Standardizing germination protocols for diverse raspberry and blackberry species

Most blackberry and raspberry (Rubus) seed exhibit delayed or poor germination because of a deep double dormancy. The objective of this study was to improve seed scarification and germination protocols for the diverse Rubus species in two subgenera often used for breeding improved cultivars. We also defined the germination potential of freshly collected seed and the seed characteristics of 17 Rubus species in three subgenera. Only four of 17 species had ≥50% germination of immediately-germinated, non-scarified seed, indicating primary seed dormancy. Seed-coat thickness was better correlated with seed size (R = 0.82) than was hardness (R = 0.71). Dry seed of three species each in the subg. Idaeobatus and Rubus were scarified with concentrated sulfuric acid (98% H2SO4) or sodium hypochlorite (14% NaOCl) followed by germination treatments of deionized water (DI), smoke gas solution, gibberellic acid (2.03 mg/L GA3) with potassium nitrate (34 mg/L KNO3) or GA3 alone. Germination after H2SO4 scarification was significantly better than NaOCl for four of the six species despite equal reduction in the seed coat by the scarification treatments. H2SO4-scarified seed had maximum germination in 6–8 months compared to 12 months for NaOCl-scarified seed. Scarification treatments were not uniform for the subgenera. Increased H2SO4 scarification durations monitored by viability testing with 2,3,5 triphenyl tetrazolium chloride (TZ) were very effective in determining optimal scarification timing. R. georgicus and R. occidentalis H2SO4-scarified seed treated with GA3 + KNO3 or smoke germinated significantly better than the other treatments; GA3 + KNO3, smoke and DI water were equally effective for the other four species.

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► Blackberry and raspberry seed have a deep double dormancy and often poor or slow germination.
► Scarification with sulfuric acid is more effective than with sodium hypochlorite.
► Seed-coat thickness and hardness affect optimum scarification duration.
► Tetrazolium testing is very effective for determining optimum scarification duration.
► GA3 + KNO3 or smoke gas treatment during stratification improved germination for most species.