Friends of the Arboretum Newsletter
Fall 2008 – Volume 12, Number 2
By Mark Weathington, Assistant Director and Curator of Collections
Hydrangeas are among the best known garden plants around. Despite being grown in the west for well over two centuries, there is still a legacy of confusion surrounding these worthy landscape shrubs and vines. About 25 species in total are generally recognized, with most coming from Asia and Central and South America. The main shrubby species available to gardeners are Hydrangea macrophylla, H. paniculata, and H. serrata from Asia and H. arborescens and H. quercifolia from the United States. These plants are grown for their large heads of showy sterile flowers.
Hydrangeas owe much of their popularity to their flamboyant colors, impressive floral display, and ease of care. Once established, they are long-lived garden stalwarts. Establishment is important; new plants should be situated in a somewhat shady location with a rich, well-drained, organic soil and watered regularly for the first two years. H. paniculata is fine in full sun and slightly poorer soils; H. quercifolia will tolerate some sun, but will tend to droop in the hot afternoon. In general, hydrangeas are fairly drought tolerant once well-established, but look so poor under dry conditions that they probably shouldn't be grown where supplemental irrigation or a moist spot isn't assured. The exception is H. paniculata which is an exceptionally tough plant as is, to a lesser extent, H. quercifolia.
Pruning hydrangeas has been a mystery for many people for many years. In general, hydrangeas do not need to be pruned, as evidenced by large, untended specimens around old homesteads. If pruning is a must, there are a few simple rules to follow. For H. macrophylla and H. serrata, flowers are formed on the previous year's growth, so pruning hard during the winter will greatly reduce or entirely eliminate flowering. Flower buds are generally formed along the stems, but only the apical or tip buds actually develop flowers. If these buds are removed, the lower or axillary buds will often develop flowers. Many hydrangeas can therefore be pruned back halfway or more in fall or late winter and still put on an impressive floral display. Before pruning, examine the stem for large (~0.5") buds, as not all cultivars produce these axillary buds. Another pruning method is to remove about a quarter or a third of the oldest stems at ground level each year on established plants. This is a good way to keep the plant healthy and vigorous and works well for both of these types of hydrangeas. Some of the newer remontant, or re-blooming, hydrangeas can be pruned hard over the winter and still produce flowers reliably.
Pruning the other forms of hydrangea is a much simpler matter. H. paniculata flowers on new wood. It can be pruned hard in winter and will develop new shoots and flower heads that are often so large that the stems will bend under their weight. If pruned lightly or not at all, smaller, more numerous flower clusters are produced. H. arborescens flowers on new wood and should simply be cut to the ground in winter. The other southeastern native, H. quercifolia can be left alone or pruned lightly in late fall or winter for shape and to control size. All of these hydrangeas can be deadheaded after flowering or spent flowers can be left for winter interest, based on personal aesthetics.
Flower color in the H. macrophylla/H. serrata selections is always another dilemma for gardeners. In the simplest of terms, the availability of aluminum in the soil determines flower color. In basic (high pH) soils, aluminum is tightly bound to calcium and is unavailable to the hydrangea, so the flowers will be pink. In typical southeastern soils, there is plenty of aluminum available and a relatively low pH, so flowers will tend toward blue. Many soils fall somewhere in the middle and flower color can be an attractive mauve. In the Lath House at JCRA, the soil is acidic due to the large amount of pine bark which would lead one to expect blue flowers. However, the lack of mineral soil means there is very little aluminum present and most of the flowers are purplish to pink. Additions of aluminum sulfate will help blue your hydrangeas, while a couple of cups of lime over the root system will pink them up. Some hydrangeas will stay bluer even where aluminum is not readily available and others will be pink even when it is.
H. arborescens 'Balsam'
Discovered by Michael Dirr, Ph.D. , in the North Carolina Balsam Mountains, it has large sterile flower heads held on sturdy stems. Although similar to the more common 'Annabelle', the smaller flower clusters on stouter stems make it a tidier garden plant. Between the Butterfly Garden and the Finley-Nottingham Rose Garden.
