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

Seed (= seed plus endocarp) morphology and physical dormancy were studied in seven North American Rhus species: R. copallina, R. glabra, R. typhina, R. aromatica (var. aromatica), R. microphylla, R. trilobata, and R. virens (var. virens). Seeds of Rhus glabra and R. typhina, of subgenus Rhus, were gray, 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 brown, >4 mm long, approx. 4 mm wide, and weighed approx. 14.5 mg (R. trilobata) to approx. 23 mg (R. aromatica and R. virens). Dormancy in all seeds was 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 five seedlots of R. trilobata, R. microphylla, and R. virens incubated on moist 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, imbibition among seedlots of R. aromatica varied from 28 ± 2% to 69 ± 5%, whereas 93 ± 2% to 100 ± 0% of the seeds from all seedlots of R. microphylla, R. trilobata, and R. virens did so. Neither dry laboratory storage for up to 4 yr (even 29 yr in R. aromatica) nor dry heating at 100°C or at 120°C effectively broke dormancy in any of the species tested (R. aromatica, R. glabra, R. trilobata, R. virens). Immersion in boiling water was the best method to render seeds of R. glabra and R. typhina permeable, 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 loss 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 Lobadium.