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| Light on the subject of vines |
| 21 April 2004 by Leonie Joubert |
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| How UV will impact on the asparagus flavours in your Sauvignon Blanc, we just don?t know |
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The hole in the ozone layer above the Antarctic reached record size last spring, the United Nations? World Meteorological Organisations (WMO) reports, in spite of strict government controls over the use of ozone depleting chemicals.
The good news is that the ozone?s condition is not expected to get much worse and if it were going to impact on Cape vines, then they?ve survived the worst of it already. Ozone forms a layer of atmospheric gas around the earth ? thinner at the equator, thicker at the poles ? at about 20 to 50km up. It?s like a natural planetary sunscreen, filtering out harmful ultraviolet sunlight which would otherwise burn plants, animals and, as we know, you and me.
Visible light ? catalogued nicely in rainbows ? is essential 'food' for plants. Using the energy of light, a plant transforms this into chemical energy. That, combined with carbon dioxide absorbed from the air around it, gives the plant what it needs to grow and survive. This friendly spectrum of light vibrates in wavelengths of between 400 and 700 nanometres (nm). Anything smaller than that becomes ultraviolet (larger becomes infrared) and is potentially very harmful.
Ultraviolet (UV) light (less than 300 nm) is generally absorbed by the ozone or filtered through cloud cover. If it gets through, however, the effects can be detrimental, as climbing skin cancer statistics have shown since a hole was first discovered in the ozone layer in the late 1970s.
Professor Kobus Hunter, vine physiologist and viticulturist with ARC-Nietvoorbij, says prolonged exposure to unnatural levels of UV light by a grape vine will definitely impact on its physiology. But how, exactly, is extremely difficult to say.
'All the elements in a grape ? the sugar, acid, light absorbing pigments, skin colour, phenolics, enzymes ? are ultimately dependent on photosynthesis and sunlight. They all develop at different ratios to one another and these ratios are, amongst others, dependent on the climatological conditions that they experience,' Hunter explains.
'If a vine is exposed to light outside of its normal photosynthetic range then photosynthesis will be impacted upon, which will then effect the base and secondary products in the grape. But it?s very difficult to say how this ratio of production changes as conditions change.'
So how exactly UV light will impact on the asparagus flavours in your cool-climate Sauvignon Blanc, we just don?t know. A vine has an arsenal of defence mechanisms against potentially harmful climatic conditions, diseases and pests. Each variety behaves differently in any situation, further complicating an understanding of how they might respond to UV.
The photosynthesis apparatus of Cabernet Franc, for example, was found to be more sensitive to UV-B radiation (at between 280 ? 320 nm) than Chardonnay and Merlot. Either way, says Hunter, grapevines are extremely adaptable plants.
Our southern hemisphere vines, which have grown up with high UV exposure, are probably more equipped to deal with it than their northern hemisphere counterparts. And if atmospheric scientists are right about ozone depleting gases being on the decline, then our vineyards have probably seen the worst of the UV bombardment that the ozone hole problem is likely to throw at them. |
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