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


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.
Winkler
Uwe
U
Functional Ecology Group, Universität Oldenburg, Oldenburg, Germany. u.winkler@uni-oldenburg.de
Zotz
Gerhard
G
D031229:Bromeliaceae / Q000378:metabolism*; D018515:Plant Leaves / Q000378:metabolism*; D011188:Potassium / Q000378:metabolism*
D011188:Potassium