Growth & Development

Plants Don’t Want Their Children to Starve

There's more to seed survival than just seed mass.
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If you are a fan of nature documentaries, you probably learned that some animals take care of their offspring until they can be on their own. This behaviour, known as parental care, looks to increase offspring survival chances and occurs in different fashions: from male Emperor penguins incubating eggs, through frogs transporting their tadpoles, to groups of female coatis collectively guarding the juveniles. What you probably don’t know is that plants can also assist their offspring in their earliest moments! Of course, they cannot actively protect their seeds or move them if they are in danger. However, they might provide them with structures to enhance dispersal and protection towards predators or nutrients to sustain their growth. Nutrient allocation is particularly crucial from these forms of parental care in plants. When seeds germinate, they cannot absorb nutrients from the soil or carry out photosynthesis at the very beginning. Thus, they depend entirely on the nutrients that their mother plant deposited in their seeds, just like a baby bird that cannot fly and look for its own food.

Examples of how parental care is expressed in plant through seeds, including the provision of nutrients. Figure by Carlos A. Ordóñez-Parra using icon from Flaticon.com

The most typical way to study seed nutrient availability in ecology is by measuring seed mass. It is logical to think that heavier seeds store more nutrients than lighter ones. Therefore, most studies have used this measure to assess how seed nutrients are related to different plant characteristics. That is the case of Dr Tereza Mašková, who investigated the relationship between seed mass and seedling root architecture during her Master’s degree at Charles University in Prague (Czech Republic). Her research showed that species with lighter seeds produced seedlings that directed relatively more of their biomass to their roots and had wider root systems that branched earlier. Altogether, these results suggest that seeds with fewer nutrients employed more resources to produce a root system that allowed them to absorb nutrients more rapidly.

There are several problems with using seed mass to approximate seed nutrient availability. Most notably, this measurement does not provide any information of individual elements –such as carbon, nitrogen and phosphorous– which are not necessarily stored in the same proportions.

“When I was thinking about it deeply, I realised that two species with the same seed mass, which means the same absolute amount of nutrients, can really differ in terms of what is stored inside. It could be a huge difference between them!” commented Dr Mašková in an interview with Botany One.

Additionally, seed mass includes structures with no nutritional value, such as its coat and the appendages that aid seed dispersal –like wings of the maple tree seeds or the hair-like structure of the dandelion’s seeds. Therefore, using seed mass alone overestimates the amount of nutrients available for the future seedling. Given these issues, Mašková decided to study seed nutrient availability in more detail during her doctorate studies at Charles University, a research that led to her recent publication in Oikos.

Long seeds and short seeds Spiky seeds and smooth seeds. Seeds that fly through the air and seeds that go thunk on the ground.
A sample of the different seeds used in Mašková’s research. Photo by Tomáš Koubek.

The researchers measured the content of nonstructural carbon –the carbon that is not part of the structure and its available for embryo nutrition– nitrogen and phosphorous in the seeds of more than 500 herbaceous species from Central Europe and assessed how the content of these nutrients was correlated to different plant characteristics, such as its seed mass and habitat. Notably, seed nutrient content varied between species, especially nitrogen (0.8-9.9% of seed mass) and nonstructural carbon (2.1-60.7%). However, this variation was not random. For example, closely-related species tended to have similar seed nutrient content. This trend is quite clear when the type of carbon reserve is analysed: families like Poaceae (grass and cereals family) and Fabaceae (legumes) preferably store carbon in the form of starch, while others, such as Asteraceae (sunflower) and Lamiaceae (lavender), store oils. Therefore, seed nutrient content was found to be strongly shaped by species’ evolutionary history.

The authors also found evidence for the classical assumption that the total amount of nutrients is positively associated with seed mass. In other words, heavier seeds do store more nutrients than lighter ones. However, you might remember that seed mass is the sum of different structures, including those with no nutritional value! Given this investment in extra structures, the authors assessed the relationship between seed mass and how much of this was devoted to nutrient storage and found that lighter seeds invested relatively more to nutrient storage than heavier seeds. This result probably arises given that heavier, bigger seeds potentially need to allocate more resources to other functions, such as defence from predators. Still, the relationship between seed mass and seed defences is complex, and future studies are required to better understand it.

One of the most fascinating results from this research is that nonstructural carbon and nitrogen content were correlated to species habitats. Interestingly, plants preferably stored nutrients that were expected to be limiting in each habitat. On the one hand, habitat nutrient availability was correlated with nonstructural carbon, with species from nutrient-rich habitats storing relatively more carbon than nutrient-poor ones. But why carbon is expected to be limiting in a nutrient-rich environment? With no nutrient restrictions, plants are expected to grow rapidly and shadow one another. By doing so, photosynthesis might be reduced, and so reduce the synthesis of carbon products to fuel seedling development. Given this, providing seeds with carbon allows young seedlings to cope with carbon limitations in this environment.

Plant characteristics that influence seed nutrient content. Figure by Carlos A. Ordóñez-Parra using icon from Flaticon.com

On the other hand, soil disturbance was negatively associated with nitrogen content, meaning that plants from less disturbed sites produced seeds with larger nitrogen reserves. According to the authors, less disturbed habitats experience a low nutrient turnover; thus, competition for soil nutrients (nitrogen included) is higher. In this context, providing seeds with more nitrogen seems to be a strategy to cope with these initial limitations.

Altogether, these results highlight the importance of studying the content of individual elements in the seed instead of just using seed mass to assess nutrient availability. Each nutrient seems to provide essential information about how plants deal with nutrient limitation and competition. In her own words, Dr Mašková, now a post-doc at the University of Regensburg (Germany), states that “it is important to realise that seed nutrient content is an important trait. In some processes ecologists use seed mass as a proxy of maternal investment, and it’s just not the way best to do it.” She also highlights that future studies should look for variability between individuals, given that a single species might be present in a wide range of habitats. Moreover, further research is needed to understand how seed nutrient content is shaped by other seed traits and symbiotic organisms that help seedlings acquire nutrients. One thing is sure, this study paves the way for an exciting research area on how plants have evolved to prepare their seeds for the future.

RESEARCH ARTICLE

Mašková, T., & Herben, T. (2021). Interspecific differences in maternal support in herbaceous plants: CNP contents in seeds varies to match expected nutrient limitation of seedlings. Oikos, 130 (10), 1715-1725. https://doi.org/10.1111/oik.08186

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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