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.