By Dr. Dustin Wolkis, Scientific Curator of Seed Conservation
One of NTBG’s core activities is to address the current biodiversity crisis by perpetuating tropical plants and ecosystems. Around 90 percent of Hawai‘i’s plants are endemic, meaning they’re found nowhere outside of Hawaiʻi. Nearly 270 native Hawaiian plant species are represented by fewer than 50 individuals in the wild and nearly half of all species federally listed as Threatened and Endangered are native Hawaiian plants.
Fortunately, NTBG has an active seed banking program with the staff and facilities to allow for ex situ seed conservation at our Botanical Research Center on Kaua‘i. Ex situ conservation refers to work happening “off site”, as opposed to in a plant’s natural wild environment (i.e. in situ). Typically, seed banking involves drying, sealing, and storing seeds at low temperatures.
Dr. Dustin Wolkis (L) and Jena Osmani hold loulu seeds to be transferred to the nursery for propagation. Photo by Jon Letman.
In general, the longevity of a seed increases with lower moisture content and colder temperatures. That said, the seeds of different species respond differently to conditions meant to extend their longevity. A seed’s physiological response to drying and cooling is called “storage behavior.” Seeds of “orthodox” species can be dried and frozen for decades or centuries and retain a high degree of viability. In contrast, seeds with “recalcitrant” behavior do not tolerate drying or freezing and require different conservation methods.
Still other seeds, categorized as “intermediate,” are short-lived. Some intermediate seeds may tolerate drying but lose viability more rapidly than expected when frozen.
All seeds at NTBG are dried to a species-specific level of humidity, hermetically sealed in an aluminum foil pouch, then stored at a species-specific temperature. Each accession (collection) is filed alphabetically by family, genus, and species, then by its unique number. Our state-of-the-art database helps us keep track of each of more than 18 million seeds.
Left: Jena Osmani swabs pollen to check for germination. Photo by Jon Letman. Right: Storing seeds at minus 80°C. Photo by Jon Letman.
Drawing on over 30 years of data gathered by NTBG and our colleagues in the Hawai‘i Seed Bank Partnership, we have observed different patterns of longevity for different species stored at varying conditions. Based on these observations and current knowledge we continually update best seed conservation strategies. In this way Hawai‘i can serve as a model for seed conservation in other tropical, subtropical, and island systems.
Below, are five examples of native Hawaiian species (or groups of closely related species) that illustrate various seed storage requirements that we are addressing at NTBG.
Left: ʻŌhiʻa lehua seeds. Photo by Lyon Arboretum Seed Conservation Lab. Right: ʻŌhiʻa lehua tree.
ʻŌhi‘a (Metrosideros spp.)
ʻŌhiʻa, ʻōhiʻa lehua, or lehua is the Hawaiian name for native Metrosideros spp., members of the Myrtaceae (myrtle family) which includes eucalyptus and guava. The five species and 13 varieties of ‘ōhi‘a are some of the most dominant and ecologically important trees in Hawai‘i’s rainforests and occur on most islands. ‘Ōhiʻa are among the first plants to colonize new lava flows and provide food and cover for native forest birds. Considered to be among Hawai‘i’s most bioculturally important trees, ‘ōhi‘a is intimately intertwined with Hawaiian culture.
Recently, a newly identified fungal pathogen began killing ‘ōhi‘a trees in Hawai‘i. The disease is commonly known as Rapid ‘Ōhi‘a Death (ROD) and has, over the last decade, killed more than one million trees across Hawai‘i. Fortunately, ‘ōhi‘a seeds are small and easy to collect, germinate readily, and can be dried and frozen. Working with our partners across Hawai‘i, we are collecting, storing, and exchanging seeds. When a seed collection grows large enough, the seeds are divided and stored at conventional as well as ultra-cold (minus 80°C/minus 112°F) temperatures to better understand the effects of temperature on seed longevity.
Left: Loulu (Pritchardia napaliensis) seeds. Photo by Lyon Arboretum Seed Conservation Lab. Right: Loulu tree.
Loulu (Pritchardia spp.)
Loulu (also known as hāwane or wāhane, which are also the names for the seeds of these species), Hawai‘i’s only native fan palm, is currently represented by at least 23 endemic species found on Hawai‘i’s main high islands except Kaho‘olawe. It also grows on Nihoa, a one sq. km island with high basalt cliffs roughly 150 miles northwest of Ni‘ihau. Pollen records show that loulu were abundant across the islands but are now rare. Today, loulu, like other palms in Hawai‘i, are threatened by a pest called the coconut rhinoceros beetle (CRB). Loulu seeds are some of the biggest in the Hawaiian flora, take around 400 days to mature, and have a storage behavior not yet fully understood.
