Friday, January 25, 2013

Some Surprises in Portulaca oleracea

Portulaca oleracea, flowering branch.
Photo courtesy of Attila Kapitany.
Portulaca oleracea L. is one of the most commonly seen and easily recognizable species in Australia. The species has an almost worldwide distribution. It is generally a very easy species to identify because the bud is always laterally compressed (as if pinched) and carinated (i.e. forming a keel-like ridge). However some closely related species, such as P. lutea Sol. ex Seeman and P. molokiniensis Hobdy from Hawaii, also possess this character.

P. oleracea, showing ovate leaves, small flowers with less than 20 stamens,
and buds laterally compressed 
and carinated.  
Photo courtesy of Attila Kapitany.

There are native and introduced forms of P. oleracea in Australia, and the populations have become so intermixed in coastal and suburban areas today that it could be an impossible task to separate them. It is only in the outback areas that native forms are still prevalent today. However I am sure that some exotic seeds have been spread far and wide by farm and road machinery. But in the majority of cases, the individuals that make up a population generally appear fairly uniform at any given location. 

This "typical" form of P. oleracea is often found as a weed
in many suburban gardens throughout the world. The form
frequently has fibrous roots and lacks a tuberous taproot.
Photo courtesy of Attila Kapitany.

One of the most common variations is leaf and stem colour, with populations often having colours that appear to mimic the surrounding soil colour. In other cases, leaf and stem colour may change due to pigmentation that is probably activated by exposure to extremes of light, heat, and cold. The leaves may start off green but later develop deep purple to almost black tonings. In Central Australia, where the soil colour is predominantly red, the plants there are more often found in pink, orange, red or light purple colours.

P. oleracea is one of the easiest of all Portulaca species to
recognize. In exposed situations (above) it typically has a
ground-hugging habit, thick spreading branches and rather
flat, ovate leaves. Some plants may be decumbent (middle),
or upright during wet seasons or cloudy weather (below)
 or when growing in shaded situations.

Photos courtesy of Attila Kapitany.

A perennial form that is sometimes found on inland clay plains is presumed to be a native. It typically has a rather tardy growth habit, short branches, fairly compact leaves, and a tuberous taproot. A contrasting form that is considered native to desert areas is a large and sprawling, red-stemmed annual with broad, flat leaves. Both are commonly referred to as 'Munyeroo', however this name may also be applied to Portulaca intraterranea Black, a species found on red sand dunes with shorter branches and significantly larger flowers.

I once thought of P. oleracea as a common and rather boring weed species, of some interest perhaps as a food plant, but generally lacking any appeal as an ornamental. But my opinion was to change when I started seeing interesting variations in P. oleracea, particularly in inland populations. I realized that this species deserved a second look. Around the same time, the author Attila Kapitany was making similar observations while conducting field research for his book Australian Succulent Plants. Some of his photos of atypical plants have been reproduced here.

Of particular interest to me were specimens of P. oleracea found on inland plains and claypans with very thick, crowded and overlapping leaves, rather short, thick stems, and a significant tuber. Many of these plants resembled P. lutea and P. molokiniensis from Hawaii, apart from the generally smaller flowers.

A truly bizarre form of P. oleracea.
Photos courtesy of Attila Kapitany.

The odd-looking plant in the above photographs was collected from a salinity scald. The blisters on the stems are an interesting feature, as well as the shortened, cascading stems and tightly packed leaves. Note too the small stalked glands on the upper margins of the leaves. The plant possessed a large tuber and swollen stems and a well developed caudex. It lived in cultivation for some 5 years, even winning a prize at a plant show! 

Short swollen branches, compact terminal foliage,
 and well-developed tubers on a variant of P. oleracea.
Photos courtesy of Attila Kapitany.

Such "salinity-adapted" plants as this one clearly have some attributes that are genetically-fixed. However it is not always possible to reproduce a plant with the same characters from seed. Furthermore, these kinds of plants occasionally lose many of their characters once they are transplanted to more favourable conditions. The plant above did maintain many of its original characters in cultivation, however it was grown quite hard in a small bonsai container. I have planted similar plants into my garden and they have soon become quite rampant. They always seem to retain some of their original characters though, such as a distinct taproot, shorter more swollen stems and smaller leaves. Their offspring also frequently retain some of the characters of the parent plants, but it is generally not possible to recreate all of the characters seen above in the absence of salinity.

