RESEARCH ARTICLE Effects of Sowing Date on Phenotypic Plasticity of Fitness-Related Traits in Two Annual Weeds on the Songnen Plain of China

Background Phenotypic plasticity of fitness-related traits is vital for plant species to adapt to variable en-vironments. Chenopodium glaucum L. and Amaranthus retroflexus L. are two common weed species globally. Understanding the plasticity in life-history traits, especially in repro-ductive allocation, within and among these species is important for predicting their success and for managing them in different environments. Methodology/Principal Findings Seeds of the two plant species were sown every 10 days from 26 Jun to 15 Aug. Life-history and fitness-related traits of both phenology and morphology were measured, and dry bio-mass of roots, stems, leaves, and reproductive tissues was determined at physiological ma-turity. Length of reproductive and total life period of the two species differed among six sowing-date treatments. Later germinating plants led to relatively reduced total life period, size, and earlier reproduction than earlier germinating plants. The ratio of reproductive bio-mass to total plant biomass increased with later planting dates in C. glaucum but declined i