Different environmental conditions, different results: the role of controlled environmental stress on grape quality potential and the way to monitor it

Environmental stress, such as water deficit or limited nitrogen availability, reduces grape yield, but generally promotes grape quality potential for red table wine production. Limited nitrogen uptake limits grape yield but enhances grape quality potential for red table wine production, because it reduces berry size and enhances phenolic compound synthesis. Water deficit stress has one negative effect (reduction of photosynthesis), and many positive effects (shoot growth cessation, reduction of berry size and stimulation of phenolic compound synthesis). Mild water deficit stress increases berry quality potential, despite reduced photosynthesis. This can be explained not only by a reduced competition for sugars between shoot growth and fruit ripening, but also by reduced berry size. For red Bordeaux wine, vintage quality is well correlated to the drought of the vintage. Many tools are available for monitoring vine water status. Among them, two are of particular interest: stem water potential and carbon isotope discrimination measured on grape sugars (13C/12C ratio or δ13C). Stem water potential is better related to vine transpiration than leaf water potential. Stem water potential is a more precise indicator of vine water status than pre-dawn leaf water potential in soils with heterogeneous humidity, which is the case in irrigated soils. 13C/12C ratio in grape sugar (δ13C) is related to vine water status during grape ripening and can be measured with mass spectrometry. This indicator can be implemented to assess the severity of vine water deficit stress in dry farmed vineyards without any measurement in the field. For companies buying grapes from deficit irrigated vineyards it can be used to control the irrigation management on a mere sample of grape juice. Grape production can be made economically viable through high yields, high quality allowing high selling prices or low production costs. Strategic winery management implies deciding which of these parameters should be optimised. High yields can be obtained by fertirrigation of vines planted in fertile sites. High quality red table wine allowing high selling prices are produced by vines grown under environmental stress. Moreover, specific terroir expression is related to environmental stress. Production costs can be reduced by mechanisation and low labour input vineyard design. Introduction The vine is a resistant species to environmental stress. Historically, in the Old World, fertile land was preferentially used for grazing and annual crops. Shallow soils, stony soils and steep slopes were used for vineyards or olive tree plantations (van Leeuwen and Seguin 2006). Although yields tend to be moderate to low in these conditions, grape quality potential for wine making is generally high. Traditionally, vine vigour, yield and grape quality potential were controlled through site selection. By trial and error the best locations for quality production were selected. Today, vineyards are planted in a broader range of environmental conditions. The effects of environmental stress on yield and quality parameters are better understood and they can be more easily managed. Soil fertility depends mainly on water and nitrogen availability, generally in relation to soil depth (the deeper the soil, the more fertile it is; Coipel et al. 2006). A limitation in nitrogen uptake reduces vine vigour, berry weight and yield and increases berry sugar, anthocyanin and tannin content (Kliewer 1971, Choné et al. 2001a, Hilbert et al. 2003). Vine nitrogen status can be monitored through petiole, leaf blade or must analysis (Kliewer 1991, van Leeuwen et al. 2000a). Must total nitrogen or must yeast available nitrogen are practical and accurate indicators. Vine nitrogen uptake can be reduced through cover crop (Soyer et al. 1996) and increased through fertilisation. A limitation in water uptake also reduces vine vigour, berry weight and yield and increases berry anthocyanin and tanin content (Hardie and Considine 1976, Matthews and Anderson 1988,1989, van Leeuwen and Seguin 1994, Koundouras et al. 2006). The effect on berry sugar content is yield-dependant; in low yields, vine water deficit enhances berry sugar content and in high yields it depresses berry sugar content (Tregoat et al. 2002). Vine water status can be assessed through (i) soil water monitoring, by means of neutron moisture probes (Seguin 1986) or Time Domaine Reflectometry (Koundouras et al. 1999), (ii) water balance modelling (Lebon et al. 2003), or (iii) the use of physiological indicators (van Leeuwen et al. 2001a, Cifre et al. 2005). Among physiological indicators two are of particular interest: stem water potential (Choné et al. 2001b) and the 13C/12C ratio measured on grape sugar at ripeness (δ13C or carbon isotope discrimination, van Leeuwen et al. 2001b, Gaudillère et al. 2002). Vine water deficit can be increased by withholding irrigation water and increasing leaf area on a per hectare basis. Vine water deficit can be reduced by selecting drought resistant rootstocks (110Richter), by limiting leaf area, by limiting crop, by selecting drought resistant varieties (Grenache, Carignane) and by irrigation. Grape production can be made economically viable through i) high yields ii) high quality allowing high selling prices iii) low production costs. To obtain high yields, vines should be planted in fertile soils and irrigated if rainfall is limited during the growing season. High quality for red table wine production is obtained if vines are grown under environmental stress (van Leeuwen et al. 2004). When environmental stress is moderate, high quality can be combined with reasonably high yields. When environmental stress is severe, a low yield is necessary to obtain high quality. Low production costs can be obtained either by mechanisation or by low labour input vineyard design, such as unirrigated bush vines. In such vineyards, production cost per ton of grapes is low, even in low yields.