Salinity is certainly one factor that can induce greater succulence in some species of plants. It is in fact a powerful evolutionary force that is undoubtedly directing variation and speciation in many plants throughout the world. However in some saline desert areas in Australia it is possible to find typical and atypical forms of P. oleracea co-habiting the same environment. The variation cannot be easily explained by soil chemistry alone, considering that both varieties are occupying the same saline habitat. 

In these situations it is possible that the more succulent form has existed there for thousands of years, gradually adapting to a hostile environment and changing climate, while the sprawling form is potentially a more recent arrival. The other possibility is that both forms are native but have adopted divergent survival strategies. The annual may be exploiting the infrequent wet seasons opportunistically, while the highly succulent form is a perennial that has adapted to survive the long periods between wet seasons. Further study of such populations may reveal individuals that are intermediate between the two; for instance some sprawling forms may possess swollen taproots allowing them to survive for more than one season.

Whether they be native or naturalized, it is at least clear to me that different survival strategies are at work in different habitat populations throughout Australia. This long-term adaptive process is producing some surprising variations in P. oleracea which may in some cases eventually lead to speciation. It is likely that some undescribed species allied to P. oleracea are already in existence.

A remarkable, bright red form of P. oleracea with "jelly bean" leaves.
The green plants beside it are a Calandrinia species.
Photos by Attila Kapitany.

Variation can only be expected in a species like P. oleracea that has such a very wide range of distribution. The species also occupies many different habitat types. The variations in P. oleracea clearly deserve more study, but whether any sense can ever be made of them is another matter. There are certainly some (poorly understood) geographical variants in existence which are forming fairly consistent populations in certain habitats, while plants growing side by side in other habitats can be very different in morphology. The variations appear to be due to a range of complex combinations of genetic and environmental factors. 

Some of the morphological variations do appear to be genetically-fixed, consistent, and stable, while others are more sporadic, or less consistent, and may be the result of significant environmental stresses, such as salinity, high pH levels or continuous predation. Plant mutations that result from naturally-occurring soil chemicals can, on the one hand, be viewed as a symptom of disease. But, on the other hand, the ability to change as a result of environmental factors is frequently regarded by scientists as a sign of genetic adaptation and evolution. 

Unfortunately the variations evident in P. oleracea have so far defied most attempts at classification, and few botanists have ever been courageous enough to study the species in any depth. Science still has much to understand about this species.

Another atypical form of P. oleracea with very clustered foliage
 on short stems and flowers to 1 cm diameter. Such plants resemble
P. lutea from Hawaii and some are approaching the range of
 P. intraterranea, however the stamens often number less than 20.
This keeps them within range of P. oleracea (according to the keys).

Photos courtesy of Attila Kapitany.


Attila Kapitany said...

I failed leaving a comment yesterday - so I'll try again!
You certainly are covering Portulaca oleracea with a more deserving overview than it just being 'an insignificant weed'. you've shown great diversity and analysis which can only lead to a greater appreciation of our desert flora.
Great work! Attila

grey_gum said...

Hi Attila, Thank you for your positive comments, and for the assistance you provided with photos and for taking the time to answer every one of my email questions. Much of the credit here goes to your excellent photos, without which I would have been struggling to explain some of the variations. A picture is, as they say, really worth a thousand words. One question I forgot to ask you... Was the bright red one with "jelly bean" leaves found in a saline or non-saline habitat?

grey_gum said...

I have just received an email (thanks Martin!) pointing out that the blisters on the stems of the salt-adapted plant are salt blisters. They are a strategy that the plant has adopted to contain the excess salt. The leaf glands serve a similar function, but rather than contain the salt their function is to "bleed" it out and away from the sensitive leaf tissues. Many other halophytic plants have adopted these morphological features to process the excess salt in their environment, such as Philotheca sporadica, some Lepidium spp., Sesuvium portulacastrum, Sarcocornia spp., Dysphania spp., Atriplex spp., to name but a few.

Attila Kapitany said...

At this stage I can't say whether the red 'jelly bean'leaved plant was in saline environment or not. Will explore my notes and get back to you if I have helpful information.