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The steamy, shady side of seed success and survival
One of the aspects of angiosperm biology that fascinates me is the great range of variation in seed physiology. Here I focus on the research of van Klinken et al. (Indooroopilly, Queensland, pp. 875–883), working with the leguminous shrub Parkinsonia aculeata, seeds of which exhibit physical dormancy. The seeds are very hard with a palisade layer in the seed coat that is both tightly packed and water-repellent. Although dormancy breakage in hard-seeded species is not well understood, the authors propose, based on the ecology and distribution of P. aculeata, that wet heat is the main factor in this species. This would mean that dormancy is broken during the wet season in the plant’s native habitat. To test this idea, a comprehensive seed burial experiment was carried out (depths from 0 to 20 cm) under different shade conditions for different lengths of time. The mean air temperature was 28 ºC (giving a mean temperature at the soil surface of 43.6 ºC in open ground; this value decreased with both soil depth and shading). Over 1000 mm of rain fell in the 14-week period, most of it in the first 6 weeks; the soil was at 100 % field capacity between weeks 2 and 6, but fell to 27 % between weeks 6 and 14. Analysis of the germination data showed very clearly that, as predicted, wet heat is the key factor in breaking dormancy. Thus, dormancy was broken very quickly in seeds ‘buried’ at 0 cm in open ground but less quickly as depth of burial increased. Shading had a major inhibitory effect on dormancy breakage so that in both ground-cover and canopy-cover sites, most of the seeds were still dormant after 14 weeks, the actual figure increasing with depth of burial. These features thus ensure wet-season germination and provide both a system for detecting gaps in the canopy and a capacity for seed banking.
Professor J. A. Bryant
University of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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