Fruit size variation and its effects on germination and seedling growth in Xanthium strumarium

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Abstract
Fruits of X. strumarium L. from a population in southwestern Ontario [Canada] exhibited an eightfold range in size. The effects of fruit size on seed size, germination and seedling growth were investigated. Correlations between fruit size and seed size were strongly positive. In general, germination percentage and germination rate were both correlated positively with fruit size, although germination rate appeared to level off for the largest fruits. Cotyledon area and primary root length of newly emerged seedlings also were positively correlated with fruit size, although both variables and especially primary root length showed smaller increases over larger fruit sizes. The weights of newly emerged seedlings were highly positively correlated with fruit size. However, this relationship was not maintained for older seedlings. Weights of 15- and 30-day-old seedlings were constant over the majority of fruit sizes. This trend resulted from a negative correlation between seedling growth rate and fruit size which could, in part, be explained by a decline in the ratio of cotyledon are a to the weight of newly emerged seedlings with fruit size.