H. arborescens 'Eco Pink Puff'
Burgundy flower buds open to reveal pink fertile flowers (no large bracts). A different look than the typical hydrangea, but a good garden performer. Discovered by Don Jacobs at Eco Gardens in Georgia. Toward the east end of the Mixed Border.
H. arborescens 'Emerald Lace'
Another odd native hydrangea, 'Emerald Lace' has lacecap flowers with dissected and irregularly cut foliage. Sometimes sold erroneously as 'Green Dragon'. Two specimens, one behind the weeping winged elm across from the east end of the Mixed Border, the other in the center of the Mixed Border.
H. macrophylla 'Big Daddy'
A large mophead hydrangea with huge inflorescences. Changes color readily with pH from light pink to bright blue. Next to the Ruby C. McSwain Education Center.
H. macrophylla 'Blaumeise'
One of our best performers, this lacecap is sited in sun here at the JCRA. This form tends to stay blue even under low pH conditions but will eventually turn rosy in the absence of available aluminum. In the Conifer Collection.
H. macrophylla 'Masja'
A dwarf mophead with medium-sized flower clusters that fits in well beneath larger shrubs and small trees. Flower color often is midway between deep red and blue. In the Lath House.
H. macrophylla 'Oregon Pride'
Stout, dark stems support richly colored mophead flowers, typically rose to purple colored. A vigorous grower with attractive foliage. In the Lath House.
H. paniculata 'Dharuma'
A dwarf form more suitable for smaller suburban gardens than other cultivars of this species. Next to the McSwain Center.
H. paniculata 'Greenspire'
Long, somewhat narrow inflorescences start lime green before fully turning white, then aging to a pale green. A vigorous growing shrub. In the District X Garden Club of North Carolina Wall Garden.
A presumed H. macrophylla × serrata hybrid with typically pink flowers aging to a beautiful red in fall. Autumn foliage color is often a very nice burgundy. In the Paradise Garden.
H. quercifolia 'Flemygea' (Snow Queen™)
A heavy flowering selection with somewhat smaller leaves and shorter stature than the species. Medium sized, dense flower spikes are held upright. Fall color is deep burgundy. In the Lath House.
H. quercifolia 'Little Honey'
A yellow-leaf form of the popular dwarf 'Pee Wee'. The color holds well in moderate sunlight and flowers are produced at a young age. Fall color is rosy-purple. At the corner of the Bobby G. Wilder Visitor Center.
H. serrata 'Grayswood'
An old, but very reliable lacecap with larger sterile florets than the species. Pink to blue flower color, aging to rose. Reddish fall color. Near the boardwalk in the Mixed Border.
H. serrata 'Kurenai'
New foliage emerges gold before darkening to chartreuse as the summer progresses. Pink to blue lacecap flowers appear in June. Fall color is often bright orange to burgundy. Behind the Wilder Visitor Center.
H. serrata (Wilson 7820)
A dwarf, early-flowering form that was wild collected by Ernest "Chinese" Wilson. Typically pink flowered, blue only with very low acidity and plenty of aluminum. A heavy flowering and reliable performer. Right of the McSwain Center doors.
JCRA 2008 Summer Interns
By Denny Werner, Ph.D., Director
The JCRA was fortunate to have seven wonderful students join us for the 2008 summer internship program. These students engaged in a diversity of activities and responsibilities during their internship program that broadened their horticultural knowledge outside a classroom setting. We sincerely appreciate all they contributed to the programs and gardens of the JCRA during their time with us.
Meet the Interns
Bradley Cole recently completed his degree at NC State University in agricultural extension education, with a concentration in horticultural science. Bradley interned with the Wake County Cooperative Extension Service in spring 2008, and has worked for Eastern Landscapes in Coats, North Carolina, from 2005-2007. He is an Eagle Scout, and is a Friend of the JCRA. Bradley is from Coats, North Carolina.
David Hoffman is an intern "early bird" in that he is interning with the JCRA prior to beginning his studies in horticultural science at NC State this fall semester. David is active in the Scouts, and he has considerable horticultural experience working for his parents at Hoffman Nursery. I believe he was happy to work with something besides grasses at the JCRA this summer! David is from Rougemont, North Carolina.
