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Correlation analysis suggest that moisture availability is more important than length of growing season as a selective force for seed weight in Amaranthus retroflexus populations from eastern and central North America. Larger seeds are associated with drier environments, probably due to their capacity to establish seedlings from deeper soil horizons, where moisture is more reliable. Seed weights correlate better with long—term moisture availability than with more recent moisture patterns, indicating that populations do not track short—term climatic fluctuations very closely. A previously reported strong relationship between length of growing season and A. retroflexus seed weight is confounded by high correlation between dryness and short growing season, as well as by genotype interaction with abnormal photoperiods. Populations in flood— or furrow—irrigated regions do not have smaller seeds than those in the driest nonirrigated regions. This may be due to enhanced dispersal of large seeds by irrigation systems.
David J. Schimpf (Tue,) studied this question.