Seed morphology and physical dormancy of several North American<i>Rhus</i>species (Anacardiaceae)

Abstract

Abstract 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, &gt;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 o C or at 120 o 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 H 2 SO 4 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 H 2 SO 4 , whereas the opposite is true for those of subgenus Lobadium .