Physiological dormancy and germination requirements of seeds of several North American Rhus species (Anacardiaceae)

Fourteen seedlots of five species of Rhus were surveyed for presence/absenc e of physiological dormancy and/or for germination requirements of nondorma nt seeds. Physiological dormancy was present in the four seedlots of R. aro matica studied, but not in either of the two seedlots of its close relative R. trilobata, which is in contrast to previous reports. Neither were seeds of R. glabra, R. typhina, nor R. virens physiologically dormant. Stratific ation at 5 degrees C for 1 week or incubation in 500 or 1000 mg/l solutions of gibberellic acid broke physiological dormancy in > 90% of the R. aromat ica seeds. Maturation desiccation acted as a switch from a developmental to a germinative mode in R. aromatica embryos, whereas it was not required fo r germination of R. glabra or R. virens (R. trilobata and R. typhina not te sted). Seeds of all five species incubated on a moist substrate became full y imbibed in 2 d, at which time moisture content was approx. 70-80% of thei r initial weight. In general, germination of non-dormant seeds was rather i nsensitive to temperature and light. Seeds germinated equally well in light and in darkness over a daily (12 h/12 h) temperature range of 15/6-35/20 d egrees C. Over a 4 week period, the best germination percentages were obtai ned at 25/15 and 20/10 degrees C, whereas 35/20 degrees C appeared to be su praoptimal, though not always significantly so. If the incubation period wa s extended to 30 weeks, germination percentages were as high at 15/6 degree s C as at 25/15 and 20/10 degrees C.