Seasonal dormancy patterns and stratification requirements in seeds of Verbena officinalis L.

Dormancy patterns and germination responses to incremental stratification were investigated in Verbena officinalis L. (Verbenaceae), a widely-distributed ruderal, but considered as an originally mediterranean plant. In an initial experiment, samples of seeds buried in the field were exhumed at monthly intervals over a period of 38 months and tested for germination in light and darkness at 25/15 degreesC and 1515 degreesC. Seeds were strongly dormant when fresh and mature and came out of dormancy during winter. Seeds entered conditional dormancy in spring, while main dormancy relief occurred in winter. There was no seed mortality, and no germination occurred in the soil during burial. The results imply, that V officinalis germinates preferably in late spring and has the potential to accumulate a persistent seed bank. The effects of stratification temperature and duration on dormancy were investigated in a second experiment. Dry-stored seeds were stratified wet at a range of temperatures from 3-18 degreesC in darkness for periods of up to 28 weeks, and tested over a range of constant temperatures (3-25 degreesC) and at 1515 degreesC in light, and at 25 degreesC and 1515 degreesC in the dark. Seeds came out of primary dormancy when kept at temperatures between 3-12 degreesC, while higher temperatures were ineffective in breaking dormancy. There was no induction of secondary dormancy during the stratification period of 28 weeks. We conclude that a gradual increase of the ambient temperatures and/or an increase in amplitude size is required for dormancy induction. Weakly dormant seeds required either high constant mean (> 15 degreesC), or fluctuating temperatures for germination. Only a small fraction of the seeds acquired the ability to germinate in darkness with incremental stratification. There is no indication of germination and dormancy traits that can be interpreted as relict adaptations to mediterranean conditions.