Scientists from the Lions Ōtari Plant Conservation Laboratory and Te Papa have followed and documented the changes orchid seeds undergo from germination to adult form. A description of these rarely seen stages will help conservation scientists identify tiny orchid seedlings growing on moss- and lichen-covered tree trunks and determine whether new generations are becoming established. Curator Botany Carlos Lehnebach and Research Technician Jennifer Alderton-Moss discuss it below.
With almost 30,000 species across the world, orchids display a remarkable diversity of shapes, colours, sizes, and habitat preferences. In temperate parts of the world, orchids are normally ground dwellers (i.e. terrestrial), while in the tropics they grow as epiphytes – a word that literally means to be on a plant. Epiphytic orchids grow on shrubs or trees without either harming or helping their hosts. Growing off the ground can protect plants from browsers and increase their access to sunlight, but the trade-off is that they do not grow in soil – meaning they rely on rainwater to deliver essential nutrients such as nitrogen and phosphorus.
Orchids can employ outside help to cope with this precarious way of life. They allow fungi to live within their roots, a symbiosis known as mycorrhiza (myco = fungus, rhiza = root). These mycorrhizal partners extend beyond the plant’s immediate habitat, accessing nutrients via thread-like structures called hyphae. This orchid-fungus relationship is important to the plant survival but plays an even more crucial role during seed germination.
The drawbacks of travelling light
Orchids have tiny seeds – they are normally less than 1mm long! Despite their shapes and sizes varying from species to species, they all share the same feature: the embryo is not surrounded by a layer of food reserves, as it happens in other seeds. Instead, the embryo is enclosed by a thin, almost transparent, net-like coat. This feature is great for being carried by wind into the canopies of trees, but not for supporting the development of a new plant.
In the wild, orchid seeds will only germinate with the help of mycorrhizal fungi, which provide them with the nutrients necessary to grow. Whether the fungus gets anything in return is unclear. What is clear is that to grow orchids from seeds, we must also learn how to grow their fungal partners.
Rebels against the norm: epiphytic orchids in a not-so-tropical country
Aotearoa New Zealand is home to nine species of epiphytic orchids, which is unusual for a country with a temperate climate. Unlike many of our terrestrial orchids, which go dormant and spend part of the year ‘hibernating’ underground as a tuber, epiphytic orchids can be seen year-round, clinging to the trunks or branches of trees.
Although some epiphytic orchids are common across the country, at least three species are currently of conservation concern either because they are known only from a few sites or because their population numbers are declining. The latter is the case of Drymoanthus flavus, a species currently classified as At Risk – Declining.
Historically, the decline of D. flavus populations has been attributed exclusively to overcollection by plant collectors. Over the past few years, however, we have observed widespread mortality in some local populations. If we can understand how to propagate this orchid and then return the seedlings safely to the wild, we could help not only prevent, but also reverse this decline.
And the transformation begins: germination, growing and greening
After years of research, we have now identified suitable fungal partners for both Drymoanthus flavus and D. adversus, a widespread and common relative of D. flavus. Thanks to these fungi, members of the Ceratobasidiaceae and Tulasnellaceae families, we have been able to observe the germination process in both orchid species.
The photographs below illustrate the changes D. adversus goes through from seed to protocorm (a pre-seedling stage unique to orchids) (top left corner and bottom right corner, respectively). These changes are only visible under the microscope, and as you can see from the scale bars, these seeds were only about half a millimetre long (or 500 µm) when first planted.
They begin as a slender embryo (the dark brown part in the centre of the seed) and seed cover. Within a few weeks of sowing, they imbibe (take in water) and swell, stretching and tearing the seed cover. They begin greening, and a few rhizoids (hair-like roots) will grow.
Within two months, they are fully green and have a ridge starting to appear at the ‘top’ – at the opposite end of the young protocorm, the remains of the seed cover can be seen. The rhizoids are now mostly found on the lateral side of the protocorm – we suggest that these play a key role in how the protocorms will hold on to their host trees.
After three months, the ridge now runs all the way down the ‘back’ of the protocorm. There are more rhizoids, but localised to the ‘front’. They have started hunching forward – we variously compare them to lentils, kidneys, torpedo bugs, or some kind of sea creature. These protocorms are now ready to begin the seedling development process.
While D. flavus progresses through the same developmental stages described above, it generally grows a bit slower than D. adversus.
Life outside the lab: time to toughen up!
After three months of living in a controlled and protective environment, the hardening-off process begins. For D. adversus, this means moving the protocorms from a jelly-like medium onto pieces of bark inside airtight containers (which keep the moisture in) and exposing them to natural temperatures. On these pieces of bark, the protocorms continue their development to become seedlings.
At this stage, the protocorms have adopted a wedge-like shape, with a ridge at the upper side and a tuft of rhizoids in the lower side. A small, leaf-like structure develops at one end of the protocorm ridge. Behind this structure, the first true leaf will develop.
Once one or more leaves have formed, true roots will begin to appear. Once both leaves and roots have grown, this animal-like green blob, with green limb-like roots and bunny ear leaves, can be considered an orchid seedling.
Studying the transformation of these orchid seeds has been very useful. It has helped us to refine germination and cultivation techniques, identify the fungal partners preferred by each orchid species, and gain a deeper understanding of the orchid life cycle – from seed to mature flowering plant. It has also opened our eyes to a hidden microscopic world, giving us the superpower to spot tiny orchid seedlings hiding among mosses, liverworts and lichens. Can you spot them too?
Acknowledgements
We thank our collaborators Dr Karin van der Walt and Phoebe Smith for their valuable contribution to this research. We are thankful for funding provided by Te Tahua Taia Nga Taonga Lotteries Environment and Heritage Fund, Stanley Smith Horticultural Trust, and Australia Pacific Science Foundation (APSF 24024) towards different aspects of this project.
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