Jessica Kitzmiller is a senior at Catawba College in Salisbury, North Carolina, majoring in environmental science and minoring in biology. She will be graduating in December 2008. Jessica has experience at Noah's Ark Wildlife Center, a non-profit organization founded and run by her parents that has rehabilitated wildlife since 1997. Jessica has done some interesting work for her senior research project, testing mercury levels in bird feathers. She currently lives in Raleigh.
Jeffrey Malcolm is a horticultural science major. He has returned to acquire his B.S. degree in horticulture after a break of nearly ten years in his studies. During that time, Jeff worked in a number of landscaping positions. He was senior groundskeeper at Davidson College for six years, groundskeeper at the Methodist Home for two years, and a landscape technician at Paramount Carowinds for a brief period. He worked as a groundskeeper at Meredith College in 2007, and interned in the Department of Horticultural Science's greenhouse conservatory this past semester. Jeff is currently living in Apex, North Carolina.
Stephen Panasci is a sophomore majoring in horticultural science with an emphasis in landscape design at NC State, and minoring in Italian. He related that he developed his interest in plants and biology from his uncle, who is a biology teacher. Stephen worked as a counselor at a youth camp in summer 2007. Stephen has a wide variety of artistic skills and has expertise in computer design programs. He is from Maplewood, New Jersey.
Russell Reagan is currently majoring in horticultural science at NC State, and will graduate this fall. He is minoring in biology. Russell has been employed at the Phytotron here at NC State this past year, and previously has worked as an irrigation installer with Atlantic Irrigation and Landscaping. Russell is captain of NC State's Men's Rugby Club. Russell is from Mooresville, North Carolina.
Michelle Rose is currently enrolled in the horticultural science program, with an emphasis in landscape design. Michelle has prior degrees in business management (B.A. ) and psychology (B.A. ). Michelle has also taken course work at Central Carolina Community College in the sustainable agriculture program. She is a passionate gardener, and manages the educational garden at her church. She is a Friend of the JC Raulston Arboretum, and lives in Cary, North Carolina.
2008 JCRA Intern Sponsors
"Four Friends" intern sponsors
JC Raulston Arboretum
North Carolina Commercial Flower Growers Association
North Carolina Nursery and Landscape Association
Raleigh Garden Club
Georgina and Dennis Werner
Support a 2009 Intern
Interested in sponsoring an intern during summer 2009? Please contact Denny Werner at (919) 513-7006 or email@example.com or Anne Porter at (919) 513-3829 or firstname.lastname@example.org.
The Scree Garden
By Charlie Kidder, JCRA Volunteer
The Scree Garden is one of the newer gardens at the JCRA, having been planted in September 2006. Before we take a look at the plants in the Scree Garden, we should answer the question I hear most often, "What is scree?"
In case you've forgotten most of your geomorphology (the study of land forms; for example, hills, mesas, cuestas, tombolos, and other arcane terms), scree is rocky debris that collects at the base of a slope. Due to the action of freezing and thawing, rocks break into smaller pieces and eventually slide downhill to form areas of scree, sometimes also known as talus slopes. Scree particles generally range from fist-sized down to gravels. There is usually an absence of fine particles and organic matter in scree; therefore, drainage is very sharp.
Scree is most conspicuous and common in arid areas where mechanical weathering from frost action is more common, as opposed to the chemical weathering of rock that occurs in humid climates. Still, scree is not totally absent from climates such as North Carolina's, where it is found in some areas of the mountains.
The scree beds at the JCRA are a rough attempt at duplicating natural scree. The area immediately west of the Ruby C. McSwain Education Center was chosen since it receives sun most of the day, with the afternoon rays reflecting off the building's wall. This mimics the hot, sunny, and dry conditions that plants growing in natural scree would experience.
To create the Scree Garden, the turf in this area was killed, and the clay soil was broken up to improve drainage. Then a special soil mix, three parts sandy loam and one part PermaTill, was built up in berms on top of this subsoil. The berms reach a maximum height of about 4' and a width up to about 12'. A mulch of one to two inches of PermaTill was applied before planting.
Plants in the Scree Garden come from a variety of areas where the climate is dry and/or drainage is exceptional. The agaves hail from the U.S. Southwest, Mexico, Central America, and the West Indies. There are 200–250 species of Agave; they are currently represented by about 20 taxa in the Scree Garden. The various species hybridize freely, and several exist in many varieties and forms, as well.
