Hardseededness isn’t just about delaying germination
Ken Thompson has privided the following summary of a paper recently published in early view , to which several other ISSS members have also contributed:
‘Physical dormancy is defined by the existence of water-impermeable
(“hard”) seed coats that need to breached before the seed can absorb water and germinate. In nature, seed coats are usually ruptured due to attrition, fire or large temperature fluctuations, and this trait has been interpreted as a mechanism of seed dormancy. For at least a century, this germination-regulating role has been the only accepted explanation for the existence of “hard” seed coats. Surprisingly, no other reasons for their evolution have been considered by seed biologists. However, hard seeds are actually very rather poor at regulating germination; “physiological dormancy” does a much better job and, not surprisingly, is much more common.
Importantly, many aspects of plant evolution such as pollination, plant chemistry and seed dispersal have clearly been driven by interactions with animals. We propose that hard seeds are no exception, and have evolved for quite a different purpose,to enable seeds to hide from seed predators. Using two hard-seeded species, common vetch and black locust, we present GC-MS data showing that intact hard seeds release very low levels of volatile compounds (compared to imbibed, soft seeds with water-permeable coats). A model seed predator, the Roborovski desert hamster, is unable to locate buried hard seeds, but has no difficulty finding imbibed soft seeds. Furthermore, hamsters can detect buried objects dosed with an artificial cocktail of the volatile chemicals released by soft seeds, demonstrating that hoarding mammals rely on smell to locate buried seed caches.
The results are consistent with the revolutionary hypothesis that the germination-regulating function of hard seeds is largely a by-product of powerful selection by small mammal predators that detect seeds primarily by olfaction. This hypothesis has profound implications for seed dispersal by scatter or larder hoarding mammals, because caches of hard seeds are more likely to escape re-discovery and pillaging. In addition, it provides plausible explanations for some hitherto unexplained features of hard seeds.’
Paulsen, T. R., Colville, L., Kranner, I., Daws, M. I., Högstedt, G., Vandvik, V. and Thompson, K. (2013), Physical dormancy in seeds: a game of hide and seek?. New Phytologist. doi: 10.1111/nph.12191 (if you have access to a New Phytologist subscription)
The paper was also featured in ‘Science’ magazine – vol. 339, p1125, 8 March 2013