SEED COTYLEDON SIZE AND NUTRIENT CONTENT PLAY A MAJOR ROLE IN EARLY PERFORMANCE OF SPECIES ON NUTRIENT-POOR SOILS/

We examined the importance of seed size in the early establishment of seedlings of four cotyledonous species restricted to nutrient-impoveri shed soils in south-western Australia. Experimentally removing cotyled ons from seedlings increased mortality and reduced root-penetration, g rowth rate and final weight of plants after 12 wk, in proportion to se ed size. The impact of removal of the cotyledons was least in the smal lest-seeded, Eucalyptus loxophleba Benth., grown in the more fertile o f two soils, and greatest in the largest-seeded, Hakea psilorrhyncha R . M. Barker, whose growth was otherwise unaffected by soil type. In th e smallest-seeded species, cotyledon content of most mineral nutrients increased over time, especially in the more fertile soil, while chlor ophyll content showed some decrease by the end of the experiment. In t he other species, most nutrients (especially N, P, K and Cu in the les s fertile soil) were translocated from the cotyledons to the growing p lant, and chlorophyll content declined markedly. The translocated N, P , K, Mg and Cu in these species made up a substantial part of the tota l plant content by 12 wk after germination, while total Ca and Fe tend ed to increase in the cotyledons via substantial soil uptake. The resu lts indicate, contrary to reports for some other species, that the cot yledons have an important nutritional function in the early establishm ent of seedlings in these species. The results also show that, in the larger-seeded species, young seedlings rely to a greater extent on nut rient support from the cotyledons than from the soil. We propose that large seeds might be an adaptation for enhancing establishment in nutr ient-poor soils.