Response of Amaranthus retroflexus L. seeds to gibberellic acid, ethylene and abscisic acid depending on duration of stratification and burial

Freshly harvested, dormant seeds of Amaranthus retroflexus were unable to germinate at 25 and 35 A degrees C. To release their dormancy at the above temperatures, the seeds were stratified at a constant temperature (4 A degrees C) under laboratory conditions or at fluctuating temperatures in soil or by outdoor burial in soil. Fully dormant, or seeds stratified or buried (2006/2007 and 2007/2008) for various periods were treated with exogenous gibberellic acid (GA(3)), ethephon and abscisic acid (ABA). Likewise, the effects of these regulators, applied during stratification, on seed germination were determined. The results indicate that A. retroflexus seed dormancy can be released either by stratification or by autumn-winter burial. The effect of GA(3) and ethylene, liberated from ethephon, applied after various periods of stratification or during stratification, depends on dormancy level. GA(3) did not affect or only slightly stimulated the germination of non-stratified, fully dormant seeds at 25 and 35 A degrees C respectively. Ethylene increased germination at both temperatures. Seed response to GA(3) and ethylene at 25 A degrees C was increased when dormancy was partially removed by stratification at constant or fluctuating temperatures or autumn-winter burial. The response to GA(3) and ethylene increased with increasing time of stratification. The presence of GA(3) and ethephon during stratification may stimulate germination at 35 A degrees C. Thus, both GA(3) and ethylene can partially substitute the requirement for stratification or autumn-winter burial. Both hormones may also stimulate germination of secondary dormant seeds, exhumed in September. The response to ABA decreased in parallel with an increasing time of stratification and burial up to May 2007 or March 2008. Endogenous GA(n), ethylene and ABA may be involved in the control of dormancy state and germination of A. retroflexus. It is possible that releasing dormancy by stratification or partial burial is associated with changes in ABA/GA and ethylene balance and/or sensitivity to these hormones.