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Rapid take-off to evade bank robbers
Amongst the hazards faced by seeds in
natural seed banks is consumption by herbivores and attack by pathogens. It is
this latter situation that has interested Beckstead
et
al. (Spokane and Provo, USA, pp.
907–914), focusing on Pyrenophera
semeniperda, a pathogen that invades seeds of the grass Bromus tectorum. Newly shed seeds of B. tectorum are dormant, and require a
period of warm, dry weather before they will germinate. Thus, the seeds do not
germinate in summer but do so in the wetter autumn weather. However, some seeds
fail to germinate even then, especially in drier habitats. Instead, they may
become secondarily dormant or simply remain ungerminated until the spring, the
two classes contributing to the ‘carry-over’ seed bank. Thus, there are banks
of B. tectorum available as hosts or
as food, especially in drier habitats. The authors studied the effects of
artificial and natural inoculation with P.
semeniperda on seeds varying in primary dormancy and natural inoculation of
seeds in the carry-over seed bank. The results showed that seeds that germinate
slowly were much more likely to be killed by the pathogen in both artificial
and natural inoculations than seeds that germinate quickly. Thus, a large
proportion of a fully after-ripened seed population, in which nearly 100 % of
the seeds germinated between day 3 and day 5 of incubation, escaped the
pathogen whereas slower germinating populations suffered significant losses.
Further, banked seeds were much more vulnerable to the pathogen than unbanked
seeds while seeds in drier habitats were up 50 times more likely to be killed
than seeds in mesic habitats. Finally, the authors note that B. tectorum, introduced into the
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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