Your Rhododendrons are probably looking a little sad right now, amidst the cold wind and snows of January. They are probably drooping their leaves, maybe they have lost their beautiful dark green glossy leaf color, or maybe they have lost their leaves altogether, sleeping away the dreary days.
But on the other side of the world, in Indonesia, tropical Rhododendrons known as Vireyas are having a heyday. Right now, at the top of Mt. Kinabalu in Borneo, Rhododendrons are growing, blooming, and living their best lives as epiphytic plants. Much like orchids, Vireya Rhododendrons make their living in the crooks of tall trees, taking advantage of the tree’s strong trunk to place itself in the tropical sunshine without taking the time and effort to grow tall themselves. Vireyas are certainly a revelation to many of us here in the northern hemisphere, shattering our commonly held perception of Rhododendrons, but as it turns out, Rhododendron is a whole genus of plants, and it’s one of the most important plant genera on earth, at that. From its remarkable history as one of the first ornamental woody plants in cultivation, to its unique chemical properties, that quirky sad looking plant in your yard has secrets to share, and scientific contributions to make. Rhododendron has a lot to teach us, so what research is your Rhododendron doing today?
The diversity of Rhododendron is an important reason why it enjoys a rather epic claim to fame in the horticultural world, and much of the research on Rhododendron has focused on breeding and horticultural practices. Some of the oldest written records of breeding and cultivation concern Rhododendron, making it one of the earliest ornamental woody plants prized by humans. In particular, a group of Rhododendrons known as evergreen azaleas have been cultivated in Japan since at least the Kamakura period (1185-1333 AD), and there are intensive hybridization records from the Edo Period (1600-1868 AD). The love of azaleas goes back even farther than these horticultural records, being featured in Man’yōshū, the oldest know collection of Japanese poetry, written between 600-759 AD. Still today, azaleas are honored in Asia as part of seasonal festivals such as the Bunkyo Azalea Festival, which draws visitors from all over the world to view the blooming of a stunning botanical collection of 300-year-old azaleas.
While Europe and North America do have their own species of Rhododendron, it was botanical gardens and exploration of Asia that introduced the western world to the horticultural potential of the genus. Rhododendron hirsutum is considered the first species to be introduced to European gardens, brought from China by Flemish botanist Charles l’Ecluse in 1656. In 1734 John Bartram of Pennsylvania introduced N. American species to Europe, and later more extensive 19th century exploration brought many new species and cultivars. Thus, Rhododendron became central to the Victorian English Garden, as their unique ability to hybridize kicked off the British love affair with Rhododendron, which we still see expressed in botanical gardens all over the world. This horticultural history strongly shapes our modern view of Rhododendron, and frankly, many think of Rhododendron only as a finicky garden plant that is hard to keep happy. In reality, genus Rhododendron presents one of the most fascinating and robust examples of diversity as a mode of survival in the face of change. Why should we care? If biodiversity and horticulture aren’t enough to get you excited about Rhododendron, I’m here to tell you that the science of Rhododendron has the most incredible potential to enrich human lives from many perspectives.
The fascinating evolutionary history of Rhododendron is what created the biodiversity of this genus and makes it a hot topic for studies in evolutionary biology, presenting the opportunity to investigate classic theories of evolution, such as the role of climate change in speciation. The first Rhododendron is thought to have arisen approximately 68 million years ago, in China. Today, there are around 1000 recognized species, distributed across all the continents of the northern hemisphere, with about half of these species native to China. In addition, more recent evolution has resulted in ~300 Vireya species, which radiated from the cold mountains of China into the tropical mountain ranges of Southeast Asia. Recent genetic and paleo-biogeography research has provided good evidence that global change has driven this adaptive radiation of Rhododendron, with such spectacular events as the rise of the Himalayas and glacial-interglacial cycles resulting in the evolution of new species, each with its unique habitat type and physical features.
These physical features that impart stress tolerance are the subject of research in the Medeiros Lab at HF&G. We are building on a strong tradition of Rhododendron physiology research that began in the 1950’s, with Rhododendron being one of the very first plants studied for the cold-hardiness of their wood. In many ways, cold-hardiness is the strength and weakness of Rhododendron, as they have evolved wood traits which make them exceptionally suited for mountainous climates, but this comes with a trade-off because these same traits make them slow-growing. From this perspective, Vireyas solved this problem by taking to the trees as they moved into the tropics, but our research shows that for Rhododendrons’ cold-hardiness traits are part of what make them hard to grow, and are bound to make some species more vulnerable to climate change. Thus, the well-known propensity for Rhododendron to die in our gardens makes them a fascinating study system from the perspective of comparative eco-physiology, which explores how the traits of species help them survive and thrive in some climates but excludes them from others. Specifically, our work focuses on understanding how leaves, roots, wood, flowers, and even microbes, evolve in concert to shape the unique habitat preferences and stress tolerance of each species.
