ATB-Colloquium: 

Final solutions to long-standing mysteries about the ecophysiologies of an American epiphyte and its primary host

Speaker: Dr. Craig Martin, University of Kansas

The outer surfaces of many epiphytic bromeliads, especially species of Tillandsia, are densely covered with large, reflective scales or modified hairs, termed trichomes, which have been shown to replace the functions of roots, as many of these plants lack functional roots. Such functions include uptake of liquid water and of elemental nutrients dissolved in that water. This foliar absorption of water and nutrients is quite rare in plants. The trichomes are multicellular and comprise two main parts: a stalk of living cells that connects with the living tissues of the leaf, and a “shield” composed of many, non-living, thick-walled cells. When the leaf is wetted, the shield cells absorb water, which then moves to the living stalk and then into the leaf. In the past several decades, several plant ecophysiologists, including the USA scientist Craig Martin and the Italian scientist Amalia Virzo De Santo, while studying the ecophysiology of Tillandsia species, made a surprising observation. At night, when air humidity increased, the epiphytes absorbed water from the air. This result, considered impossible for nearly all plants, was measured with Tillandsia usneoides in the field in North Carolina, USA by Dr. Martin and with several species of Tillandsia in the laboratory by Dr. Virzo De Santo. In both cases, the water absorbed at night was lost during the subsequent day, when atmospheric humidity declined. While Dr. Virzo De Santo and others claimed that this humidity-driven nighttime uptake of atmospheric water vapor was beneficial to the plant’s physiology, Dr. Martin argued that hydration of the living cells of these epiphytic bromeliads resulting from this uptake of water vapor was theoretically impossible. While theoretical arguments are powerful tools in science, they cannot provide definitive answers to questions without experimental evidence, and there existed no direct experimental evidence for or against the potential physiological benefit of water vapor absorption by the trichomes of epiphytic bromeliads.

With this in mind, Dr. Martin came to Potsdam to collaborate with his friend and colleague Dr. Werner Herppich at the Leibniz Institute for Agricultural Engineering Potsdam-Bornim (ATB). The two scientists devised an experiment to test the above ideas.

Using plants of Tillandsia usneoides, kindly provided by Dr. Michael Burkart of the University of Potsdam Greenhouse facility, they, along with another ATB scientist, used a tiny probe to directly measure the turgor pressure of a living epidermal cell very near a large trichome before and after the plant was exposed to an increase in air humidity. If the water vapor subsequently absorbed by the nearby trichome were to benefit the physiology of the plant, the turgor pressure of the nearby living cells must increase when the humidity increased.

In several experiments using different plants of T. usneoides, the American-German scientific team never measured an increase in turgor pressure of the epidermal living cells after exposure of the plant to high humidity.

This was the first time the question of the biological importance of the unusual uptake of water vapor had been experimentally tested, and the results confirmed theoretical predictions.