Biologists are eager to classify life, and seeds are no exception. One simple way to classify them is by their ability to remain viable after losing water content. Those sensitive to this loss, even if only a minute portion, are called recalcitrant seeds. These seeds are known to germinate quickly to escape dehydration. Palm species, however, seem to have missed this memo: not only do some have recalcitrant seeds, but they are also dormant, meaning they must experience a specific series of conditions before they can germinate.
This is the case for Mauritia flexuosa —the iconic buriti palm of South American swamps— which has recalcitrant seeds yet remains viable in the soil for over a year without germinating. This seemingly contradictory combination is even more striking given that buriti palms also grow in savannahs, where at least one season is marked by severe drought —conditions that could easily be fatal. This paradox has sparked the interest of seed scientists, who are eager to understand the mechanisms that allow buriti seeds to survive such stressful environments.
Brazilian researcher Guilherme Dias has devoted his PhD to helping solve this mystery. In a recent study published in Environmental and Experimental Botany, Dias and colleagues investigated how buriti embryos respond to changing water conditions. The team exposed embryos to waterlogged as well as moderate and severe drought conditions in the laboratory, then carried out various tests to assess how these conditions affected the embryos’ structure, stored compounds, and metabolism.
The authors found that while a significant proportion of the seeds died when exposed to the severe drought conditions, 40% of the seeds remained viable, suggesting considerable tolerance. One characteristic that might contribute to this tolerance is the high concentrations of mucilage in the endosperm —the seed’s reserve tissue— region surrounding the embryo. Full of water-retaining carbohydrates and proteins, this mucilage creates a buffer zone that protects buriti embryos by preventing water from escaping too quickly under stress.
But mucilage isn’t the whole story. One of the most exciting insights from this research is that seeds hydrated after dispersal became more tolerant to drought conditions. For instance, those hydrated before exposure to drought showed lower levels of oxidative stress markers—chemical signals that plant physiologists use to detect cell damage. Additionally, this post-dispersal hydration triggered the activity of an enzyme known as endo-β-mannanase, which breaks down mannan reserves in the endosperm. According to the authors, this enzymatic breakdown may help seeds absorb water more effectively by balancing the concentration of newly released sugars. In short, hydration after dispersal seems to activate a cascade of metabolic changes that boost the seed’s ability to cope with stress.
Hydration also appears to stimulate the production of antioxidant substances, especially phenolic compounds. These molecules—best known for their abundance in berries and herbs—are widely recognised for helping plants manage environmental stress, including drought. A rise in antioxidant levels after dispersal may better prepare the seeds for upcoming dry spells. This could be especially important, as the study showed that the seeds’ enzymatic antioxidant systems were insufficient to handle the oxidative damage caused by drought on their own.
Altogether, the research by Dias and colleagues shows that buriti embryos have built-in defences that help them endure the fluctuating water conditions of the Brazilian Cerrado. Even so, they are far from invincible: under the most extreme drought conditions, more than half of the seeds perished. Still, these mechanisms are usually enough for the seeds to tolerate moderate drought—a remarkable feat given their recalcitrant nature. Thus, the study offers valuable insights into the paradoxical combination of recalcitrant and dormant seeds in palm species, a long-standing mystery in seed science.
READ THE ARTICLE:
Dias, G.P., Ribeiro, L.M., Mazzottini-dos-Santos, H.C., Nunes, Y.R.F. and França, M.G.C., 2024. Water stress resilience in Mauritia flexuosa (Arecaceae) embryos: New insights into the persistence of recalcitrant seed banks. Environmental and Experimental Botany, 226, p.105930.
Carlos A. Ordóñez-Parra
Carlos (he/him) is a Colombian seed ecologist currently doing his PhD at Universidade Federal de Minas Gerais (Belo Horizonte, Brazil) and working as a Science Editor at Botany One and a Communications Officer at the International Society for Seed Science. You can follow him on Bluesky at @caordonezparra.
Cover picture: Mauritia flexuosa fruits. Photo by Kristi Denby (Wikimedia Commons).
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