Field studies were conducted to determine the effect of emergence timing on the fitness of the next generation as represented by seed mass, maturation, and afterripening of common waterhemp cohorts. Five natural cohorts were documented both in 2009 and 2010. Different maternal environments resulting from varied cohort emergence timings did not influence seed maturation time and seed mass, but had an inconsistent effect on seed afterripening. Here are our major findings. (1) Waterhemp cohorts needed similar amounts of time to generate viable seeds (20 to 27 d after flower initiation) and the seeds produced were of similar size (2.0 to 2.35 g), and (2) waterhemp has strong primary dormancy that may be released within 4 mo during the afterripening process, depending on the dormancy level. Seeds produced by later cohorts were more sensitive to the afterripening period, suggesting more flexibility in life strategy. Seeds from the 2009 cohorts had similar afterripening patterns; newly harvested seeds had strong primary dormancy (<10% germination), which was gradually released during dry storage and reached the maximum germination (>80%) rate 4 mo after harvest (MAH). However, germination then dropped to 40% 6 and 8 MAH, suggesting the induction of secondary seed dormancy. Strong primary dormancy at harvest for 2010 seeds was sustained in dry afterripening, perhaps because of higher dormancy level, which was the result of less-favorable parental environments brought by 10 to 30 times higher population densities and 2.5 to 5 times higher accumulative precipitation than in 2009 (see Wu and Owen 2014). We also tested the soil seed-bank seed population densities for each waterhemp cohort and found that early cohorts greatly influenced the seed population densities at the soil surface level and the turnover rate of the soil seed bank. Results from this research will provide insights into better management of waterhemp, targeting a better understanding of the seed bank.
Nomenclature: Common waterhemp, Amaranthus tuberculatus (Moq.) Sauer.