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Basil puts on sun-block to screen out UV-B
Imagine a tomato salad: sliced tomatoes, drizzled with olive oil and vinegar, sprinkled with black pepper and freshly torn leaves of basil, and even accompanied by slices of mozzarella - delicious! But wait, is the basil perhaps less aromatic or flavoursome than usual? If so, it may have been grown under glass or plastic rather than out of doors. Sweet basil, Ocimum basilicum, is an important herb and a source of plant oils. It is a complex mixture of the latter that gives the herb's aroma and flavour. Different cultivars differ in the detail of their oil composition, but in general terpenoids are the major class of compound while some varieties also contain significant amounts of phenylpropanoids. A very interesting piece of work carried out by Daphne Ioannidis and colleagues (Mediterranean Agronomic Institute of Chania, Crete and University of Reading, UK, pp. 453-460) has demonstrated the essential role of exposure to UV-B in the production of these oils. They grew plants under glass (which screens out UV-B) at both Reading and Chania; for experimental treatments, the light was supplemented with UV-B giving a daily exposure equivalent to that received 'on a summer's day in the Mediterranean'. The results were very clear. Developing leaves produce more essential oils than mature leaves, but in both significantly larger amounts are produced in plants exposed to UV-B. Furthermore, this is not just an effect on biochemical production: development of the oil glands themselves is stimulated by UV-B. In control plants many of the glandular trichomes fail to fill properly and do not burst. By contrast, plants grown under supplementary UV-B exhibit filling and then bursting of the glands, releasing a 'volatile shield' that is thought to contribute to the protection of leaves against exposure to ultra-violet radiation. The lesson for herb growers is clear: UV-B is not all bad!
Professor J. A. BryantUniversity of Exeter, UK
j.a.bryant{at}exeter.ac.uk
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