Agaves are usually grown for their sculptural form and foliage color, rather than the flowers, since most species are monocarpic; that is, they flower once in their life, then die. They go out with a bang, however. In the larger species, a 15' stalk shoots up in less than two months and bears dozens if not hundreds of flowers.
Even the few species of agave in the Scree Garden display an amazing variation in shape and color. An Agave potatorum hybrid sports very broad gray-green leaves with purplish tips and spines. On the other end of the foliage spectrum, A. aff. tenuifolia has leaves that appear almost grass-like. And there is a foliar bonus on some species: as the leaves unfold from the central rosette, they bear the imprint of the adjoining leaves, adding considerable interest when viewed at close range. (Not too close, mind you!)
There are several plants related to the agaves in the Scree Garden. Yuccas (Yucca spp. ) are also in the Agavaceae family and are more widely known to most easterners, since a few species are native to this part of the country. The Nolinaceae family is represented by Nolina and Dasylirion species, both with elongate grass-like leaves, but with varying degrees of toothiness on the margins. The foliage of all of these plants flutters in a strong breeze, producing an almost hypnotic effect.
But perhaps you aren't a big fan of the spiky nature of the Agavaceae and Nolinaceae. If you prefer something of a frillier texture, check out the ferns growing in the Scree Garden. Yes, there are ferns that like sun and dry conditions. Pellaea calomelanos sports blue-gray foliage that allows it to withstand intense sunlight; P. ovata has distinctive zigzag rachises and hails from Texas, Mexico, Hispaniola, and Central and South America.
Not all these ferns have origins in dry climates, however. Cheilanthes lanosa, with the delightful common name of hairy lip fern, is indigenous to the Midwest and eastern United States. , including the North Carolina Piedmont and mountains. By growing on top of rock outcrops, it too lives in a xeric environment.
But don't think the Scree Garden is only about the varying textures and foliage tones. There is also color—that is, other than green—to be found in the Scree Garden. Several cultivars of rain-lilies, Zephryanthes and Habranthus, put out their delicate pastel flowers after showers.
And if you want shocking colors, you can't miss the many salvias (Salvia spp. ) in the Scree Garden. In fact, don't even try to miss the salvias! The various cultivars of Salvia greggii put out dark red, hot pink, magenta, or pink-and-white flowers, from spring to fall, with a short break in mid-summer. Salvia chamaedryoides stands out from the crowd with its gray-green foliage and cobalt blue flowers.
We would be remiss if we did not thank Tony Avent of Plant Delights Nursery for donating many of the plants and Carolina Stalite Co. for providing their expanded slate product, PermaTill.
The Scree Garden provides year-round floral and textural interest. Creation of a scree garden is relatively simple and allows one to grow a unique palette of plants typically challenging to grow in our hot and humid climate.
By Tim Alderton, Research Technician
In the preceding article, Charlie Kidder explained to you what scree is, how our garden was constructed, and talked about just a few of the plants there. I am going to tell you a bit more about a small sampling of some of the other plants that you may encounter.
Clematis is a genus best known for large flowering vines that want their roots in the cool moist shade and their heads in the sun. So you would not expect to find Clematis growing in the inhospitable heat and drought of the Scree Garden, but many of the smaller, non-climbing species can naturally be found in just these types of conditions. These species are not as showy as that of their larger cousins, but are interesting to plant in that dry, sunny spot where few other plants will prosper. Clematis ochroleuca is one of those clematises. A native of the sandy Coastal Plain from New York to Georgia, it is perfectly at home on the well-drained soils of the Scree Garden. The foliage is simple, undivided, elliptically shaped, and opposite along the wiry stems. Cream colored sepals surround the stamens and pistils of the campanulate 0.5"–1" flowers borne at the tips of the 8"–12" tall shoots in spring and early summer. Following the flowers, tufts of silvery seedheads top the stems. Clematis fremontii, a similar species from dry rocky soils in Nebraska, Kansas, and Missouri, as well as from disjunct populations found as recently as 2006 in Georgia and Tennessee, has sepals that range in color from purple to creamy white. The young leaves and stems, as well as the flowers, have a slight pubescence covering them. Of the two plants in the Scree Garden, one has flowered white; maybe the second will be another color when it flowers. A third clematis species in the Scree Garden, C. hirsutissima, is native to much of the western United States and southwestern Canada. It naturally grows on sagebrush plains and in open ponderosa pine forests. This species has a similar diminutive size as that of the two preceding species, but with very different foliage. Clematis hirsutissima has very thinly cut foliage topped in early spring by small, purple, campanulate flowers. The entire plant is covered in a haze of fine hairs. Ours has yet to flower, but I look forward to seeing it in future springs.
