Abstract
The seed stage is often critical in determining the regeneration success of plants. Seeds must survive an array of seed predators and pathogens and germinate under conditions favourable for seedling establishment. To maximise recruitment success plants protect seeds using a diverse set of chemical and physical defences. However, the relationship between these defence classes, and their association with other life history traits, is not well understood. Data on seed coat thickness and fracture resistance, and the abundance and diversity of potential defensive compounds were collected for 10 tree species of Macaranga from Borneo. The data were used to test whether there is a trade-off in physical versus chemical defence investment, and to determine how investment varies with seed mass, and light requirement for regeneration. Across species there was no correlation between seed coat thickness and abundance of potential defensive compounds, indicating the absence of a direct trade-off between defence classes. While chemical defences were not correlated to other traits, physical defences were positively correlated with light requirement for regeneration. For a subset of five Macaranga species we evaluated the relative investment in chemical and physical defence to seed persistence in the soil, measured as the time to half initial seed viability (seed half-life). Half-life was negatively related to the ratio of potential defensive compound abundance to seed coat thickness, suggesting that species with long persistence invested in physical defence more than stored chemical defences. These results indicate that investment in seed defences are associated with species' light requirements for regeneration, rather than scaling positively with seed mass. Furthermore, chemical defences, although highly variable among species, do not appear to be critical to long term persistence of Macaranga seeds, and may be important in defending seeds from natural enemies distinct from those found in the soil.
Highlights
Natural enemies strongly influence plant population growth, plant traits, and the local and regional distribution of plant species [1], [2], [3]
Species with both relatively thin (,150 mm) and thick (.300 mm) seed coats had relatively low quantities of potential defensive compounds compared to species with intermediate seed coat thickness (170–220 mm) (Figure 2A)
We found that variation in defence investment was primarily determined by species habitat requirements
Summary
Natural enemies strongly influence plant population growth, plant traits, and the local and regional distribution of plant species [1], [2], [3]. Many of these effects are most strongly exerted at the seed stage [4], [5]. Seed defences are often sub-divided into traits that provide physical or chemical protection. While physical defences can provide effective mechanical protection against vertebrates and invertebrates) [17], [18] seedprotecting structures may still be consumed by predators and seeds may be susceptible to pathogen infection [19]. To discourage predators and pathogens seeds may develop other means of protection based on chemical defences
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