Growth & Development

How Do Seeds Save for a Rainy Day?

Timing is everything. For seeds it's a matter of life or death.
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Imagine you are a seed in the soil: would you want to germinate as soon as you feel some water in the surroundings? Some could think: “Sure! I want to germinate and start growing as soon as possible!” Yet, that water you feel could come from be a sporadic, isolated rain in the middle of the dry season. That could mean, you end up growing up in a quite harsh environment with little chances of survival. That possibility could make you think it’s better to wait. So, you remain in the soil waiting for the rainy season, and the ground is full of water. Still, you realize that one of those adventurous plants that germinated earlier survived and now is shadowing you! You waited all that time for nothing.

This simple decision ¬–to germinate or not germinate– might seem trivial for most people. For plants, it is a matter of life or death. And if that wasn’t stressful enough, imagine that you live in an environment where water is not readily available all year long, such as in seasonal forests and savannas. In these ecosystems, rainfall is concentrated in just one part of the year. Under this scenario, you could think that plants would always want to play safe and keep their seeds with them until the rainy season starts. But if you have visited one of these places in the tropics or walked around your city during the summer, you might have seen some plants producing fruits or releasing their seeds, implying their seeds are being dispersed in a season that is not ideal for their germination.

A typical savanna landscape in Itirapina Ecological Station (Brazil). Photo by Diego F. Escobar.

How plants deal with seasonality and balance the risks associated with germination is a question that has triggered the interest of many researchers, including Dr Diego F. Escobar. He has studied germination since he was a Biology student at Universidad del Valle (Colombia). In an interview with Botany One, he shared that his interest in seasonality began after reading a seminal study by Dr Nancy C. Garwood. The paper explored the mechanisms behind germination timing in a seasonal forest in Panama and highlighted that seed dormancy was crucial for achieving this. Dormancy is an innate seeds property that blocks germination under favourable conditions until some prior requirements are met. It can take many forms: from impermeable coats that prevent water uptake to an unbalance of substances that prevent embryo development and growth. Curious to test whether dormancy was equally crucial in other seasonal ecosystems, Escobar took his interests to his PhD at Universidade Estadual Paulista (Brazil), where he would meet Dr Patricia Morellato, also intrigued by Garwood’s paper and eager to see if the same mechanisms operated in Brazilian seasonal ecosystems. In the end, they took their research to the Cerrado, the largest savanna in South America.

The first chapter of Escobar’s thesis explored the relationships between seed dormancy and dispersal season, finding similar results to Garwood’s in Panama: seeds dispersed during the dry season tended to be dormant, while those scattered during the rainy season were not. However, there are many kinds of savannas in the Cerrado. Some are the typical open, grass-dominated savanna, and others have a closed tree canopy. Could the same pattern be found in these different vegetations? This question was explored in the following study by the authors, where they compared the seed and germination traits in species from open and closed savannas. Although differences were expected between these vegetation types due to their contrasting microclimates, the proportion of dormant species in each savanna was statistically the same. Therefore, seed dormancy seems to be a vital mechanism in seasonal ecosystems, no matter the local conditions. While these results are pretty exciting, they raised a simple but compelling question: what happens in species with nondormant seeds? In the end, species that produce dormant seeds were as abundant as those with nondormant ones. Are they condemned to a life full of risks by germinating in the dry season? Sure, they might disperse their seed just before the rainy season starts, but that is all they can do? All these questions led to the third chapter of Diego’s PhD, which was recently published in Oikos.

The authors evaluated the dispersal ecology and germination behaviour of 82 Cerrado species, focusing on 1) dispersal season and syndrome, 2) the presence of seed dormancy and 3) temperature requirements for germination. Each of these features represents different ways plants can control when to germinate and balance risks. We have already discussed that dormancy is a way to ensure germination at the most suitable times. Dispersal season is a rather logical one: there is no risk for your seeds to germinate during the dry season if you release them during the rainy season! On the other hand, germination requirements establish the environmental conditions where seeds can germinate. So, if seeds have evolved to germinate in a narrow temperature range, germination will be restricted to the part of the year where these temperatures occur. Contrastingly, if germination occurs under a wide range of conditions, seeds can germinate almost anytime.

