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  • Author or Editor: Derek N. Peacock x
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Many lesser-known wild Rubus species from Ecuador, the People's Republic of China, and North America have been obtained on recent U.S. Dept. of Agriculture plant-collecting expeditions. In this study, the seed size of 43 Rubus species was measured. An 80-fold range in seed weight was observed within the genus. Asian species in the subgenera Idaeobatus and Malachobatus had the lightest seed, ranging from 0.3 mg (R. eustephanus Focke ex Diels) to 1.2 mg (R. coreanus Miq.). The seeds of ≈80% of the species examined weighed <2 mg. Seeds of European species in the subgenera Idaeobatus and Rubus (formerly Eubatus of Focke) ranged from 1.3 to 3.0 mg. The South American Orobatus included several of the heaviest-seeded species. Rubus megalococcus Focke (subgenus Rubus) had the heaviest and largest seed weighing 24.2 mg. Seed weight was not related to ploidy level in wild species. Seed weight and length were positively correlated. Seed flatness was not related to seed length. Several of the smaller-seeded Asian species, such as R. minusculus A. Lev. & Van., R. hirsutus Thunb., and R. eustephanus, had more drupelets per fruit than did those of larger-seeded species. This heritable trait may be useful in breeding for increased fruit size.

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We contrasted the effect of liquid nitrogen (LN2), sulfuric acid (H2SO4), and a nontreated control on the germination of six Rubus species. We also were interested in determining if LN2 could be an effective mechanical scarifying agent for these species. Seeds of each species were treated with three 3-minute dips in LN2 with alternating 10-minute thaws, with H2SO4 for 30 minutes, or left untreated. The percent germination of R. multibracteatus A. Leveille & Vaniot, R. parviflorus Nutt., R. eustephanos Focke ex Diels, R. leucodermis Douglas ex Torrey & A. Gray, R. ursinus Cham. & Schldl., and R. chamaemorus L. treated with LN2 was not significantly different than the control. Germinated seedlings from the LN2 treatment of each species showed normal development upon planting, indicating that long-term cryogenic preservation of these Rubus species seeds may be possible. The H2SO4 treatment significantly increased the rate and percentage of germination in R. parviflorus, R. eustephanos, R. leucodermis, and R. ursinus over that of the control and the LN2 treatment. The alternative LN2 application techniques that have been attempted thus far have not significantly improved Rubus seed germination compared with that of the control.

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Recent USDA plant collecting expeditions to Ecuador; the People's Republic of China, and within North America, have obtained a number of lesser known wild Rubus species. These, and additional species, are preserved as seedlots with some plant representatives, at the USDA-ARS National Clonal Germplasm Repository-Corvallis. In this study, the seed size of 40 Rubus species was measured and contrasted. The average weight of the largest-seeded species of the study group, R. megalococcus Focke, an Ecuadorean blackberry, was 24.2 mg; European blackberry, R. procerus Muller, was 3.0 mg. The average weight of other European and North American blackberry and raspberry seed ranged from 2.7 to 1.3 mg. Asian raspberry species tended to be the smallest, ranging from R. coreanus Miq. at 1.2 mg to R. eustephanus Focke ex Diels at 0.3 mg. Several of the smaller seeded Asian species such as R. formosensis Kuntze, R. minusculus A. Leveille & Vaniot, R. hirsutus Thunb., and R. eustephanus had many drupelets, which may be a heritable trait to benefit yield through breeding for increased fruit size.

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During research to develop a new germination protocol for Rubus being conducted at the National Clonal Germplasm Repository in Corvallis, we observed mixed responses to sodium hypochlorite (NaOCl) as a seed scarifying agent. For R. parviforus Nutt., scarification with NaOCl resulted in 34% germination. Fewer than 1% of the seedlings showed any negative effects after exposure to 2.6% NaOCl for 24 hours. But in R. ursinus Cham. & Schldl., R. multibracteatus A. Leveille & Vaniot, R. swinhoei Hance, and R. setchuenensis Bureau & Franchet, the percentage of injury observed ranged from 40% to 100%. In these cases, although embryonic tissue did not appear necrotic, the radicle and plumule failed to elongate after emergence. The epicotyl or primary leaves did not develop, and the radicle failed to form root hair. The cotyledons, apparently unaffected, opened and were a healthy green. NaOCl did not kill the embryo, but deterred development of the embryonic axis. As a result of the NaOCl scarification the cotyledons expanded yet the seedlings eventually died.

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