Institute for Limnology in Plön, and the Instituto Nacional de Pesquisas da Amazônia (INPA), in Manaus.

Morphological adaptations: the role of gas spaces and suberin
Under anaerobic conditions, some of the trees under investigation show morphological responses to prolonged flooding, such as formation of adventitious roots, the presence of air spaces in the root cortex, and the development of apoplastic barriers in the root exodermis or hypodermis. Our findings suggest a direct relationship between the suberization of the roots and the leaf shedding behaviour. For example, the deciduous tree species lack the ability to develop suberin deposits in the root hypodermis of young growing root tips. The chemical composition of isolated hypodermal cell walls is object of recent investigations in co-operation with the University of Bonn, Institute of Botany.

Oxygen deficiency and energy status: alternative energy metabolism
The capacity to withstand anaerobic conditions requires flood response mechanisms which aids to maintain the energy level for cell metabolism and nutrient uptake. Under hypoxic conditions, respiration is restricted and the maintenance of cellular energy metabolism is mainly achieved by fermentation. The effect of hypoxia on the adenylate energy charge (AEC) and changes in the expression of respiratory and fermentative enzymes are investigated in trees which are subjected to long flooding periods. Ethanol produced in the roots under hypoxic conditions by fermentation is transported to the leaves via the transpiration stream in the xylem, oxidized, and subsequently emitted as acetaldehyde via the stomata. Emission of acetaldehyde and other volatile organic compounds (VOCs) by trees in response to flooding is studied in cooperation with the Max-Planck-Institute of Chemistry, Department of Biogeochemistry .

Rhizosphere conditions: oxygen as a detoxifying compound
Quantification of oxygen levels within the roots and the rhizosphere is performed using oxygen electrodes in agar-embedded plants. In flooded soils, reduction of radial oxygen loss (ROL) is of adaptive value, since it enhances longitudinal diffusion of O2 and, in turn, allows for a higher respiration rate of root cells. Investigations on the oxygen distribution in the roots and the rhizosphere of the aerenchymatous species S. martiana showed that the radial oxygen leakage out of the roots was able to create a several millimetre thick oxygenated zone around the roots, suggesting a role of ROL in detoxifying reduced phytotoxins in the rhizosphere. These investigations are carried out in co-operation with the Max-Planck-Institute for Limnology, Tropical Ecology Group.

Phytotoxins: what to do with divalent iron
Cellular iron homeostasis has to be strictly controlled in order to prevent iron deficiency or toxicity when the extracellular concentration of iron varies. In flooded soils, due to the decrease in soil redox potential, extracellular iron can reach phytotoxic concentrations by reduction of insoluble Fe(III) oxides to Fe2+. Excessive and uncontrolled uptake of Fe2+ leads to toxicity symptoms such as blackening of root tips, inhibition of root growth, root flaccidity and necrotic spots on the leaves (`bronzing´). These deleterious effects are mainly attributed to oxygen stress induced by free radical generation through Fenton chemistry. Our investigations focus on morphogenetic and endogenous mechanisms that help to avoid the build-up of toxic iron concentrations in the plants when subjected to high Fe2+ levels.