The role of large seed size in shaded conditions: experimental evidence

1. Large seeds are thought to provide an adaptive advantage to seedlings growing in shaded environments. Four hypotheses that could account for the possible mecha- nisms underlying this advantage were tested using 23 species ranging in seed size from 0 04 to 22.2 mg. The hypotheses were: (i) seedlings from large seeds are able to survive longer in shade than seedlings from small seeds; (ii) seedlings from large seeds divert relatively more resources to shoot development when grown under shaded conditions compared to seedlings from small seeds; (iii) seedlings from large seeds always have larger shoots than small-seeded seedlings growing under shaded conditions due to the large initial reserve size of large seeds and the reduction in growth rate differences between small and large seeds under growth-limiting condi- tions; and- (iv) seedlings from large seeds can support a stronger etiolation response than seedlings from small seeds.

2. There was no difference in survival from emergence with seed size under the treat- ments no shade, 50% shade, 80% shade and 95% shade. However, large-seeded species survived significantly longer than small-seeded species under 99% shade.

3. The hypothesis that seedlings from large seeds divert relatively more resources to shoot development than seedlings from small seeds was rejected as there was no significant difference in allocation to shoots with seed size under any shade treatment.

4. Seedlings from large seeds were significantly heavier and had significantly longer shoots than small-seeded species after 6 weeks under 95% shade only. Under the other shade treatments of no shade, 50% shade and 80% shade there was no relationship between seed size and seedling size after 6 weeks growth. Small-seeded species showed a greater reduction in relative growth rate under shade compared to full light than large-seeded species.

5. Seedlings from large seeds had a stronger etiolation response to 95% shade than seedlings from small seeds. We suggest that large seed size provides additional resources to support etiolation compared to small seed size. Consequently the longer shoots of large-seeded species under 95% shade can be attributed to a larger initial seedling size and greater stem elongation. Seedlings from large seeds are able to main- tain a height advantage over small-seeded species due to the suppressed relative growth rates of small-seeded species. 6. We suggest that: (i) large seeds enable seedlings to tolerate shade for longer by pro- viding a larger initial energy reserve and that this may be an advantage in habitats where gaps in the canopy are regularly created; and (ii) large seeds provide seedlings with increased height relative to small-seeded species and that this height may be an advantage in habitats where there is a steep gradient of light such as in herbaceous vegetation, or for seeds germinating below litter.