Study sheds light on how seeds survive in tropical soils

  • Researchers have identified three different strategies by which seeds from pioneer tree species avoid predators and await the right time to germinate.
  • Pioneer tree species are the first to grow after a clearing is opened in the forest. The seeds from these species are also the most abundant within the soils.
  • To avoid early germination, many seeds in the soil become ‘dormant’, an ability that allows them to survive for relatively long periods until the right conditions to germinate arise.
  • A recent study suggests that the means by which tropical seeds from pioneer tree species keep away their predators and remain dormant are evolutionarily connected.

Life is not easy for seeds. Avoiding ending up as food for animals and finding the right time and spot to germinate are two common problems.

Until recently, scientists studying seeds approached these two issues separately. Some worked on the mechanisms by which seeds keep their predators away, while others focused on how they prevent germination until the right conditions arise, also known as dormancy.

Now it turns out that both areas might be more connected than previously thought, with defense and dormancy traits actually working together as part of a larger strategy to survive. This is one of the conclusions of a recent paper published in Ecology by a group of researchers working at the Smithsonian Tropical Research Institute.

The study looked specifically into pioneer tree species responsible for re-establishing the forest after a disturbance, such as a fire or logging.

“They are very good competitors when they have enough light. This limits their chances of germination, as there aren’t a lot of clearings in the forests and they occur randomly,” says Paul-Camilo Zalamea, first and corresponding author of the study. “Having seeds that are able to persist until a clearing opens is a good evolutionary strategy.”

In the study, the researchers measured thickness, permeability, toxicity and other defense traits in seeds from 16 of pioneer tree species from a lowland tropical forest in Panama. They also evaluated dormancy through an experiment in which they buried the seeds and assessed how many of them could germinate 30 months later.

When they put all the data together, the analysis showed two interesting things. First, defense and dormancy mechanisms seem to have evolved together. Second, all the seeds that were studied fall into one of three categories, called “dormancy-defense syndromes” in the jargon.

Each of the three syndromes is characterized by a specific combination of defense and dormancy traits. Some seeds remain dormant by being highly impermeable, and they tend to have a harder coat and to be thicker.

A second group, that includes seeds that remain dormant by using chemical signals, includes seeds that are permeable and produce chemical compounds that induce dormancy and deter predators. The last category corresponds to seeds that are also permeable, but don’t display dormancy nor produce chemicals to protect themselves.

“We believe that seeds from this third group form associations with microorganisms that allow them to persist in the soil for some time,” explained Zalamea.

Shyam Phartyal, a seed researcher at Nalanda University, in India, who was not involved in the study, highlights the importance of the findings.

“This is first detail and an in-depth study carried out to disentangle how seeds of tropical pioneer tree species persist in the soil, from few days to few decades, without getting damaged by predators and pathogens,” he explained.

Phartyal notes that the idea that dormancy and defense traits are evolutionary connected is not new. It was predicted in 2011 by some of the authors of this paper, and was “later supported for temperate weed species” in a 2016 study. However, this new work in tropical latitudes broadens the picture.

“The paper highlights that dormancy-defense syndromes vary considerable among different habitats. This means seeds are not employing only one defense system uniformly across species and habitats,” Phartyal explained.

While these results are relevant from a basic science perspective, as they advance our knowledge on how plants have evolved, they are also relevant from a restoration point of view. Pioneer species are crucial to restore damaged ecosystems, and what happens to their seeds can determine their chances of survival.

“Most of the seeds in the soil are from pioneer species,” said Zalamea. “Late successional species [the ones found in more mature ecosystems] tend to germinate as quick as possible and remain as seedlings until the right conditions allow them to grow.”

This means that knowing how seeds persist in the soil also sheds light on how degraded ecosystems can be naturally restored.

“We have put dormancy in a wider ecological context and showed that there are evolutionary reasons why dormancy and defense mechanisms are related,” said Zalamea.

Banner image: Seedlings in Brazil. Photo courtesy Guilherme Rodrigues.

Ignacio Amigo is a freelance journalist based in São Paulo, Brazil. You can find him on Twitter at @IgnacioAmigoH.

Citation: Zalamea, P.C., et al. (2018). Dormancy‐defense syndromes and tradeoffs between physical and chemical defenses in seeds of pioneer species. (2018). Ecology. 99 (9). 1988.

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