Whilst looking for an example of a plant which uses C4 photosynthesis for the coming term’s plant physiology lectures in Durham, I came across a familiar-looking image. A plant which glories in the name of Borszczowia aralocaspica looks very like the pretty plant I speculated about in February’s blog post – CAM, CAM, CAM. Both have tiny, fleshy, cylindrical leaves and Borszczowia is described as a halophyte (salt-tolerant plant) growing in the deserts of Central Asia.
My mystery plant
Somewhat to my surprise, Borszczowia turns out to be a member of the Chenopodiaceae, a family which in the UK includes familiar ‘weeds’ such as Fat-hen and Good-King-Henry, but also the glasswort or Salicornia which we sometimes buy from the fishmonger in the market as a tasty treat with fish.
Anyway, it turns out that there is a relative of Borszczowia called Halogeton glomeratus, found from Afghanistan to Kashmir and Ladakh, and my mystery plant seems to fit the bill. What is more surprising is to find out that Halogeton is regarded as an invasive, toxic weed by the US department of Agriculture. Introduced into Nevada from Asia during the early 1930s, Halogeton is now held responsible for the deaths of large numbers of sheep in the western states. It spreads easily along roadsides and in arid and semi-arid regions, outcompeting more desirable species.
The plant’s toxicity comes from sodium and potassium oxalates which it accumulates to deter herbivores but another problem is its ability to accumulate salts from the soil. By sequestering salts safely in its tissue (in cell vacuoles and specialised water storage cells) Halogeton, like other halophytes, can tolerate levels of salinity in the soil which most plants cannot; something else it has in common with Salicornia. It can tolerate alkaline soils for similar reasons. This means that, when the plants die, the accumulated salts leach out and are deposited on the soil surface, inhibiting the germination and establishment of non-halophyte seedlings. Halogeton thus gets a head start in the competition for light and water.
The plant is well adapted for a harsh, arid environment with its reduced, fleshy leaves. It sports a tap root up to 50 cm long to help anchor it in disturbed ground and to make sure it has access to whatever water is present deep in the soil. What look like pretty pink flowers are actually bracts, with a tiny flower located in the centre of each cluster, rather like Poinsettia or Bougainvillea ‘flowers’.
Halogeton glomeratus flowers
The plant produces many, very tiny seeds which are easily dispersed by traffic and animal movement, especially where the ground is disturbed by road maintenance. It is no coincidence that we found it alongside the notoriously-unstable Zoji La pass in Kashmir, which has to be regraded every spring when the road reopens after winter snows have melted. The seeds germinate very rapidly on coming in contact with water.
Zoji La pass
Incidentally, it looks like I was wrong in guessing that Halogeton is a CAM plant. In fact, it is known to use C4 photosynthesis, a modified version of the standard process designed to help concentrate carbon dioxide when its concentration is low and when temperatures are high. Unlike CAM plants though, the stomata still open during the day rather than at night. More of this anon, perhaps…
USDA Plant Guide: http://plants.usda.gov/plantguide/pdf/pg_hagl.pdf
USDA (1965) : Ecological and physiological factors influencing the chemical control of Halogeton glomeratus. Technical bulletin No. 1325.