Tuesday, May 7, 2013

Seasonal Tuber Development in Portulaca filifolia

In an earlier post I wrote about the apparent uncertainty of tuber formation in the species Portulaca filifolia (Muell.). On the basis of further observations, I  now believe that the development of tubers is seasonal and tubers are most commonly encountered in the autumn and winter, but are generally uncommon during the summer months.

The species Portulaca filifolia germinates reliably in my sand bed each spring and within a few weeks I often have hundreds of individual plants. They receive full sun and natural rainfall and are not provided with supplementary water or fertilizer. By the autumn I generally remove all of the plants and cultivate the ground again, allowing a period of fallow before new seedlings emerge.

By the end of each growing season, most of the lower leaves of mature plants have already shriveled and fallen away and the stems and terminal leaves wilt slightly and turn a pale yellow or reddish colour. The branches tend to droop over, often becoming prostrate or almost so. At this time the seedlings also suspend all above-ground growth. Their leaves become bunched on one or a few short branches which tilt forward and remain perpendicular to the stem axis throughout the dry season.

This autumn the plants removed from my sand bed ranged in age from seedlings to mature individuals. I found that around 98% of the population had well-developed, oblong-shaped tubers on their root systems. In some cases the main taproot was tuberous, but in other cases the tubers were located near the middles or ends of the root strands.

The tubers were up to 35 mm long and 5-8 mm wide, and although pliant could be easily snapped in two. The inner part of each tuber was creamy or light brown in colour and quite juicy, having the texture, smell, and flavour of freshly dug potatoes. The tubers on the seedlings were mostly single, about 5-10 mm long, and more or less turnip-shaped. They were generally white or creamy in colour and slightly translucent compared to the tubers of the more mature specimens.

The exception were the few large plants that had passed their prime and were close to expiring. I assume that those plants must have expended all of their energy and resources in vegetative growth and seed production through the warmest part of the growing season. They had not wasted any of their energy and resources on tuber development late in the season. Plants that had recently expired also showed no signs of tuber formation. However, all of the mature plants and seedlings possessed tubers.

Tuber development on P. filifolia plants in Autumn 2013. Photo by Ian Menkins.

It seems to me now that tuber development is most likely a winter survival strategy. Plants that develop tubers are able to weather the dry, cold conditions of winter, then re-shoot in spring to rapidly resume flower and seed production. The winter is just an inconvenient hiatus for these plants. The old adage "the early bird catches the worm" aptly sums up this strategy.

There is a distinct advantage for any plant that does not have to waste time and energy starting out as a seed at the end of every winter. Seeds are just one survival strategy employed by plants, but a plant with a tuber can not only survive the winter, it can also re-shoot immediately after the first rains of the growing season.

The reason why tubers were absent in December 2012 was because the growing season had been much shorter. The winter had been dry so plants had germinated late, then grown rapidly in response to an increase in rainfall, soil moisture, day length, and temperatures. The plants directed all of their resources and energy into vegetative growth and flower and seed production. Compare the above photo with the ones on my previous post here. It is possible that some of these plants would have gone on to develop tubers if left to grow through until the autumn.

The following line graph illustrates the predicted pattern of tuber formation in an average season.

Tubers develop as a result of complex relationships between the external environment and the internal genetics and biochemistry of a plant. We can presume that the age of the plants in relation to a combination of seasonal factors, including soil and air temperature, day length, humidity, and soil moisture trigger a biochemical response in the plant. These chemical changes cause changes to occur to the plants metabolism and morphology at the cellular level.

The benefit of tuber development is that it provides plants that are still capable of producing seed at the end of summer with an extended life span. Tuber formation is therefore a survival strategy utilized at a time when seasonal changes force all above-ground vegetative production to cease.

I am certain that there will be exceptions to the rule, as not every season is identical. I have in the past found that during occasional wet winters many plants with tubers have rotted out. Success of any survival strategy depends a great deal on average seasons, but this is apparently a gamble the plants take in the interests of long-term survival. The fact that P. filifolia is such a widespread and generally common species suggests that it is very well adapted to the seasonal yet somewhat unpredictable nature of the Australian climate. The development of tubers provides an extension to the plant's life cycle. Depending on the seasons, the species P. filifolia is capable of either completing its life cycle as an annual or continuing as a short-lived herbaceous perennial.

It is interesting that similar patterns of tuber development that are influenced by planting times and environmental factors have also been noted in commercial crop production, for example in potatoes. This article (click to access) from the The Journal of Animal & Plant Sciences is a case in point. I realize that Portulaca tubers are not the same as potatoes, and of course the former has root tubers whereas the latter has underground stem tubers. Another difference is that seedlings of P. filifolia can also produce tubers and this development conforms to the same seasonal and environmental conditions that govern tuber development in the mature plants of that species. But I suspect that some of the same rules may apply.  Another paper with similar findings is Kleinkopf et. al., "Physiology of Tuber Bulking".

The papers raise many more questions. For instance, I wonder if an increase in nutrient levels (particularly Nitrogen) during the growing season could result in a decrease in tuber formation in Portulaca filifolia.

There may also be seasonal micorrhizal associations with some species of Australian Portulacaceae, but as far as I know this possibility has never been investigated in the Australian species. In the absence of studies, it could be unwise at this time to speculate about how such an association may effect nutrient levels and/or influence tuber development.