Violets are often thought of as the scourge of many a garden, popping up in the lawn and beds. When pulled, the plant returns from either the rhizomes or unseen hundreds of seeds that hide in the soil. One species, though, was welcomed into the Scree Garden. Viola pedata, the bird's foot violet, is not the thug that many of its relatives are. This small clumping species is a native to Wake County and can be found throughout much of the eastern half of North America. Where it gets its common name is obvious when one sees the deeply cut, three- to five-lobed, bird's foot-like leaves. In the wild, they are commonly found growing in dry, rocky, upland settings. There are two in the Scree Garden. Half- to one-inch flowers are perched over the 2"–4" tall foliage from April to June, with sporadic flowering in the fall. The first is the more common, pale lavender-blue flowered form, and the second is a cultivar called V. pedata 'Eco Artist Palette', which has smaller flowers, with the three lower petals being pale lavender blue and the two upper petals a deep royal purple.
Blazing hot dry conditions are a great place to find ferns, right? Of course! These primitive plants have had plenty of time to fit into every niche in the world, including the xeric conditions of rock outcroppings, screes, and deserts, as well as the moist, shady soils we are so accustomed to. There are several in the Scree Garden that have been right at home for the last few years now. A favorite of mine is Pellaea wrightiana. The unreal powder-blue fronds appear and feel more like that of the cheap plastic foliage plants at your favorite fast food restaurant than that of a living fern. This semi-evergreen fern is largely native to the southwestern United States and northern Mexico, but there are a few disjunct populations in western North Carolina and South Carolina. This little fern grows to about 6"–8" and is right at home in tight crevices or well-drained soil in full sun.
A plant that almost everyone has grown or is growing in their garden is phlox, so it is not surprising to find some species are even present in the Scree Garden. The genus Phlox is almost entirely from North America, aside from a few species that wandered over the Bering Land Bridge into northeastern Asia. Species growing in the Scree Garden are not the common tall garden phlox, so prone to mildew in the heat of summer, but are the smaller spring-flowering species. Phlox nivalis 'Camla' is the first of these phlox to flower. P. nivalis is a native of the Southeast with disjunct populations in Texas, Utah, and Michigan. At first glance, it looks much like that of P. subulata, but is differentiated by having woody stems and subtle differences in the shape of the flowers. It is found growing in sandhills and dry pinelands. P. nivalis 'Camla' covers itself in pale lavender flowers beginning as early as late December and January, but is typically at its peak in March and early April. A second species is Phlox bifida 'Betty Blake', a Midwesterner found growing in dry sandy soils of both open forest and plains. 'Betty Blake' forms a tight, non-spreading dome of slightly ridged foliage that looks like a foot-wide green hedgehog when not in flower. But in April, the dome is covered in lavender blue flowers that have petals so deeply cleft that they appear to have ten petals instead of five.
The preceding plants have all had close association with the eastern United States, but there are many more plants in the scree from more distant places. One such genus, Sinningia, is a close relative of Saintpaulia—better known as the African violet. Sinningia, a Central American and South American genus, includes the florist gloxinia, Sinningia speciosa hybrids. There are two of note we have been growing in the Scree Garden for a few years now; S. sellovii and S. 'Butter and Cream'. Both of these are perfectly adapted to the well-drained, hot conditions of the Scree Garden. Hairs covering their succulent leaves, stems, and flowers help prevent evaporation and large tubers up to 6"–8" across store water and nutrients below ground. Sinningia sellovii, a species from southern Brazil and Argentina, has 0.75" long by 0.25" wide tubular flowers of deep salmon dangling from arching inflorescence up to 18" long on plants that measure up to 3' tall. Sinningia 'Butter and Cream', a hybrid of S. aggregata and S. tubiflora, has creamy white, 1.5" long by 0.5" wide flowers hanging from upright stalks 18" tall, and has light fragrance.
