Seed/cotyledon size and nutrient content play a major role in early performance of species on nutrient‐poor soils

Abstract

summaryWe examined the importance of seed size in the early establishment of seedlings of four cotyledonous species restricted to nutrient‐impoverished soils in south‐western Australia. Experimentally removing cotyledons from seedlings increased mortality and reduced root‐penetration, growth rate and final weight of plants after 12 wk, in proportion to seed size. The impact of removal of the cotyledons was least in the smallest‐seeded, Eucalyptus loxophleba Benth., grown in the more fertile of two soils, and greatest in the largest‐seeded, Hakea psilorrhyncha R. M. Barker, whose growth was otherwise unaffected by soil type. In the smallest‐seeded species, cotyledon content of most mineral nutrients increased over time, especially in the more fertile soil, while chlorophyll content showed some decrease by the end of the experiment. In the other species, most nutrients (especially N, P, K and Cu in the less fertile soil) were translocated from the cotyledons to the growing plant, and chlorophyll content declined markedly. The translocated N, P, K, Mg and Cu in these species made up a substantial part of the total plant content by 12 wk after germination, while total Ca and Fe tended to increase in the cotyledons via substantial soil uptake. The results indicate, contrary to reports for some other species, that the cotyledons have an important nutritional function in the early establishment of seedlings in these species. The results also show that, in the larger‐seeded species, young seedlings rely to a greater extent on nutrient support from the cotyledons than from the soil. We propose that large seeds might be an adaptation for enhancing establishment in nutrient‐poor soils.