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Oxford Companion to Wine

Oxford Companion to Wine

By Jancis Robinson

Published 2006

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sunlight, the ultimate energy source of all life, and of wine itself. Through a process known as photosynthesis, part of its energy is used by grapevines to combine carbon dioxide from the air with water taken up from the soil, to form sugar in grapes. This is the building block for other plant products, as well as being the immediate source of energy for all of a plant’s biochemical processes, via its respiration back to carbon dioxide and water.

In climatology, the traditional measurement of sunlight was as hours of bright sunlight but it is now generally measured in terms of total energy using electronic sensors. This is more pertinent in viticulture since photosynthesis of grapevine leaves is ‘light-saturated’ at around one-third full sunlight, although a canopy of leaves will photosynthesize at higher rates with increased sunlight. Photosynthesis is reduced at very low light levels in a shaded canopy. It is also reduced by high (> 35 °C) or low (< 15 °C) temperatures, and by water stress.