Seed morphology and physical dormancy of several North American Rhus species (Anacardiaceae)

Seed (= seed plus endocarp) morphology and physical dormancy were studied i n seven North American Rhus species: R. copallina, R. glabra, R. typhina, R . aromatica (var, aromatica), R. microphylla, R, tribolata, and R. virens ( var. virens). Seeds of Rhus glabra and R. typhina, of subgenus Rhus, were g ray, approx. 3 mm long, 2 mm wide, and weighed approx. 7 mg, whereas those of R, aromatica, R. trilobata, and R. virens, of subgenus Lobadium, were br own, >4 mm long, approx. 4 mm wide, and weighed approx. 14.5 mg (R. triloba ta) to approx. 23 mg (R. aromatica and R. virens). Dormancy in all seeds wa s due to a water-impermeable endocarp, but depth of dormancy varied greatly among species and seedlots. After 4 weeks, 29-34% of the seeds from all fi ve seedlots of R. trilobata, R. microphylla, and R. virens incubated on moi st substrate had imbibed, compared to 0-14% of the seeds of all 16 seedlots of R. aromatica, R. copallina, R. glabra, and R. typhina. After 1 yr, imbi bition among seedlots of R. aromatica varied from 28 +/- 2% to 69 +/- 5%, w hereas 93 +/- 2% to 100 +/- 0% of the seeds from all seedlots of R. microph ylla, R. trilobata, and R. virens did so. Neither dry laboratory storage fo r up to 4 yr (even 29 yr in R. aromatica) nor dry heating at 100 degrees C or at 120 degrees C effectively broke dormancy in any of the species tested (R. aromatica, R. glabra, R. trilobata, R. virens). Immersion in boiling w ater was the best method to render seeds of R, glabra and R. typhina permea ble, yet it was ineffective for those of R. aromatica, R. trilobata, and R. virens. In contrast, a 1 h-soaking in concentrated H2SO4 led to complete l oss of endocarp impermeability in the latter three species, but mostly was ineffective in R. glabra and R. typhina. Thus, there seems to be a tendency for seeds of subgenus Rhus to respond well to boiling in water, but not to soaking in H2SO4, whereas the opposite is true for those of subgenus Lobad ium.