By Matt Walker
Editor, Earth News
Standing the test of time
Plants have a unique ability to defy the ill effects of ageing, say two scientific experts.
Instead of physically declining with age, as humans do, perennial plants avoid a progressive deterioration in their physiological functions.
As a result, older plants are no more likely to die than younger ones, and are just as capable of reproducing.
The experts publish their claims about this extraordinary ability of plants in the journal New Phytologist.
The distinction between a plant's organs, and it as a whole organism, is the key to understanding how it can defy ageing, they argue.
For example, as a plant ages, its organs such as leaves and flowers do show signs of physical deterioration and of growing old.
Older trees are no more likely to die than young ones
This can manifest itself in reduced rates of photosynthesis, growth, and lack of vigour in flower buds.
But that does not mean the plant overall suffers.
Also, as a plant gets older it tends to increase in size, and growth tends to slow.
Larger plants need more water and nutrients, which might limit their growth, and hence the growth of new organs such as leaves and flowers.
But that does not necessarily lead to a deterioration of the plant at the organism level.
And that means that an old tree is no more likely to die than a younger one, say ecologist Josep Penuelas of the Autonomous University of Barcelona, Spain and plant biologist Sergi Munne-Bosch of the University of Barcelona, Spain.
Often cells age as they accumulate genetic mutations over time.
But writing in the journal, the researchers describe how such mutations do not appear to damage a plant's meristem, the part that is actively growing via cell division, which usually occurs at the tip of a shoot or root.
For example, one study found no evidence of accumulation in somatic mutations in the meristems of bristlecone pines (Pinus longaeva), regardless of whether the pine was 23 years old or up to 4713 years old.
Other plant species, such as the endangered Tasmanian shrub known as King's Lomatia (Lomatia tasmanica), appear capable of almost mind-boggling lifespans.
The plant, which reproduces exclusively asexually, appears to have continually cloned itself for at least 43,600 years, the researchers say.
Some studies have shown an increase in tree mortality rates in the western US, southern Europe and globally.
But these are caused by temperature increases and a lack of water, rather than aging.
"Furthermore, no study has demonstrated so far that increased mortality at old age is associated with a physiological deterioration of meristem function in plants," the researchers write.
In short, older plants die due to outside influences, such as storms, wildfires or pollution, not because they are aged.
And fewer older plants survive simply because the chances of being killed by an external factor increases with time.
Some studies have even shown that plants produce more flowers and seeds as they get older and bigger, and when a plant reaches its maximum size, its reproductive ability remains constant.
"It very rarely decreases with age," the researchers write.
"In most cases, mortality in perennials seems therefore not to be caused by the progressive deterioration of physiological functions associated with age, as occurs in humans."
"The unique combination of modular development and dormancy evolved by perennial plants allows them to defy ageing."