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Developing optimum scarification method for Acacia koa seeds

Project Collaborators

  • Anna Sugiyama, Department of Forestry and Natural Resources, Purdue University
  • James B. Friday, Department of Natural Resources and Environmental Management, University of Hawai‘i, Mānoa
  • Christian P. Giardina, Institute of Pacific Islands Forestry, United States Department of Agriculture Forest Service
  • Douglass F. Jacobs, Department of Forestry and Natural Resources, Purdue University
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Reforestation and forest restoration projects require massive quantities of seeds. Seeds of many legumes, including Acacia koa (koa), which is one of the most ecologically, economically, and culturally important endemic tree species in the Hawaiian Islands, require that seeds are scarified (treatment to damage the seed coat to allow water uptake) in order to germinate. Manual nicking is a reliable method to scarify koa seeds, but it is not an efficient method if thousands of seeds are to be scarified. An alternative strategy to scarify koa seeds is to use hot water. Hot water treatment is a broadly used scarification method that is relatively cheap, quick, and safe, but the optimum conditions for hot water treatment are often unknown. Overly benign conditions would be less effective whereas overly severe conditions would potentially damage or kill the seeds. Previously reported instructions for scarifying koa seeds using hot water indicate to pour near-boiling water (90°C, 195°F) over the seeds in a volume ratio of 5-10 parts water to one-part seed and soak for 1-3 minutes, followed immediately by cooling water (Wilkinson and Elevitch 2003, Elevitch et al. 2006). Part of the uncertainty in scarification methods for koa may result from a large variation in its seed morphology (Ishihara et al. 2017), which occurs across a wide range of environments. Understanding optimum scarification conditions would improve efficiency in restoration and nursery practices, as well as reduce costs and labor.

The goal of this study is to test whether optimum scarification conditions for koa seed would differ by families (seed source trees) and whether optimum conditions can be predicted by variables such as elevation. We used seeds from different families along an elevational gradient and tested different water temperatures and exposure times after pre-trials. After these treatments, we recorded the number of imbibed (absorbed water) seeds, the number of seeds that germinated, and the number of seedlings that showed abnormal development (potentially due to damage by hot water). Preliminary results indicate that the traditional scarification method of treating seeds with 90°C water for 1-3 minute was ineffective for koa seeds from most families, often resulting in less than 50% imbibition. Effectiveness of any given scarification condition greatly vary among families, but overall, seeds from families in low elevation tend to require more severe scarification conditions. We will continue monitoring seedling development after these different scarification treatments in order to make recommendations for optimum scarification conditions that would maximize imbibition and germination, while at the same time minimizing seedling abnormality.

References

Elevitch, C. R., K. M. Wilkinson and J. B. Friday (2006). Acacia koa (koa) and Acacia koaia (koai'a), ver. 3. Species profiles for Pacific Island agroforestry. C. R. Elevitch. Holualoa, HI, Permanent Agriculture Resources (PAR).

Ishihara, K. L., M. Corpuz, C. W. Morden and D. Borthakur (2017). Botany, ecology and diversity of Acacia koa in the Hawaiian Islands. American Journal of Agricultural and Biological Sciences 12(2): 66-78.

Wilkinson, K. M. and C. R. Elevitch (2003). Growing koa: a Hawaiian legacy tree. Holualoa, HI, USA.