From dust-like orchid seeds to the massive double coconuts, the variation in seed size is one of nature's most striking features. Large seeds, such as those from oak trees, pack a lot of resources to kickstart their growth, but this also makes them a target for animals that see them as a convenient snack. But what happens when animals eat part of the seed? Does losing some of their nutrient reserves hurt the seed's chances of survival?

Published in Journal of Ecology, the research uncovers that being big isn't just about growth potential, it's also about resilience and survival.

Researchers from the Wuhan Botanical Garden of the Chinese Academy of Sciences and Royal Botanic Gardens, Kew, UK collected acorns from 20 Fagaceae species across the UK, Spain, and China in 2020 and 2021. They simulated what happened when seeds lost parts of their nutrient reserves due to animal feeding by carefully cutting away up to 96% of the acorns' reserves without damaging the embryo. These seeds were then planted and their progress was tracked from germination to seedling growth.

The results were quite revealing. Surprisingly, partial granivory did not significantly affect seed germination. Even when a large portion of the cotyledons was removed, many seeds still germinated just fine. This suggests that seed are equipped with more nutrients than just what they need for germination, with much resilience against partial consumption. 

As for seeding growth, it was a different story. As the level of granivory increases, the seedlings have a harder time emerging and establishing. These seedlings struggle to grow with fewer and smaller leaves, they must invest the limited resources in photosynthesis by expanding leaf area as much as possible, and thus compromise leaf mechanical defences. It is a clever strategy but comes with a trade-off: thinner leaves are more vulnerable to herbivores.

The size of the seed also played a role. Large seeds have evolved their size not just to provide extra resources for growing seedling, but also as a defense mechanism. By tolerating partial predation, these seeds can survive and germinate while still benefiting from being dispersed by animals. Interestingly, the larger the seed, the more likely it is to survive partial consumption. This makes large seed size an effective adaptation for balancing the competing demands of dispersal and predation.

This study provides valuable insights into the relationship between seed size and defense against granivory. It shows that large seeds in the Fagaceae family have a strategy that can tolerate partial consumption. Partial damaged seeds still have a chance to germinate and grow, although the seedlings may face challenges. This research not only deepens our understanding of plant evolution, but also has implications for forestry and conservation. 

This research is supported by the National Natural Science Foundation of China, and the Future Leader Fellowship in Plant and Fungal Science from the Royal Botanic Gardens, Kew, the Swedish Research Council, the Swedish Foundation for Strategic Environmental Research MISTRA (Project BioPath), the Kew Foundation and the Chinese Academy of Sciences’ President's International Fellowship Initiative. (Online: 16 January)

Flowchart of the study design: An acorn is separated from its cupule and pericarp, and then artificially deprived of varying amounts of nutrient reserves. The embryo and its remaining seed reserve are planted, and seed germination and seedling growth are monitored. Organs of the seedling are weighed or burned for measuring biomass allocation (Image by CHEN Sichong)