Tag Archives: epiphytes

Plant parts doing unexpected things: Part 2 (or, Root research all up in the air)

Image: Wikimedia Commons.

Image: Wikimedia Commons.

Our suitably erudite – albeit neophyte – botanical generation who knew about the functions of plant stems when quizzed previously (see Plant parts doing unexpected things: Part 1, posted previously) would probably do equally well when asked about the main roles of roots*. However, what they may be surprised to learn is that some roots photosynthesise (yes, like stems or leaves). We’re not talking about ‘typical’ soil-surrounded roots, but the so-called aerial roots of epiphytic plants perched high above the ground on trees – for example certain orchids. These photosynthetic roots dangle in the air that surrounds the epiphyte and its host plant. Whilst a photosynthetic capability is unusual for a root that is typically subterranean, you might expect that gain of this function might be at the expense of another, more typical root role, say absorption. But no, such roots still retain the capacity to absorb water from their surroundings. However, rather than rely on the assistance of root hairs as for their terrestrial, soil-rooted relatives, nature has equipped these aerial roots with an additional tissue, the velamen. The velamen is a remarkable multi-layered epidermis-like structure whose specially thickened cells not only absorb water from the humid air or rain water, but also help to reduce transpiration from the internal root tissues when the velamen cells are dried out. There is still much to uncover about the role of the velamen in the biology of epiphytes, but an interesting discovery has been made by Guillaume Chomicki et al., and one that relates not to the plant’s water relations but to the integrity of the root’s photosynthetic process. Recognising that levels of damaging ultraviolet B (UV-B) radiation are high in the epiphytes’ habitat, and knowing that UV-B screening compounds such as flavonoids help to protect leaves, the team wondered how similarly challenged, photosynthetic roots might be protected from UV-B harm. Using a nice combination of molecular and structural techniques – gene expression analyses, mass spectrometry, histochemistry and chlorophyll fluorescence – they demonstrated that UV-B exposure resulted in inducible production of two UV-B screening flavonoids within the living (i.e. young) velamen of Phalaenopsis × hybrida, but which compounds persist in the cell walls of the functional – dead – velamen tissue. Furthermore, and interestingly, this root mechanism of UV-B protection is apparently different from that employed by leaves. A case of same destination, different routes? Not bad for a dead tissue one could easily write-off as merely acting like a sponge!

* Which, for completeness, are generally assumed to be: anchorage of plant in soil, absorption of water/minerals from the soil, storage of reserve materials, and conduction of water/nutrients to/from the stem – Ed.

“It’s an ill wind…

Hauck et al Annals of Botany vol 108

Lichens don't like it as much as they used to...

… that blows nobody any good” is an old English idiom that suggests that most bad things that happen have a good result for someone, somewhere. And Markus Hauck and colleagues in Göttingen, Germany, have illustrated this nicely – and rather literally – with some work looking at lichens growing in forests in Germany. In the days before global warming hit the headlines, “acid rain” was the big environmental concern in Europe, with westerly winds from the Atlantic picking up polluted air from industrialised regions (of the UK in particular) and blowing it up across Scandinavia and northern Germany, where it would be washed out in rainfall. This was a time when the UK was fuelled by coal, and that coal contained a lot of sulphur – so the smoke was heavy in sulphur dioxide, which dissolved in the clouds to produce the aforementioned “acid rain”. From the early 1980s onwards, there was widespread concern that large areas of forests in northern Europe were being damaged and public concern led to legislation to reduce pollution. The result has been that over the last 20 years sulphur dioxide emissions have fallen back to levels not seen since the early days of industrialisation in the 19th century. But what Markus Hauck and his team have found is that one of Europe’s most common lichens, Lecanora conizaeoides, actually quite liked this “ill wind”, as it thrives in acidic conditions. Their study in the Harz Mountains of northern Germany shows that the lichen has undergone a dramatic decrease in abundance within only 15 years, from being the most dominant species of its type to a point where it can now be described as rare. Their analysis suggests that this is attributable to just a very slight decrease in the acidity of the bark on the trees on which the lichen lives, a change of only 0.4 pH units. To put that into context, that is about half the difference in acidity that you would find between a glass of orange juice and one of tomato juice. So for this lichen at least, a fair wind is not as welcome as an ill one. Full details of the work can be found in the August issue of Annals of Botany.

Increased pH and dieback of lichen

Increased pH and dieback of lichen
Increased pH and dieback of lichen

Lecanora conizaeoides is an epiphytic lichen adapted to very acidic conditions, and a reduction in atmospheric SO2 in western and central Europe has coincided with a dramatic decline in its distribution. Hauck et al. study lichen diversity and bark chemistry in northern Germany and compare the results with comparable data collected 13–15 years previously. Neither competitors nor parasites that might have benefitted from the reduction in SO2 are likely to account for the decline in L. conizaeoides cover on trees, and the results instead suggest that an increase of only 0.4 in bark pH is responsible.