The effect of endocarp and endocarp splitting resistance on warm stratification requirement of hawthorn seeds (Crataegus monogyna)

Crataegus monogyna seeds were studied to uncover the mechanisms behind the exogenous dormancy. Seeds with or without endocarp were warm stratified for 0, 28, 56, 84 and 112 days and then subjected to a combined chilling and germination test, a conductivity test, a moisture content test and a test of the mechanical pressure necessary for splitting the endocarp. Seeds with endocarp needed 112 days of warm stratification followed by cold stratification to reach a germination percentage of 75%, while seeds without endocarp could germinate without any warm stratification preceeding the cold stratification. The mechanical resistance to splitting of endocarps decreased during the warm stratification period. This offers a new method for quantification of the warm stratification process. The increasing germination percentage with increasing warm stratification was strongly correlated with the percentage of seeds with split endocarps. After imbibition of seeds with endocarp the seed (embryo and testa) water content was approximately 30%, compared to a water content of above 40%, when seeds were imbibed without the endocarp. The moisture content of seeds without endocarp did not improve with increasing duration of warm stratification, but initially reached a higher level than in seeds with intact endocarps. Splitting of endocarps released a possible mechanical, water deficiency or oxygen availability restraint on the embryo.