Kielmeyera coriacea seed capsule looking almost like a brown moth on the end of a twig.
An open capsule of Kielmeyera coriacea Mart. (Calophyllanceae) a species with anemochorous (wind-dispersed) seeds. Photo by Diego F. Escobar

Now, to understand the relationship between dispersal syndrome –the means employed to transport seeds from one place to another– and risk, we will imagine ourselves as seeds again. Would you want to be dispersed as long as possible from your mother plant, or would you like to stay near them? Both alternatives are tempting. If you stay near your mother plant, you can be sure you are in a place ideal for your growth. In the end, your mother plant got there and was able to thrive and have you! In that scenario, you would place safe and wouldn’t need any particular structure or assistance for your dispersal; you just need to fall to the ground. However, what if there were better places to grow far from your mother plant? You might want to take the risk and develop some structures to be dispersed by the wind or attract some animals that could carry you. But, you might get to that distant place and find yourself in a harsh place to live.

Interestingly, Escobar and his colleagues show that dispersal season, dormancy and temperature requirements can be combined to produce different strategies that restrict germination to the rainy season. Three main combinations were identified, including the two described in their previous studies: dispersal of dormant seeds during the dry season and release of nondormant seeds during the rainy one. The third strategy was based on the association between dispersal season and germination temperatures. They found that seed dispersal season and temperature germination requirements were highly correlated so that species dispersed during the dry season required warmer temperatures to germinate. In other words, the seeds dispersed during the dry season are unlikely to germinate right away because temperatures typical of this season were below the species’ temperature range. The most fascinating aspect of this result is that the relationship between dispersal season and temperature requirements was independent of seed dormancy. So, this research shows that seed dormancy is not a prerequisite to avoiding germinating during the dry season!

On the other hand, the authors also found that dispersal syndrome was significantly associated with seed dormancy and germination synchrony. For instance, autochorous species –those with no specialized means for dispersal– tended to produce dormant seeds with asynchronous germination. Yet, those with animal- or wind-dispersed seeds are typically nondormant and germinate synchronously. These contrasting combinations illustrate how plant evolution seems to balance risks. For autochorus species, seed dormancy ensures germination occurs under the most ideal conditions. On the other hand, given that all seeds produced by one of these plants are expected to accumulate below it, asynchronous germination reduces the competition between sibling seedlings. As a result, this combination prevents that fatal scenario where a seed germinates and another seedling impedes its growth. The same logic to balance risks applies to seeds dispersed by animals or the wind. As mentioned before, there was a probability that seeds that travelled far from the mother plant never reached an ideal place for their growth. So, if they arrive at a site with ideal conditions for germination, one would expect them to germinate right away!

Altogether, the research carried out by Escobar and his colleagues has provided fascinating insights into how plants from the Brazilian Cerrado deal with seasonality and germination-related risks. Notably, this research clearly illustrates that –in Escobar’s own words– “one just can’t assume that dormancy is the only mechanism to achieve this!”. As stated in their paper, “the adaptive value of each individual trait can only be understood by taking other traits into account“. While this pattern should be maintained in other seasonal ecosystems, Escobar confessed he is curious to know what happens in more humid ecosystems, such as rainforests. Hopefully, future seed ecologists will look at Escobar’s research and be interested in testing these ideas in the ecosystems of their preference, just like he was inspired by Garwood’s ideas in the past.

ORIGINAL ARTICLE

Escobar, D. F., Rubio de Casas, R., & Morellato, L. P. C. (2021). Many roads to success: different combinations of life‐history traits provide accurate germination timing in seasonally dry environments. Oikos, 130(11), 1865-1879. https://doi.org/10.1111/oik.08522


Carlos A. Ordóñez-Parra (he/him) is a Colombian MSc. Student at the Plant Biology Program at Universidade Federal de Minas Gerais (Brazil). Besides his research in seed functional ecology, he is interested in science communication and has written for Pesquisa Javeriana and Revista Javeriana at his former university in Colombia, and Plant Science Research Weekly – the weekly roundup published by Plantae.org. Follow him on Twitter @caordonezparra.

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