In one loulu species, Pritchardia remota, seeds were found to tolerate an intermediate level of desiccation (i.e. neither orthodox nor recalcitrant), yet there have been anecdotal reports of some loulu seeds germinating after 20 years in conventional storage. At NTBG, we have stored loulu seeds in a variety of ways, and finding optimal storage conditions is a focus of future research.
Left: Kokiʻoʻula (Hibiscus clayi) seed. Photo by Lyon Arboretum Seed Conservation Lab. Right: Kokiʻoʻula flower.
Koki‘o‘ula (Hibiscus clayi)
The endangered Hibiscus clayi or koki‘o‘ula is a red-flowering native Hibiscus species endemic to Kaua‘i. Like many species in the Malvaceae (mallow family), koki‘o‘ula have seed coats that are impermeable to water. These seeds can be kept frozen and in dry storage but, with a few exceptions, they require hand pollination.
Hibiscus clayi seeds are brownish-black, shaped like a mix between an egg and a kidney bean, almost half a centimeter long, and have a few soft, star-shaped and simple hairs on their surface. The seeds produce mature fruit in just 40 days.
Alani (Melicope clusiifolia)
Alani (Melicope spp.)
Alani refers to the majority of 54 species of native Hawaiian Melicope (in the Rutaceae or citrus family) which occur across all the Hawaiian islands except for Kaho‘olawe and Ni‘ihau, and include the iconic mokihana (Melicope anisata), a shrub endemic to Kaua‘i beloved for its anise-scented fruits which are woven into lei. A second melicope species (M. clusiifolia), pictured above, grows alongside mokihana and bears similar fruit but those unfortunate enough to mistake the two plants are ridiculed at having collected what is derisively referred to as “kūkae moa” (chicken feces).
Alani seeds are on the larger side (around half a centimeter long) and are glossy black and oval-shaped. They are exceedingly difficult to germinate and the best means of storing the seed remains a mystery. However, we continue to actively explore new methods which we hope will allow us to preserve and protect alani species, some of which are abundant, while others are extinct.
Left: ʻĀlula seeds. Photo by Lyon Arboretum Seed Conservation Lab. Right: A flowering ʻālula.
Hawaiian lobeliads
The largest example of adaptive radiation1 in the Hawaiian flora are the lobeliads and their relatives, members of Campanulaceae (bellflower family). Currently, 161 species and subspecies make up six genera, five of which are endemic to Hawai‘i. One species, ‘ālula or pua ‘ala (Brighamia insignis) was last seen growing wild on Kaua‘i in 2012. Although recent drone surveys have confirmed that the species is extinct in the wild, decades of successful collection and cultivation efforts have made ālula a powerful symbol of plant conservation.
Hawaiian lobeliads also include ‘ōhā wai, the various species of Clermontia; hāhā (another name for Clermontia), and the majority of Cyanea, and Delissea species; lobelia species such as pānaunau (Lobelia yuccoides); and koli‘i, the majority of Trematolobelia species. Hawaiian lobeliads also represent the family with the largest number of endangered Hawaiian plants and extinctions. Seed conservation is critical for this family.
Hawaiian lobeliad seeds range in size from 0.5 mm to almost 1.5 mm long, may be wrinkled or ridged across their widths, and can be white or blackish brown depending on the species.
These seeds generally tolerate drying but their viability declines faster when frozen rather than if stored at moderately cool temperatures (above 5°C/41°F). We dry these seeds and store them at an ultra cold minus 80°C and, when possible, keep a portion of the same accession at multiple temperatures.
Through NTBG’s efforts to monitor seed longevity, we can maximize viability for future use of seeds in a variety of conservation and restoration efforts, while improving our ability to understand seed conservation biology.
Going forward, NTBG continues to collect, study, and save seeds of some of the world’s most endangered plants. We are also making progress in our ability to store pollen, fern spores, and are beginning to store bryophyte (mosses, hornworts, and liverworts) spores as well. As we enhance our storage methods and improve our ability to save seeds, we are making a positive contribution to safeguarding one of the most unique floras on Earth.
[1] The diversification of a group of plants or animals into new forms to fill varyng ecological niches