A very different genus, Pulsatilla, has species throughout much of the temperate northern hemisphere. Pulsatilla, a member of the Ranuculaceae, is sometimes lumped into the genus Anemone. The similarities to Anemone are seen in the spring flowers, but the long, fuzzy, Dr. Suess-like seed heads distinguish them. Pulsatilla halleri, a sub-alpine species from populations scattered across Europe, has finely pinnate cut leaves. Up-turned flowers surrounded by sepals that range from pale purple to dark purple are 1.5"–2" across. All parts of the plant are covered in fine hairs, making them touchably soft and silky in appearance. The second species, P. cernua, is found in temperate east Asia, including parts of Russia, China, Korea, Mongolia, and Japan. Nodding flowers enclosed in fuzzy purple-red sepals are held just above the coarse pinnate leaves. Both species appreciate the heat and gritty soils of the Scree Garden.
South Africa is noted for having one of the most diverse flora of any area in the world, so it is not surprising that we grow many plants in the Scree Garden from there. Included in the plants from South Africa is Aloe cooperi. This is unlike the aloe that sat on your grandmother's kitchen window, ready at a moment's notice to treat that burn from grabbing the hot handle of a pan. Aloe cooperi grows as an upright clump of grassy, slightly succulent foliage to 18" tall, much like that of Kniphofia, another South African. During the summer months, 24" tall stalks topped in orange flowers tipped in green are held like a torch above the foliage. In the winter, it dies back only to resprout in the spring. Wild A. cooperi can be found growing in both moist grasslands as well as dry rocky areas, making it a perfect choice for the Scree Garden.
Much more can be found throughout the year in the Scree Garden from the winter-flowering bulbs to the salvias of summer. Find the plants of both North Carolina and distant regions of the globe all at home in the Scree Garden. Take time to stop and smell the Berlandiera the next time you come to the garden.
Hormonal Basis of Dwarfism in Plants
By Vikramjit Singh Bajwa, Graduate Research Assistant, Department of Horticultural Science, NC State University
Variation in height between plants within the same species is commonly observed. Have you ever pondered the basis for this observed variation? Variation in plant height is controlled by both genetic and environmental factors. These factors can limit plant growth either independently or in combination by affecting important physiological processes in the plant. For example, temperature is an environmental factor that can directly affect photosynthesis, respiration, and transpiration, thereby influencing overall plant growth. Other environmental factors that can limit plant height include moisture, light, and nutrition, among other things.
Genetically, plant height can be described as a quantitative trait controlled by a number of genes. These genes are often located on different chromosomes and can affect different physiological processes that interact to influence plant height.
Plant hormones play a major role in plant growth and development. Dwarfism can result from disrupted function of these essential plant hormones. The two major hormones that control dwarfism in plants are 1) Brassinosteroids (BRs) and 2) Gibberellins (GAs). In this article, I will focus on the role of these two plant hormones on dwarfism.
Brassinosteroids (BRs) are essential steroid plant hormones. BRs are ubiquitously present in the plant kingdom where they control a wide range of physiological processes, including promotion of cell expansion, cell division, and organ elongation in young monocot and dicot plant tissues. The first study on BRs was reported by John Mitchell and his USDA coworkers in 1973, where they reported that organic extracts of Brassica napus (rapeseed) pollen (subsequently shown to be a rich source of BRs) promoted stem elongation in plants. After this discovery, significant research was initiated to understand the chemistry and physiology of these plant hormones. Michael Grove, PhD. , and others (1979) purified 4 mg of brassinolide (BL), the most bioactive form of BRs, from 40 kg of Brassica napus pollen to verify its structure. They determined that the structure of BL was analogous to steroid hormones in mammals and insects. Recently, the focus of BR research has been to understand BR biosynthesis and signal transduction pathways in plants using molecular genetic tools. Signal transduction is the process by which plant cells perceive and respond to a signal. The signal, such as a plant hormone, initiates biochemical events, which lead to functional changes within the cell. The stimuli that transmit the information to plants are both external and internal, such as light, mineral nutrients, gravity, water status, soil quality, mechanical tensions, temperature, growth regulators, and pathogens.
