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Nitrogen allocation in Pentaclethra macroloba cotyledons in five developmental stage classes

Nitrogen-fixation ability — the ability to incorporate atmospheric nitrogen into organic compounds — and toxicity to seed predators are important factors explaining the dominance of Pentaclethra macroloba in the forest of the La Selva Biological Station, Costa Rica. Because the nitrogen contained in the cotyledon — the fleshy seed leaf that serves as a nutrient source during seed germination — is necessary for growth and because herbivory rates (which may be related to concentrations of toxic nitrogenous compounds in cotyledons) have been shown to increase with seedling age, I hypothesized that nitrogen contained in the cotyledon would be increasingly allocated to different parts of the plant with age, and therefore predicted that P. macroloba cotyledon nitrogen concentration would decrease with cotyledon age. I collected eight P. macroloba seeds in each of five developmental stage classes and measured cotyledon mass, root mass, shoot mass, and cotyledon nitrogen concentration for each. Nitrogen concentration generally decreased as the seedling developed, but it increased at the stage class in which roots appeared. The pattern of decreased cotyledon nitrogen concentration seems to correlate with decreased root:shoot mass ratio, indicating that nitrogen from the cotyledon is allocated to other parts of the plant as development progresses.