Morphophysiological epicotyl dormancy in seeds of three Psychotria species from Sri Lanka: First record for Rubiaceae

To increase our knowledge of the diversity of seed dormancy and germination in Rubiaceae, we investigated seed desiccation sensitivity and germination of three Psychotria species. Seeds of P. gardneri, P. nigra and P. zeylanica germinated to high percentages at <15% seed moisture content. Intact seeds of P. zeylanica and P. nigra imbibed water and thus do not have physical dormancy. More than 50% of the seeds of P. zeylanica, P. nigra and P. gardneri took 33, 53 and 110 d, respectively, at 25°C for the radicle to emerge, and embryo growth occurred before and after radicle emergence. Thus, seeds have morphophysiological dormancy. Shoot emergence of P. nigra and P. zeylanica seeds was delayed 50 and 80 d after radical emergence, respectively; thus, seeds have epicotyl morphophysiological dormancy (eMPD). This is the first report of eMPD in Rubiaceae. Since warm stratification promoted both radicle and shoot emergence in seeds of P. zeylanica and P. nigra, the level of eMPD is non-deep simple. Hence, dormancy of the studied Psychotria spp. can be described as C1bBb (radicle)-C1bBb (epicotyl), i.e. the embryo is underdeveloped and grows prior to radicle emergence and epicotyl emergence under warm temperatures (Bb), and both the radicle and epicotyl have non-deep simple physiological dormancy broken by warm temperatures (C1b). In two Psychotria species studied in detail, radicle emergence occurs at the beginning of the rainy season and plumule emergence at the peak rainy season when conditions are most favourable for rapid seedling development.