OVERTON — Short treatments with a particular type of ultraviolet light results in more marketable bedding plants and vegetables, according to preliminary work done by a Texas Agricultural Experiment Station horticulturist at Overton.
Dr. Brent Pemberton has found that exposing plants such as impatiens, tomatoes and cucumbers to ultraviolet B, (UV-B) treatments for a few hours slows their foliar growth and stem length without reducing color or other favorable characteristics.
“Simply put, it acts as a growth regulator and stunts them somewhat, but with bedding plants this may be desirable,” said Pemberton, who is based at the Texas A&M University Research and Extension Center at Overton.
Bedding plants are often grown from seedlings in greenhouses. The growing conditions in a greenhouse cause many plants to outstrip their containers, growing too tall to be easily transported by the time their color matures. Plants overly tall for their containers may also be aesthetically displeasing, which can hinder their marketing.
Height control of greenhouse-grown plants has long been an issue in the bedding plant industry. Chemical growth regulators are often used on ornamentals but are not considered safe on vegetable crops. Using UV-B to regulate growth is a relatively safe alternative that seems to work on some bedding plant species as well as select vegetables, Pemberton said.
While some plant varieties show a promising response to UV-B, others don’t. For example, Pemberton has found the technique does not work on annual periwinkles. While pansies, an important East Texas greenhouse crop, show some positive response to UV-B, much work remains to be done.
Two years ago, after attending the International Horticulture Conference in Japan, Pemberton began preliminary work with UV-B on bedding plants. Most work with UV-B at that time involved attempts to predict how advanced global warming and ozone layer depletion conditions might adversely affect weeds, small grains and forage crops.
Having worked for years with bedding plant growers, Pemberton saw a potential advantage in the stunting effects of UV-B exposure.
Back at the Overton Center and working with a limited budget, Pemberton began experiments with newly transplanted seedlings. Using florescent lighting fixtures similar to those used in tanning salons, he set up exposure trials in a walk-in cooler, then moved the plants to one the center’s greenhouses.
With encouraging results and a modest grant from the Texas Ornamental Enhancement Endowment , which is funded by ornamental plant production industries in association with Texas A&M University, Pemberton plans to expand the work. The next step is to refine the exposure times of UV-B for different varieties.
Pemberton also wants to investigate exposing plants at an earlier growth stage. How the UV-B radiated plants grow out in different types of greenhouses also needs to be evaluated, he said.
Glass-paned greenhouses filter out much of the sun’s ultraviolet spectrum while sheet plastic-covered greenhouses do not, and this may alter the long-term effects of UV-B-exposed plants.
Pemberton warned that UV-B should be used with caution. Relatively short exposures can damage the cornea of the eye, cause conjunctivitis and damage the skin. He typically turns the UV-B lamps off before entering the growth chamber or wears protective goggles.
The sun emits several frequencies of ultraviolet radiation. Longer frequencies the ground and are important to normal plant growth. Shorter frequencies are mostly absorbed by the ozone layer in the upper atmosphere. Scientists are concerned that disturbances of the ozone layer by various air pollutants could allow more of the shorter, more dangerous frequency of ultraviolet to reach the ground.
“Destruction of the ozone layer is a serious issue. What we’re trying to do is take something negative and find a positive use for it,” Pemberton said.
Pemberton also conducts annual bedding plant trials, looking at the performance of sometimes 200 species under East Texas conditions during the spring and fall. The bedding plant industry means big money for Texas. Wholesale total more than $250 million annually in East Texas alone, according to extension surveys. Yet until the early 1990’s, Texas nursery operators had no way to check out how new varieties performed except through trials on the West Coast or Upper Midwest.
“Growing conditions vary greatly from state to state,” Pemberton said. “As big as the industry is, there are few places from Georgia to California that perform outdoor trials other than Overton. Except for our trials here at the center, there are no greenhouse trials closer than Chicago.”