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Sunburn Protection for Plants

Too much UV-B radiation can cause cause sunburn in plants, as well as people, and stunt plant growth. One strategy developed by plants to protect themselves against UV-B is under examination by scientists at Industrial Research.

With the progressive depletion of the ozone layer, this knowledge could have important future applications for maintaining New Zealand's agriculture and horticulture in an environment with increasing UV-B. It could also help extend the range of habitats in which particular crops could grow.

Plant scientists Drs Ken Markham, Ken Ryan and Stephen Bloor are leading a project investigating the ways in which plants protect themselves against harmful solar UV-B radiation to see how these can be enhanced to cope with increased levels of UV-B.

Markham says they already knew that flavonoid compounds on the outer cell layer of a plant leaf could act as a sunscreen for plants, and that for some plant, the level of flavonoids increases with increasing UV-B radiation. [Sunburnt Plants, April 96]

"We develop a method to protect plants against increasing UV-B. We assumed that the flavonoids acted as good UV absorbers by creating a filter to protect the plants," he says.

Their approach was to first construct a plastic house, known locally as "The Ultraviolator", to grow plants. The house comprises three compartments each with a differing UV level. One compartment has all UV blocked out, one receives just ambient UV, and the other has fluorescent UV tubes to enhance the natural sunlight. Detectors measure UV levels in the compartments.

By growing three widely different types of plants in the plastic house -- a liverwort, petunias and rice -- the researchers sought to cover the plant kingdom so they could be confident that any finding would be broadly applicable to a wide range of plants.

As expected, when UV levels increased, the plants grew more poorly, exhibiting smaller leaves and stunted growth. The second step was to analyse the flavonoids present. Again, as expected, with increased UV levels the plants' flavonoids increased in concentration. However, the most interesting find was that the ratio of flavonoids present changed. Previously scientists had just measured an overall increase in flavonoids.

The IRL team found that of the two major flavonoids, kaempferol and quercetin, the latter became more prevalent. Ryan says they found this in all three types of plants.

"This suggested to us that if we could choose plant varieties with naturally higher quercetin levels we would have better UV defence. Or, alternatively, we could genetically modify plants to produce higher levels of quercetin themselves."

To test this, they obtained genetically modified petunias from Crop and Food Research in Levin. The transgenic plants have selected flavonoid enzymes modified or removed.

"We found that one of these petunias had naturally higher concentrations of flavonoids. These plants were also able to increase the ratio of quercetin to kaempferol more readily than the normal variety. As predicted, these transgenic plants were more resistant to UV."

The next step is to investigate why quercetin is more effective than kaempferol as a sunscreen. Possible reasons include the ability of flavonoids to act as antioxidants by scavenging free radicals which destroy plant tissues; quercetin is better at absorbing the free radicals than kaempferol. Another possibility is that quercetin is better at harmlessly dissipating the absorbed UV energy.

Over the coming summer the three scientists will be working with AgResearch to look at different varieties of clover and their UV sensitivities.

Markham says that if the increasing UV levels in many parts of the world pose a serious threat, they hope to be in a position to help people grow UV-resistant plants.