While diversity of microbes and roots are certainly interesting from an academic perspective, flowers are undoubtedly the most charismatic aspect of Rhododendron diversity. Another important area of Rhododendron research is centered around the fact that their flowers also provide important floral resources for birds and insects. Typically, Rhododendrons are some of the first and most abundant flowers to appear in spring, when many other plants are still dormant. Rhododendron floral diversity can be quite high in some locations, for example at the Shingba Rhododendron Sanctuary in Sikkim, a region of the Eastern Himalaya in India, 40 Rhododendron species can be identified in just a 100-acre parcel, all blooming on a slightly different timeline. On that same 100 acres, one survey found 2545 birds belonging to 100 species and 22 families, some of whom prefer Rhododendron nectar and pollen above all others. The important role for Rhododendrons in natural ecosystems is just beginning to be explored, and while many species have very restricted geographic distribution along with strict habitat requirements, thicket-forming Rhododendrons can take over the landscape by shaping ecosystem structure and function where they occur. By forming dense, impenetrable thickets and collaborating with specialized soil fungi, these species can shut out light, reduce water infiltration and saturate the ground with low nutrient leaf litter, making the habitat perfect for their own growth, but inhospitable to other plants. While in some cases this may become problematic for humans as thicket-forming Rhododendrons can force out other plants we enjoy, this also occurs in native habitats, representing one more aspect of their diversity and providing a testament to their secret super-powers of survival.
In addition to the ecological value arising from this diversity, this also makes Rhododendron a fascinating study system in genetics. Massive DNA sequencing projects are currently under way, primarily taking place in China, making Rhododendron one of few woody plant genera with full genome sequences available for multiple species. This gives exceptional power to address questions on the genetic foundations of biodiversity, stress tolerance, hybridization, host-symbiont interactions, floral color and form, and even genetic engineering of medicinal compounds. This frontier is just now opening, and we are the lucky folks to see this fulsome view of Rhododendron, for the first time in human history.
Wait…did you say medicinal compounds? That’s right, one of the richest current areas of Rhododendron research is in medicinal chemistry. It’s important to note first when discussing this topic that you should NEVER consume any part of a Rhododendron, they are highly toxic when consumed in their natural form, and much of the scientific literature on Rhododendron concerns poisoning by accidental or purposeful consumption. But their toxic compounds are thought to be key features in the stress tolerance of Rhododendron, and thus they have a very high diversity of bio-active compounds, so many species have been used in traditional medicine wherever they grow. One of the most promising species, Rhododendron tomentosum, a native of tundra habitats in North America, is undergoing extensive investigation for its anti-microbial, anti-fungal, anti-HIV, and anti-cancer effects. Pharmaceuticals derived from this species even hold the promise of treatment for diabetes, with a wide array of research projects addressing its positive effect on insulin resistance. Determining how to separate the medicinal from the toxic compounds is a major undertaking being conducted in the US, Germany, India, China, and really, anywhere in the world you find scientists, such is the vast promise of Rhododendron in medicine.
This rich research landscape demonstrates that Rhododendron truly is one of the most important plant genera on earth, it feeds the human soul with beauty and mystery. Today, however, our resilient and surprising Rhododendrons are facing brand new challenges. Approximately 300 species of Rhododendron are critically endangered or threatened, with both climate change and clearing of forested lands for human uses being the primary threats to its survival. Botanical gardens like HF&G have a big role to play in conserving this unique plant group, and in conserving the plants, so too we preserve the opportunity to research and learn all of it’s amazing human uses for future generations. Is your garden plant exempt from this bright future? Why no, it is not, this horticultural hero in your garden is quietly, patiently waiting for its superpowers to be discovered. Researchers all over the world are listening to what Rhododendrons can teach us, about the environment, about ourselves…are you listening too?
Juliana S. Medeiros, PhD
My research focuses on plant anatomical and physiological acclimation and adaptations to the abiotic environment. I am interested in how phenotypic and genetic variation in plant form and function interact with variation in climate over space and time to drive ecological patterns and the evolution of plant diversity. I focus primarily on plant hydraulic traits, including xylem water transport, leaf gas exchange and the integration of leaf and xylem function. Click here to learn more about research in my lab: Medeiros Lab Webpage