The effect of nutrient addition on dimorphic seed reproduction and biomass allocation was investigated in amphicarpic peanut grass (Amphicarpum purshii Kunth). This annual, which grows in disturbed habitats on the Coastal Plain of eastern North America, produces self-fertilizing cleistogamous spikelets under ground and potentially outcrossing chasmogamous spikelets above ground. For each of three sibling families raised in the greenhouse from aerial seeds, 11 plants were fertilized twice during the growth period while 11 plants received no fertilizer. Total spikelet output and seed weight increased in response to fertilizer addition for both aerial and subterranean reproductive components, but changes were greater for aerial reproduction. Biomass allocation to subterranean spikelets and seeds was unaffected by nutrient addition. As measured by coefficients of variation, phenotypic variation for six aerial reproductive characters was about twice as great as that for six subterranean reproductive characters. Subterranean and aerial seed output were significantly related to shoot biomass for fertilized plants, but only mean aerial seed weight showed a significant relation to biomass when nutrients were not provided. These results support the hypothesis that early subterranean seed production provides reproductive insurance in peanut grass, whereas later aerial reproduction on newly formed tillers allows plants to increase reproductive output at the end of the growing period according to the availability of resources.
