PLANT OF THE WEEK

Dr. T. Ombrello - UCC Biology Department

THE FLOWERING MAPLE 

AND ITS VIRUS

Common names:         

Flowering Maple, Parlor Maple, Indian Mallow  

Scientific  name:         Abutilon striatum   

Explanation of scientific name:       

Abutilon  - a name of Arabic origin applied to this genus of plants

striatum  - striated, striped, in reference to the handsomely veined petals of this species’ flowers

While commonly called the Flowering Maple, Abutilon striatum is not at all closely related to the Maple trees most of us are familiar with.  The common name, however, reflects the similarity of this species’ leaf shape to the true Maples.  The Flowering Maple belongs to the Mallow family of plants (Malvaceae) and includes among its close relatives the Mallows, Hollyhocks, Cotton, Hibiscus, Okra, and Rose of Sharon.  Naturalized and commonly found throughout South and Central America, the Flowering Maple is native to Brazil.

Now considered an old-fashioned houseplant, Abutilon striatum was brought to Europe and North America early in the 19^th century where it proved to be an attractive indoor flowering plant in temperate regions, and a small flowering shrub in tropical habitats.  When planted outdoors as annuals in our area, Flowering Maples look spectacular and last well into autumn, tolerating light frosts without damage.  The old common name of Parlor Maple reflects its extensive use for interior decorating in past years.  Today it is much less commonly seen as a houseplant, neglected in favor of the more exotic species available to the plant enthusiast.  Flowering Maples tend to get tall and lanky with age, but their interesting foliage and bell-like flowers that come in shades of white, yellow, salmon or red, make them quite desirable.

The Flowering Maples that were first introduced into Europe had solid green colored leaves.  In 1868, however, one seedling in a shipment of Abutilons imported into England from the West Indies had leaves that showed bright yellow mottling on the green blades.  This plant was propagated vegetatively and soon replaced the solid-green leaved types as the favored ornamental of this species.  Apparently no one gave much thought to the origin of this unusual form at the time.

Early in the development of the science of Virology (the study of viruses), 1904, the mottled leaves of Flowering Maples were studied in an attempt to determine if some virus parasite was responsible for the condition.  The virus hypothesis was rejected since mottle-leaved and green-leaved Abutilons could grow side by side in European greenhouses without any evidence of transmission.  In 1933, however, it was discovered that the condition could be transmitted by the seeds of the plant.

Today we know that indeed a virus is responsible for the mottled leaves of Flowering Maples.  Commonly called Abutilon Mosaic Virus or AMV, this virus goes by the scientific designation of Marmor abutilon.  With a worldwide distribution, AMV can be transmitted from one Abutilon to another by grafting, occasionally by seed, and in Brazil by the whitefly insect Bemisia tabaci.  Since the common greenhouse whitefly of Europe and North America, Trialeurodes vaporariorum, does not transmit the virus and the Brazilian whitefly is not found in Europe, early European virologists did not recognize the transmission possibilities of this virus.

Calling Marmor abutilon a mosaic virus has a very specific meaning.  Upon close observation, the leaves’ normal green and discolored areas are usually sharply bordered at small veins.  The non-green regions are made up of infected cells while the green regions or “islands” contain no appreciable amounts of the virus.  The infection of the leaf cells occurs while the leaves develop, with mosaic symptoms visible as soon as the leaves appear.

The expression of the mosaic symptoms tends to disappear in subdued light.  If grown under very low light conditions, newly developing leaves will be completely green despite being virus infected.  Even under bright light conditions, Flowering Maples may recover from the virus infection and produce completely green leaves if the mosaic leaves are persistently removed.

Flowering Maples, although infected, do not seem to suffer in regard to vigor and flowering.  In fact, the virus is responsible for most of the species’ ornamental value.  Thus, the question arises – Is AMV a disease?  According to Webster’s definition of a disease as “an impairment of the normal state of the living animal or plant body that affects the performance of the vital functions”, one could argue that it isn’t.

Looking beyond the Flowering Maple, there are numerous cases of plants being more useful when virus infected.  The best, and probably the oldest, example is the striped tulip flower.  Today, striped tulips are commercially bred for their unusual color patterns on the petals, but the original striped tulips were solid flower colored plants infected by a virus.  In the 1600’s the condition called “Tulip-break” was first described in Holland.  The multicolored flowers instantly became very popular, and vast fortunes were paid for bulbs with “Tulip-break”.  Wild speculation followed by a loss of interest and subsequent value of the infected bulbs almost caused the collapse of Holland’s economy.  Tulipomania, as the episode came to be known, remains today as an interesting example of silly horticultural fads.  After all, today we know that people were simply buying “diseased” bulbs.