Uwe Winkler

Highly efficient uptake of phosphorus in epiphytic bromeliads

BACKGROUND AND AIMS Vascular epiphytes which can be abundant in tree crowns of tropical forests have to cope with low and highly intermittent water and nutrient supply from rainwater, throughfall and stem flow. Phosphorus rather than nitrogen has been suggested as the most limiting nutrient element, but, unlike nitrogen, this element has received little attention in physiological studies. This motivated the present report, in which phosphate uptake kinetics by leaves and roots, the subsequent distribution within plants and the metabolic fate of phosphate were studied as a step towards an improved understanding of physiological adaptations to the conditions of tree canopies. METHODS Radioactively labelled [(32)P]phosphate was used to study uptake kinetics and plant distribution of phosphorus absorbed from bromeliad tanks. The metabolism of low molecular phosphorus metabolites was analysed by thin-layer chromatography followed by autoradiography. KEY RESULTS Uptake of phosphate from tanks is an ATP-dependent process. The kinetics of phosphorus uptake suggest that epiphytes possess effective phosphate transporters. The K(m) value of 1.05 microm determined for leaves of the bromeliad Aechmea fasciata is comparable with values obtained for the high affinity phosphate transporters in roots of terrestrial plants. In this species, young leaves are the main sink for phosphate absorbed from tank water. Within these leaves, phosphate is then allocated from the basal uptake zone into distal sections of the leaves. More than 80 % of the phosphate incorporated into leaves is not used in metabolism but stored as phytin. CONCLUSIONS Tank epiphytes are adapted to low and intermittent nutrient supply by different mechanisms. They possess an effective mechanism to take up phosphate, minimizing dilution and loss of phosphorus captured in the tank. Available phosphorus is taken up from the tank solution almost quantitatively, and the surplus not needed for current metabolism is accumulated in reserves, i.e. plants show luxury consumption. Young, developing leaves are preferentially supplied with this nutrient element. Taken together, these features allow epiphytes the efficient use of scarce and variable nutrient supplies.

Aerial roots of epiphytic orchids: the velamen radicum and its role in water and nutrient uptake

The velamen radicum, a spongy, usually multiple epidermis of the roots, which at maturity consists of dead cells, is frequently described as an important adaptation of epiphytic orchids. Yet, quantitative evidence for the alleged functions, e.g., efficient water and nutrient uptake, nutrient retention, reduction of water loss, mechanical protection, or the avoidance of overheating, is rare or missing. We tested the notion originally put forward by Went in 1940 that the velamen allows plants to capture and immobilize the first solutions arriving in a rainfall, which are the most heavily charged with nutrients. In a series of experiments, we examined whether all necessary functional characteristics are given for this scenario to be realistic under ecological conditions. First, we show that the velamen of a large number of orchid species takes up solutions within seconds, while evaporation from the velamen takes several hours. Charged ions are retained in the velamen probably due to positive and negative charges in the cell walls, while uncharged compounds are lost to the external medium. Finally, we demonstrate that nutrient uptake follows biphasic kinetics with a highly efficient, active transport system at low external concentrations. Thus, our results lend strong support to Went’s hypothesis: the velamen fulfills an important function in nutrient uptake in the epiphytic habitat. Most of the other functions outlined above still await similar experimental scrutiny.

