To have true memory an organism requires brain cells to store experiences through the action of sophisticated neurotransmitters. Plants lacking brain cells therefore cannot be said to have that capacity for memory. However, there is evidence that some plants adapt their characteristics based on ‘remembered’ experiences.
From 2015 to 2019, a team of scientists in South Africa studied perennial savannah grasses that had a proven resilience to wildfires. Though they appeared to be destroyed by the flames, their roots were capable of regrowing quite quickly.
The researchers wanted to discover the long-term effects frequent wildfires have on burnt grasses, compared to those never experiencing fire. They studied grass plants on established experimental burn plots, set up in 1980 by the University of Fort Hare to assess whether controlled burning of savannah areas could restrict tree and shrub heights to benefit browsing livestock. They also took plants from control plots where no burning had occurred in those 35 years.
After studying the plants’ roots and regeneration rates, the team concluded that those repeatedly damaged by flames had developed different characteristics to those that had never had to resist a wildfire. As the study paper says, “Annually burned plants invested more of their biomass below ground compared to no-burn plants, which probably equates to them having greater stored energy reserves to initiate and support early resprouting.”
Had the grasses retained a memory of being burned, and so adapted themselves to ensure rapid regeneration after a wildfire? While that theory might assume too many human-like characteristics in a non-sentient life-form, there are various other examples of plants having at least a form of ‘memory’ encoded in their genes.
Natural defences
Wouter van Hoven published a study in 1994 after observing acacia trees being browsed by giraffes and antelopes in what was then the Transvaal province in eastern South Africa. Acacia leaves contain low levels of the chemical tannin, which has a bitter taste. As the animals feed, the damaged leaves emit ethylene gas. This is detected by the cell membranes of other acacia trees, which triggers a release of certain proteins. These proteins stimulate the trees’ stress-response genes and cause the production of more tannin to flood the leaves and make them unpalatable.
The change happens quickly. “When an intact plant is exposed to the [high levels] of ethylene, the tannin levels increase within 30 minutes,” wrote van Hoven. Is this simply an automatic chemical reaction to a sequence of events, unrelated to the cause of the ethylene gas release, or is it a genetic memory telling the tree it is under attack and a defence mechanism is required?
Scientists have a phrase – ‘epigenetic changes’ – that relates to alterations in the way that DNA instructions are carried out, to reflect damaging environmental conditions. Researchers in Norway (led by Carl Gunnar Fossdal and Paal Krokene) studied Norway spruces and found that epigenetic changes “provide the plant with a ‘memory’ of past stressful events and may be passed on to their seeds, making the next generation better prepared to face the challenges around them.”
According to the scientists, one environmental signal that can trigger conifer memories is temperature. Summers are shorter and colder the further north the trees grow, so they have to maximise the time their leaves can capture available light. In spring, new needles appear earlier on trees from seeds that grew up in lower temperatures, compared to the same species in warmer regions.
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Top image: fire in Northern Cape, South Africa (not the burn plots of University of Fort Hare). Credit: Getty