Analysis of BR mutants in plants clarified the role of these hormones in various physiological processes. Brassinosteroid mutants have been identified in different plants including Arabidopsis, tomato, pea, barley, and rice. These BR mutants show severe dwarfism, demonstrating the indispensable role of BRs in stem elongation and leaf expansion. The BR mutants are of two types: 1) BR-deficient and 2) BR-insensitive. The BR-deficient mutants contain lesions in genes involved in the BR biosynthetic pathway whereas BR-insensitive mutants have aberrations in genes involved in BR signal transduction. BR-deficient mutants can be rescued to wild type (normal growth expression) by exogenously supplying the biologically active BRs such as brassinolide (BL); however, the BR-insensitive mutants cannot be rescued by the exogenous application of synthetic BRs. BR-insensitive mutants fail to perceive the hormone and, therefore, fail to relay the signal to downstream components of the signal transduction pathway. The identification of BR-deficient mutants played a major role in understanding the BR biosynthetic pathway, whereas the identification of BR-insensitive mutants helped researchers in understanding the BR signal transduction pathway.
In recent years, a number of genes involved in BR-biosynthesis and signaling have been identified and cloned. During BR signal transduction, BR binds to a cell-membrane bound receptor known as BRASSINOSTEROID INSENSITIVE 1 (BRI1). The binding of BL to BRI1 activates BRI1 and the signal is passed to other components that occur downstream from BRI1 in the pathway. The downstream components include the transcription factors BES1 and BRZ1. These transcription factors control BR-responsive gene expression by binding to a regulatory region of the BR-responsive genes. BR-responsive genes include cell wall and growth-related genes, suggesting that BRs promote elongation of cells by modifying cell wall properties.
Research on BRs is relatively recent, having started only in last quarter of the twentieth century. Considerable research remains to be done in this area to fully understand how these hormones control dwarfism by regulating genes involved in various important physiological processes. It is important to understand the interplay of BRs with other plant hormones such as GAs, which affect plant growth and development and therefore dwarfism in plants.
The other important hormone group that controls dwarfism in plants is Gibberellins. GAs are a large family of plant hormones that affect a broad range of plant growth and developmental activities, including stem elongation, leaf expansion, seed germination, and flowering. Similar to BRs, GA-deficient and GA-insensitive mutants show severe dwarfism. The physical characteristics of these mutants helped in understanding the physiological role of GAs in growth and development. The biosynthesis of GA has been very well characterized. The GA-deficient mutants played a major role in understanding the GA biosynthetic pathway, whereas GA-insensitive mutants helped researchers in understanding the GA signal transduction pathway. The GA signal transduction pathway leads to the expression of GA responsive genes. Similar to BRs, GAs alter the bio-physical properties of cell walls by increasing the level of cell wall modifying enzymes.
Practical Applications of BR and GA Research on Dwarfism
The green revolution in the 1960s and 1970s greatly increased the yields of wheat and rice. This increase in yield was mainly due to creation of new varieties by plant breeders, and the subsequent adoption by farmers of these varieties. These new, high-yielding varieties were shorter, gave more grain yield at the expense of straw biomass, and were more resistant to damage by wind and rain. Subsequent research has demonstrated that these new varieties were shorter because of their abnormal response to GAs, hence they represent GA mutants. These varieties contain lesions in genes involved in the GA signal transduction pathway (i.e. , perception and response to GA). Recently, it has been shown that new genetic variants of rice, which have very erect leaves due to defective BR biosynthetic genes, can capture more light for photosynthesis and can store more nitrogen for grain filling, thereby increasing grain yields.
Commercial plant growth regulators (PGRs) that are used in commercial horticulture to control height, such as Topflor, contain active ingredients which effectively reduce internode elongation through the inhibition of GA activity. Topflor can be used as a growth retardant on a number of ornamental plants such as chrysanthamums, impatiens, dahlias, Easter lilies, pansies, geraniums, poinsettias, and sunflowers. A related PGR, Cutless, is also used on turf grass as a growth retardant.