A new type of a multifunctional β-oxidation enzyme in Euglena

The biochemical and molecular properties of the β-oxidation enzymes from algae have not been investigated yet. The present study provides such data for the phylogenetically old alga Euglena (Euglena gracilis). A novel multifunctional β-oxidation complex was purified to homogeneity by ammonium sulfate precipitation, density gradient centrifugation, and ion-exchange chromatography. Monospecific antibodies used in immunocytochemical experiments revealed that the enzyme is located in mitochondria. The enzyme complex is composed of 3-hydroxyacyl-coenzyme A (-CoA) dehydrogenase, 2-enoyl-CoA hydratase, thiolase, and epimerase activities. The purified enzyme exhibits a native molecular mass of about 460 kD, consisting of 45.5-, 44.5-, 34-, and 32-kD subunits. Subunits dissociated from the complete complex revealed that the hydratase and the thiolase functions are located on the large subunits, whereas two dehydrogenase functions are located on the two smaller subunits. Epimerase activity was only measurable in the complete enzyme complex. From the use of stereoisomers and sequence data, it was concluded that the 2-enoyl-CoA hydratase catalyzes the formation ofl-hydroxyacyl CoA isomers and that both of the different 3-hydroxyacyl-CoA dehydrogenase functions on the 32- and 34-kD subunits are specific to l-isomers as substrates, respectively. All of these data suggest that the Euglena enzyme belongs to the family of β-oxidation enzymes that degrade acyl-CoAs via l-isomers and that it is composed of subunits comparable with subunits of monofunctional β-oxidation enzymes. It is concluded that the Euglena enzyme phylogenetically developed from monospecific enzymes in archeons by non-covalent combination of subunits and presents an additional line for the evolutionary development of multifunctional β-oxidation enzymes.

'And then there were three': highly efficient uptake of potassium by foliar trichomes of epiphytic bromeliads.

BACKGROUND AND AIMS Vascular epiphytes have to acquire nutrients from atmospheric wash out, stem-flow, canopy soils and trapped litter. Physiological studies on the adaptations to nutrient acquisition and plant utilization of nutrients have focused on phosphorus and nitrogen; potassium, as a third highly abundant nutrient element, has received minor attention. In the present study, potassium uptake kinetics by leaves, within-plant distribution and nutrient accumulation were analysed to gain an improved understanding of physiological adaptations to non-terrestrial nutrient supply of plants. METHODS Radioactively labelled (86)RbCl was used as an analogue to study uptake kinetics of potassium absorbed from tanks of epiphytes, its plant distribution and the correlation between uptake efficiency and abundance of trichomes, functioning as uptake organs of leaves. Potassium in leaves was additionally analysed by atomic absorption spectroscopy to assess plant responses to potassium deficiency. KEY RESULTS Labelled rubidium was taken up from tanks over a wide range of concentrations, 0.01-90 mm, which was achieved by two uptake systems. In four tank epiphytes, the high-affinity transporters had average K(m) values of 41.2 microm, and the low-affinity transporters average K(m) values of 44.8 mm. Further analysis in Vriesea splenriet showed that high-affinity uptake of rubidium was an ATP-dependent process, while low-affinity uptake was mediated by a K(+)-channel. The kinetic properties of both types of transporters are comparable with those of potassium transporters in roots of terrestrial plants. Specific differences in uptake velocities of epiphytes are correlated with the abundance of trichomes on their leaf surfaces. The main sinks for potassium were fully grown leaves. These leaves thus function as internal potassium sources, which allow growth to be maintained during periods of low external potassium availability. CONCLUSIONS Vascular epiphytes possess effective mechanisms to take up potassium from both highly diluted and highly concentrated solutions, enabling the plant to incorporate this nutrient element quickly and almost quantitatively from tank solutions. A surplus not needed for current metabolism is stored, i.e. plants show luxury consumption.

Heteroblasty in bromeliads – anatomical, morphological and physiological changes in ontogeny are not related to the change from atmospheric to tank form

Heteroblasty is defined as an abrupt change in gross morphology during ontogeny, whereas homoblastic species show no or gradual changes. For Bromeliaceae, there are conflicting reports on a very limited number of species on the functional importance of this step change compared with gradual changes (ontogenetic drift). Studying a large set of species should allow more general conclusions. Seventeen homoblastic and heteroblastic species from Panama were investigated, including the entire size range of most species. Measurements included functionally relevant anatomical (water storage tissue), morphological (stomatal and trichome densities) and physiological parameters (transpiration rates, nutrient uptake rates). Size-related variation in all parameters was common, but evidence for a step change in the studied parameters could not be detected in any of the heteroblastic species. Our results caused us to question the widely held view of the course of the ontogenetic development in heteroblastic bromeliads and their functional implications. These findings suggest that the possible functional relevance of heteroblasty in bromeliads require rethinking and future investigations should employ a comparative approach with both homoblastic and heteroblastic species and including the entire size range to account for ontogenetic drift.