Air plants are peculiar herbaceous plants that can be seen growing on branches and trunks and even on poles and high-voltage cables in much of the American continent. In Spain we can find them in many medieval markets and in some flower shops.
The large group that forms the genus Tillandsia is made up of epiphytic plants—plants that, in their natural state, live on other plants. These are not parasitic species, but within the different forms of interaction, air plants use the so-called commensalism with the trees on which they settle. This means that they benefit from the relationship, in this case, finding in the trees a support on which to stay and grow, while the trees do not obtain any harm or benefit from the air plant.
With roots, but as if not
Despite what many believe, air plants actually do have roots , only they are extraordinarily small and very simplified. In most species of the plant kingdom, the roots perform multiple functions, not only support, but also absorption of water and mineral salts with which the plant is nourished.
However, in air plants, the roots are just a few filamentous rudiments whose only function is to support the plant on the bark of the tree they host —or whatever surface—, and avoid falling into the void. Tillandsia roots, therefore, not only do not touch the ground, but do not even absorb water or mineral salts.
Absorbing water from the air
Air plants aren’t given that name for no reason. During an episode of rain or fog, with an ambient humidity level close to saturation, the leaves of Tillandsia absorb water , in the form of fine droplets, directly from the atmosphere. They can be small droplets retained from fog or dew drops that form by condensation. To perform this function, it has tiny hairs, called trichomes , that cover its leaves and work like a velvet fabric would, retaining the drops of water from a diffuser.
Next, the stomata come into play, a kind of biological valve that all plants have on the underside of their leaves —some, also on the upper side—, which facilitate gas exchange and evapotranspiration . In air plants, when the environment is humid, and once the water is retained, the trichomes open and the water is absorbed into the plant.
Some species live in extraordinarily dry environments, such as Tillandsia landbeckii , which inhabits the Atacama Desert . There, it obtains moisture from condensation and night fog, but during the day it can withstand extreme temperatures and strong sunlight, without dehydrating, thanks to the great specialization of its trichomes, extraordinarily complex compared to other plants.
In the presence of moisture, special cells of the trichome, called dome cells , swell, raising a kind of plug, called the central shield, and allowing water to pass into the stoma, through permeable cells called foot cells. . When the environment is dry, the cells of the dome dehydrate, sinking the central shield and thus closing and waterproofing the stoma.
This system is so ingenious that there is research that, based on the stomata of air plants, seeks artificial valve designs that allow this unidirectional permeability.
Absorbing the good… and the bad from the air
As we already anticipated, air plants absorb their nutrients directly from the atmosphere. This is a difficulty compared to plants that do it through the roots: there is no system that allows filtering according to which chemical substances enter the body. Thus, the plants of the Tillandsia genus absorb everything, even substances that are harmful to them , which makes them an excellent biomarker of air quality .
A biomarker, or biological marker, is any biological unit —from a biomolecule, to a cell, an organism, or even an entire species— that, due to its presence, its absence, or due to certain biochemical, morphological or behavioral responses , can be used as an indicator of some phenomenon. For example, certain animals only live in polluted water, and others require clean and clear water; The presence of both could serve, therefore, as a biomarker of poor or good water quality.
The plants of the Tillandsia genus retain in their tissues those polluting substances that they receive. If these substances are bioaccumulative , as is the case with many heavy metals, they can remain in the body for a long time. Therefore, it is enough to collect air plant samples and analyze them to obtain a reliable and reproducible indicator of air quality.
In fact, an investigation carried out in 2002 by the National University of Córdoba (Argentina), with the collaboration of the University of Buenos Aires and the National Atomic Energy Commission (Argentina) analyzed the air quality in the central region of Argentina, using absorption spectrometry , based on pollutants retained by air plants. They detected, among others, traces of cobalt, copper, iron, nickel, manganese, lead and zinc and were able to infer the origin of each of these contaminants based on the location of the plants and the type of environment existing in each place.
References:
Estrella-Parra, E. et al. n.f. The Tillandsia genus: history, uses, chemistry, and biological activity. 27.
Pignata, ML et al. 2002. Atmospheric quality and distribution of heavy metals in Argentina employing Tillandsia capillaris as a biomonitor. Environmental Pollution, 120(1), 59-68. DOI: 10.1016/S0269-7491(02)00128-8
Raux, PS et al. 2020. Design of a unidirectional water valve in Tillandsia. Nature Communications, 11(1), 396. DOI: 10.1038/s41467-019-14236-5
