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Sunburnt Plants

What will the ozone hole do to New Zealand's plant life?

Ken Markham and Ken Ryan

The progressive depletion of the ozone layer means that increased levels of harmful ultraviolet-B radiation (UV-B, with wavelengths of 280-320nm) are reaching the Earth's surface. This increase will inevitably continue into the next century with potentially deleterious consequences for New Zealand agriculture, horticulture, forestry and tourism. As our economy is heavily dependent on plant-based resources, it is important that research is carried out to determine the effects of UV-B on plant life in New Zealand.

To this end, a team of scientists from Industrial Research Ltd and Crop & Food Research has been assembled to attack the problem. The explosion of knowledge in science in recent years has led modern research increasingly towards a multidisciplinary approach and the team boasts a wide variety of skills ranging from chemistry and photochemistry, to plant physiology and molecular biology. The team is investigating the means by which important plants protect themselves from modest increases in UV-B. This knowledge in turn could be used to enhance the UV-resistance of vulnerable New Zealand crop and native plants.

Of the photoprotective measures known to be used by plants, the most important is the production of protective pigments that strongly absorb the short wavelength UV-B radiation. The major protective pigments are water-soluble, colourless flavonoids which occur naturally in all land-based green plants. Exposure to UV-B radiation induces a rapid increase in gene expression of the flavonoid biochemical pathway, which results in increases in enzyme activity and pigment production. The flavonoids produced are then deposited in the vacuoles of the outer (epidermal) cells, or on the plant's external surfaces.

This increase in flavonoid production is a widespread plant response to UV-B irradiation. However, the extent of this response is limited and varies from species to species, and between varieties. Exposure to excessive levels of UV-B therefore may eventually lead to plant damage and thus ultimately to stunted growth and lower crop yields.

The ultimate focus of the FRST-funded research is to provide strategies for improving the UV-B resistance of UV-susceptible plants. One way to achieve this is simply to breed more UV-tolerant varieties. Another approach is relatively new and relies on advanced molecular biology technology available via Dr Brian Jordan of Crop & Food's Plant Pigment Group. This technique involves genetically modifying the biochemical pathways which result in flavonoid production, so that more, or different, flavonoids are produced in response to UV-B stimulation. In this way, innovative gene technology can be used both to modify the UV sensitivity of selected plants and study aspects of UV-B induced plant damage.

The first steps towards these goals are to design and construct a plant growth facility in which UV-B levels can be accurately controlled and modified, and then to develop methods for the detection of UV-B damage in plants so that the UV sensitivities of different plants can be examined.

The UV-B irradiation facility -- known locally as "the Ultraviolator" -- consists of a two-chamber plastic house. The plastic used is a special clear Teflon that transmits all visible and UV radiation. The natural solar UV-B is continuously monitored in one chamber and the brightness of a bank of fluorescent UV-B tubes in an adjacent chamber is adjusted so that a constant level of UV-B enhancement is maintained inside. The facility is possibly unique in the level of control it affords. The chambers work well and a number of different plant types have been studied to date, including barley, pea, mustard, cucumber and a liverwort.

The work is still at a preliminary stage, but a number of plants have responded to UV-B radiation enhanced to the level expected under conditions of moderate ozone depletion. Furthermore, as noted earlier, we have found that different species of plants respond to enhanced UV-B radiation to different extents. Investigations on rice and mangrove have also been carried out in conjunction with Australian researchers.

Figuring Out Flavonoids

A second major thrust has been to investigate the significance of flavonoid structure, composition, location and rate of accumulation on effective protection. Progress has been made in understanding the photochemistry of flavonoids. It has been shown for example that some flavonoids are better able than others to dissipate the absorbed UV-B energy in a harmless way. In addition, plants exposed to elevated UV-B radiations in the UV facility have responded by modifying their flavonoid profiles either quantitatively or qualitatively. Prospects for significant progress in the next two to three years are thus encouraging.

Dr Ken Markham is a scientist with Industrial Research Ltd.
Dr Ken Ryan is a scientist with Industrial Research Ltd.