The understanding of underlying molecular mechanisms involved in the signal transduction and biosynthesis of BRs and GAs can have great practical applications in the future. The alteration of genes involved in the signaling and biosynthesis of these hormones can help in generating plants with controlled growth properties which may be directly applicable in field crops, horticultural commodities, and ornamental and nursery plants. Indeed, it may turn out upon further investigation that many of the dwarf ornamental varieties currently planted in gardens throughout the world may, in fact, be mutants in the GA or BR pathways.
JCRA Cultivar Releases
By Denny Werner, Ph.D., Director
The JCRA has recently introduced a number of exciting new cultivars from its breeding program. Below is a brief description and information on availability.
Buddleja 'Summer Frost'
This brand new butterfly-bush is a selection derived from a cross of 'Lochinch' × 'Dartmoor'. It combines the silver-gray foliage and medium purple flower color of 'Lochinch' with the attractive, branched panicle architecture of 'Dartmoor'. The panicles arch slightly. As an added bonus, the flower buds themselves are lovely when they just begin to show color, and, of course, full bloom is quite beautiful. The plant is vigorous, growing 5' to 6' tall, but denser and more highly branched than most butterfly-bush cultivars. This cultivar was just recently distributed to North Carolina nurserymen for trial at the recent summer trade show, and is not yet available in retail channels. It is not patented.
Lantana 'Sunset Beach'
This new Lantana originated as an F2 selection derived from a cross of Lantana 'Miss Huff' 'Confetti'. The cross was made by undergraduate student Vance Whitaker in 2002 under the direction of Denny Werner, Ph.D. F2 seed were derived from open pollination of the F1 family, and F2 progeny were grown at the Sandhills Research Station in 2004. One selection demonstrated moderate vigor (less than 'Miss Huff') and flower color similar to the male parent 'Confetti'. 'Sunset Beach' has proven cold hardy and in Zone 8, but has not been trialed in Zone 7. However, based on the 'Miss Huff' parentage, it is likely to perform well in Zone 7. 'Sunset Beach' was recently distributed to North Carolina nurserymen for trial at the recent summer trade show, and is not yet available in retail channels. It is not patented.
Buddleja 'Blue Chip'
This new butterfly-bush was released this year, and is currently available at a limited number of garden centers. It should be widely available in spring 2009. 'Blue Chip' has a unique compact, spreading growth habit. Its growth rate is considerably less than that of other butterfly-bush cultivars, and it is easily maintained at a height of 2'–3'. The inflorescences are very dense and shortened as compared to the typical elongated inflorescence of Buddleja. Flowers of 'Blue Chip' are very fragrant, and foliage is semi-evergreen, showing retention into winter similar to its Buddleia lindleyana parent. The compact growth habit and dense foliage of 'Blue Chip' will allow it to be used in a wide range of landscape situations. 'Blue Chip' produces small, distorted anthers, and shows no evidence of pollen production. 'Blue Chip' shows evidence of seed production in a field setting, but considerably less than that of other Buddleja cultivars. A plant patent for 'Blue Chip' has been applied for.
Buddleja 'Miss Ruby'
'Miss Ruby' was selected for its semi-compact growth habit, silver-grey foliage, and unique flower color. The flower color of 'Miss Ruby' is a true pink with little to no purple contamination. The standard pink cultivar in the trade, 'Pink Delight', has distinct purple contamination in the flower. 'Miss Ruby' was named in honor of Ruby McSwain, for whom the Ruby C. McSwain Education Center is named. 'Miss Ruby' is currently included in the Royal Horticultural Society (RHS) Buddleja trials in Wisely, England, and I have heard it is being well received. 'Miss Ruby' was released in 2008, and will be available in garden centers in 2009. A plant patent for 'Miss Ruby' has been applied for.
Formatted into HTML by Christopher
Programs and Education Coordinator
JC Raulston Arboretum
Department of Horticultural Science
North Carolina State University
Raleigh, NC 27695-7522
© The JC Raulston Arboretum, September 2008