Phenotypic plasticity of life-history characters in response to different germination timing in two annual weeds

An experimental manipulation was conducted to test whether germination timing influences the post-germination life-history characters in Amaranthus retroflexus L. and Chenopodium glaucum L. Seeds were sown in spring, late spring, and summer. Life-history characters of both phenology and morphology were measured, and dry masses of roots, stems, leaves, and reproductive organs were determined. Life-history characters showed high plasticity in response to different sowing dates. Later germinating plants had relatively faster growth rates and smaller sizes at reproduction than earlier germinating plants. Delaying germination led to relatively earlier reproduction and a relatively greater allocation to reproduction. Much of the variation (60%) could be explained by a single axis of a principal component analysis. The attributes on this axis were similar to the C-R axis of Grime's C-S-R model. Further, the sowing dates of these two species were aggregated on this axis such that spring germinators tended towards the competitor strategy (C), late-spring germinators tended towards a mixed competitive-ruderal strategy (C-R), and summer germinators tended towards a ruderal strategy (R). Different germination timing led to different life-history strategies in the established phase. This kind of phenotypic plasticity in life history results from the plant adapting to regeneration strategies of different germination timing.