Effect of the ripening season on the germination response of Datura strumarium populations.

Variability regarding dormancy and germination has been observed among some European populations of Datura stramonium L. Gradual adaptive processes to local environmental and agronomic conditions are supposed to be related to this intra-specific variability. Moreover, differences could exist among seed batches produced by the same plants in different seasons according to the specific environmental conditions during their ripening. The interaction of these two phenomena probably determine the intra-specific variability of local D. stramonium populations; a specific study on this interaction could therefore contribute to expand the existing knowledge about this important weed. Seed collections of three populations of D. stramonium took place in autumn 2008 from maize fields at Legnaro (Padova, Italy), Tapada da Ajuda (Lisbon, Portugal) and Arganda del Rey (Madrid, Spain). The three seed batches obtained were sown in autumn 2008 at the CSICexperimental farm located in Arganda del Rey, so differences regarding maternal environment during life cycle of the following plants were minimized. The seeds were collected in 2009 at three subsequent moments, called Collection Times (CT 1, 2 and 3), i.e. 18th September, 16th October and 10th November, to compare germination response of seeds produced during the whole ripening period. Nine seed batches were obtained. Small portions of seed coat were mechanically removed by pricking before germination tests to facilitate seed imbibition. Seeds were incubated at different constant temperatures (8, 12, 18 and 22 °C) with a photoperiod of 12/12 h light/darkness and three 50-seed replicates for each treatment. Germinated seeds were counted and removed daily and at the end of the experiment. Non-germinated seeds were checked to assess viability with a seed-crushing test. The germination percentage of each replicate was calculated based on the total number of viable seeds. The germination percentage reached maximum values for all seed batches at the highest incubation temperatures (18 and 22 °C) while values were significantly lower at 12 °C and no germination occurred at 8 °C. Mean total germination percentages varied from 59.2 % ± 6.70 SE for the Spanish population to 50.6 % ± 6.43 SE for the Italian one, but no statistical differences were detected. The Portuguese population instead presented the lowest germination percentage (20.9 % ± 3.77 SE) at 12 °C temperature at which the greatest differences in germination response among the three seed batches in terms of CT were detected. Seed batches CT1 of each population reached a higher germination percentage than the others, while CT3 reached the lowest. Seed batches CT3, which were produced at the end of the growing season, were probably characterized by a higher degree of dormancy, especially the CT3 Italian which resulted in a lower germination percentage also at 18 °C. Seeds produced during ripening season CT3 largely increased the variability observed regarding dormancy and germination even within the single populations. Since this situation was similar among the three populations, a certain role in adaptive process could therefore be supposed.