is the source of considerable variation in vintage quality, which, in turn, forms one of the bases for the existence of an entire “industry” of wine judges, consumer magazines, and related services.

The majority of the fluctuations in fruit composition are caused by climate variability. In fact, weather differences among years, in addition to vineyard location, are by far the strongest determinants of fruit composition.9 Such climatic differences often trump seasonal differences in soil moisture (except at the far low and high ends of the moisture spectrum) in both dry-farmed and irrigated vineyards.19,25,34

Uniform fruit composition is often described as a critical factor for premium wine making and is equally desirable for the production of grape juice, table grapes, and raisins. It could be argued, however, that less uniform fruit may sometimes result in more complex wine, and this might be one of the rationales behind blending wines from different vineyard sources. In many European wine regions, such blending was traditionally done in the field, where a range of clones or even cultivars were interplanted, sometimes systematically and sometimes randomly.

Whereas traits related to color, astringency, acidity, and flavor are key quality attributes of wine grapes, table grape quality depends more on sugar:acid ratios, texture for

crunchiness or crispyness, and visual appearance, including color,24 although Muscat flavor is often also sought after. Seedlessness is important in some table grape markets and is usually desirable for raisin grapes, for which high sugar content is much more important than it is for table grapes.

Fruit composition changes over time during the berry ripening period as part of the grapevine’s developmental program and is therefore under genetic control. In addition, like phenology and yield formation, the extent of these changes may also be modulated by environmental factors and by the interaction of these factors with the genotype of the vine. It is often difficult to separate the influence of one of these factors from that of another.

For example, solar radiation affects both incident light and tissue temperature. An increase in water supply leads to an increase in shoot growth, which can decrease the proportion of exposed leaves, which, in turn, alters both light quantity and light quality, and may also lead to changes in canopy microclimate (Chapter 5.2). Furthermore, the sugar concentration is higher and the malate concentration lower in berries that have fewer seeds, apparently because veraison occurs earlier in such berries.7,28 Thus, even within a grape cluster, there is often a natural variation of sugar concentration in the range of 5° to 7° Brix that results from asynchronous development of individual berries.6 Due to such asynchronous development, neither time nor thermal time after bloom adequately reflects the developmental stage and maturity of individual berries in a sample from the entire population of berries on a vine, let alone in a vineyard.

Any factor that influences vine growth and metabolism directly or indirectly impacts fruit composition, and this leads to large variation among growing seasons in terms of fruit quality. Wine grape production is especially sensitive to climate variability. This sensitivity