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Ann Bot. Dec 2008; 102(6): 945–951.
Published online Oct 7, 2008. doi:  10.1093/aob/mcn186
PMCID: PMC2712403
Germination Responses to Water Potential in Neotropical Pioneers Suggest Large-seeded Species Take More Risks
Matthew I. Daws,1,2* Lora M. Crabtree,1 James W. Dalling,3 Christopher E. Mullins,1 and David F. R. P. Burslem1
1Institute of Biological and Environmental Sciences, University of Aberdeen, Cruickshank Building, St Machar Drive, Aberdeen AB24 3UU, UK
2Seed Conservation Department, Royal Botanic Gardens, Kew, Wakehurst Place, Ardingly, W. Sussex RH17 6TN, UK
3Department of Plant Biology, University of Illinois, 265 Morrill-Hall, 505 S Goodwin Avenue, Urbana, IL 61801, USA
*For correspondence. E-mail matthew.daws/at/ewlsciences.com.au
Received June 13, 2008; Revised July 29, 2008; Accepted August 18, 2008.
Abstract
Background and Aims
In neotropical forests, very small-seeded pioneer species (<0·1 mg seed mass) recruit preferentially in small tree fall gaps and at gap edges, but large-seeded pioneers do not. Since water availability is related to gap size, these differences in microsite preference may reflect in part species-specific differences in germination at reduced water potentials.
Methods
For 14 neotropical pioneer species, the hypothesis is tested that small-seeded species, with shallow initial rooting depths, reduce the risks associated with desiccation by germinating more slowly and at higher water potentials than large-seeded species.
Key Results
Germination occurred both more quickly and at lower water potentials with increasing seed mass. For example, Ochroma pyramidale (seed mass 5·5 mg) had a time to 50 % germination (T50) of 2·8 d and a median base potential for germination (ψb50) of −1·8 MPa while Clidemia quinquenervia (seed mass 0·017 mg) had a T50 of 17·6 d and ψb50 of −1·1 MPa.
Conclusions
These data suggest that small-seeded species germinate only in comparatively moist microsites, such as small canopy gaps, which may reduce the risk of drought-induced mortality. Conversely, large-seeded species are able to germinate in the drier environment of large gaps, where they benefit by enhanced seedling growth in a high irradiance environment. The positive association of seed size and canopy gap size for optimal seedling establishment is maintained by differential germination responses to soil water availability coupled with the scaling of radicle growth rate and seed size, which collectively confer greater drought tolerance on large-seeded species.
Key words: Germination, seed size, Panamá, neotropical, pioneer, water potential
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