Temperature requirements for after‐ripening in buried seeds of four summer annual weeds

Freshly matured, seeds of the four summer annuals Ambrosia artemisiifolia, Polygonum pensylvanicum, Amaranthus hybridus and Chenopodium album were buried in soil at (12/12 h) daily thermoperiods of 15/6, 20/10, 25/15, 30/15 and 35/20°C and at a constant temperature of 5°C. After 0, 1, 3 and 5 months, seeds of each species at each temperature were exhumed and tested at a 14‐h daily photoperiod at all six temperatures. Fresh seeds of A. artemisiifolia and P. pensylvanicum did not germinate at any temperature, those of A, hybridus germinated to 4 and 64% at 30/15 and 35/20°C, respectively, and those of C. album to 11–20% at 25/15, 30/15 and 35/20°C. Seeds of A. artemisiifolia and P. pensylvanicum, which germinate only in spring, required exposure to low (5, 15/6°C) temperature to after‐ripen completely (i.e., to gain the ability to germinate over a wide range of temperatures), and little or no after‐ripening occurred at high (25/15, 30/15 and 35/20°C) temperatures. Seeds of A. hybridus and C. album, which germinate in spring and summer, required exposure to low temperature to after‐ripen completely, but at high temperatures they rapidly gained the ability to germinate at high temperatures. Regardless of the burial temperatures and species, when after‐ripening occurred, seeds firs germinated at high and then at low temperatures. The minimum germination temperature for a species decreased with after‐ripening temperature and with an increase in the length of the burial period.