Can seeds adapt to the heat?

By Anna Mayor, College of Arts and Sciences

Gabriela Quesada-Avila, a third-year doctoral student in the Department of Integrative Biology, turned to the warm forests of Panama to examine how temperature impacts seed sprouting and gather more insight on how plant communities will respond to future climates.
 
Quesada-Avila, who is studying plant ecological responses to future climates and is on track to graduate in 2027, specializes in seed and microbial responses to elevated temperatures.  
 
“I want to find answers about who the ‘winners and losers’ will be under a warmer world,” she explained. “This information can help us select the most suited species for future forest restoration or conservation efforts.”

Searching for seeds

Quesada-Avila worked under the guidance of her doctoral advisor Paul-Camilo Zalamea, an assistant professor in integrative biology and a research associate at the Smithsonian Tropical Research Institute in Panama, to carry out the study in the Gamboa research lab, located at the edge of the Panama canal.
 
“The planet is getting warmer every day. Last year, climate scientists determined that Earth’s average temperature increased to more than 1.5 °C above pre-industrial levels for the first time and will continue to warm up,” she said. “Lowland tropical rain forests are ecosystems that are already exposed to very high temperatures.”

Quesada-Avila added that many different tree species are close to their thermal limits for processes such as photosynthesis. 
 
“We know very little about neotropical seed germination thermal thresholds. If temperature keeps increasing as fast as it has been, many plants will not have enough time to adapt to the new thermal conditions, threatening crucial ecosystem dynamics,” she added. “If temperature continues to rise at the alarming rates observed, much of the extant vegetation may not contain the evolutionary tools to persist, threatening crucial ecosystem processes across the globe.”
 
She collected 15 species of seeds from available fruiting trees, walking miles in the vast Barro Colorado Natural Monument forest — described as a “mecca for tropical biologists” since this particular island forest provides access to Panama’s lowland tropical forest.

Once she had found at least four different individuals of the same species with ripe fruits, she cleaned seeds from the fruit pulp and air-dried them in the dark for 24 hours.

After the seeds were cleaned, Quesada-Avila evaluated initial seed viability, using a colorimetric test called tetrazolium (TZ), which measures seed embryo respiration.
 
“Seed viability is useful information because it allows me to determine whether the seeds I collected have the potential to germinate,” she said.

Quesada-Avila was careful to also consider seed dormancy — the inactive metabolic state that inhibits the embryo from developing. She divided the plant species into two groups: dormant seed species and non-dormant seed species, or ‘quiescent species.’

Turning up the heat

Quesada-Avila used environmental chambers to characterize germination responses under five temperature regimes. 

She simulated day and night by cycling the seed pools through 12 hours of light and 12 hours of darkness at a range of corresponding temperatures: 68/64.4°F, 78.8/75.2°F, 89.6/86°F, 100.4/96.8°F, and 111.2/107.6°F. 
 
“Germination responses were scored on five temperature regimes every week for up to six weeks. After that, I performed another seed viability test to confirm if the non-germinated seeds were dead or dormant,” she said.

Finding the germination ‘sweet spots’

Quesada-Avila found that subjecting the seeds to daytime temperatures of 111.2 degrees Fahrenheit and nighttime temperatures of 107.6 degrees Fahrenheit was a “death sentence.” 
 
“There was no germination under this temperature for all of the 15 species,” she said. 
 
Quesada-Avila also found that temperature effects on germination are seed dormancy- and species-specific.  
 
“This means that quiescent seeds or non-dormant seeds were able to equally germinate under the other four temperatures,” she said. “But germination responses of dormant seed species were not that easy to generalize because each of them responded very differently.”

Some species showed a “stepped germination response,” meaning that as temperatures increased, the number of seeds that germinated increased as well. Other species showed very little germination under the colder temperatures. 
 
Quesada-Avila found that all the species germinate faster when subjected to 89.6 degrees Fahrenheit during the day and 86 degrees Fahrenheit at night, suggesting this range as the “germination sweet spot” for this group of species.   
 
She also found that, under the extreme temperatures — 64.4-68 degrees Fahrenheit and 96.8 -100.4 degrees Fahrenheit — germination was slower, suggesting that these temperatures might be inhibiting the seed metabolic reactions.

Seeds that surprise

“I was surprised that two of the dormant-seed species were not dead after exposing them to the highest temperature regime,” said Quesada-Avila, referring to the 107.7-111.2 degrees Fahrenheit regime.  
 
One of those seeds was the Balsa tree, which is often used to build model aircrafts, boat decks and thermal insulation panels.  
 
“The seeds of this tree did not germinate but they were not dead either, and we think they were still dormant. I was not expecting this after seeing the general trend that all of the species die at this temperature. I started to wonder if dormancy is a trait that could help trees ‘acclimate’ or ‘adapt’ to warmer temperatures,” she said.  
 
Quesada-Avila said she hopes to see seed dormancy studied more in depth to have a better understanding of how it can help seeds face stressful environmental conditions.

Taking it a step further

This summer, Quesada-Avila will return to Panama to work on a similar project, leveraging a groundbreaking experiment, where scientists from the Smithsonian Institution and the University of Leeds have warmed the soil with electric heaters at the Barro Colorado Island in the Panama Canal.  
 
“Using this setup, I will conduct a six-week seed burial experiment to examine the response of seeds and seeds that interact with the surrounding soil microorganisms to different temperatures. I will test if what I found in a lab-controlled setup holds true in the forest soil,” she said.  
 
She hopes to get more insight into how seed-fungal interactions are affected by elevated temperatures.  
 
Quesada-Avila, who earned her bachelor of science from a university in her home country of Costa Rica, is also a member of the Ecological Society of America (ESA), which organizes annual meetings and publishes several scientific journals, such as Ecology, one of the most recognized journals in the field.  
 
“Plants are the foundation of ecosystems, it is kind of a cliché to say it, but they are the reason why we can breathe,” she said. “It is time to remember something the marine biologist, writer and conservationist, Rachel Carson once said, ‘In nature, nothing exists alone!’” 
 
Learn more about the Department of Integrative Biology’s Plant and Microbial Ecology program.