Seed structure and physiology in relation to recruitment ecology in Leucospermum (Proteaceae) in fynbos
G. J. Brits A D E and J. C. Manning B C
+ Author Affiliations
- Author Affiliations
A Horticultural Division, Institute for Fruit Research, Agricultural Research Council, PO Box 5026, Stellenbosch 7599, South Africa.
B Compton Herbarium, National Botanical Institute, Private Bag X7, Claremont 7735, South Africa.
C Research Centre for Plant Growth and Development, School of Biological and Conservation Sciences, University of KwaZulu-Natal, Pietermaritzburg, Private Bag X01, Scottsville 3209, South Africa.
D Present address: Brits Nursery, 28 Flamingo Road, Stellenbosch 7600, South Africa.
E Corresponding author. Email: gbrits@iafrica.com
Australian Journal of Botany 67(4) 290-308 https://doi.org/10.1071/BT18199
Submitted: 23 October 2018 Accepted: 16 May 2019 Published: 19 July 2019
Abstract
Development of embryo envelopes in Leucospermum cordifolium (Salisb. ex Knight) Fourcade is presented in a detailed anatomical study, and their structure interpreted ecologically. To support interpretation results for at least six other species of Leucospermum are reviewed, confirming that L. cordifolium is representative of this shrubby group occurring in fynbos, a fire-prone Mediterranean-type vegetation subject to summer drought. The fate of the water-permeable seed coverings subsequent to dispersal is followed in seeds stored experimentally underground. The testa at maturity effectively excludes oxygen in intact soil-stored seeds, thus creating a less common type of physical dormancy which we term ‘anoxia PY’ (oxygen-impermeable physical dormancy). We postulate ‘fire-mediated desiccation-scarification’ testa breakage on a large scale in freshly dispersed (by ants) soil-stored young-seeds, dynamically regulated by drying of different testa layers through fire heat shock (but also by post-fire ambient climate heat), thus alleviating anoxia PY in, and causing undelayed germination of, a major part of the young-seed bank within the ensuing winter germination season. Concurrently a patchy disturbance pattern within a lesser portion of the young-seed cohort causes uneven underground desiccation-scarification by fire, resulting in variable degrees of oxygenation and thus temporally extended (erratic) germination of young-seeds. Both of these strategies are primary long term fitness traits in Leucospermum. By contrast the persistent older-seed cohort, stored underground during the inter-fire period, may become weathered by soil and climate factors (‘soil-mediated abrasion’), which alleviates anoxia PY in old seeds. Thermodormancy of these embryos (physiological dormancy, PD) is enforced in unburnt vegetation by two narrow habitat-dependent daily temperature requirements (~10°C × 20°C), and these have specific ecological functions. The complex Leucospermum system includes balanced adaptive responses to multiple selective pressures. We propose a coherent mechanistic profile for this fynbos genus, synthesising data partly from the literature of other fire-prone ecosystems.
Additional keywords: adaptive response, bet-hedging strategy, disturbance regime, fire-sensitive species, oxygen impermeability, physical dormancy, seed anatomy, seed bank dynamics, seed dormancy, seed ecology.
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