Construcción del indicador de capacidad fiscal

Plants are designed to function over a range of water availability, and several short and long term plant responses are exhibited over this range. Stress responses are known to occur as water availability decreases. Firstly, there is a reduction in growth rate and later stomata start to close and photosynthesis is reduced (Shackel, 2006).

Plants obtain water from the soil by the root system to re-supply water lost from the leaves. Therefore, plant status depends on the balance between absorption and losses of water by transpiration. Water absorption depends on roots distribution and on moisture content of the explored soil volume. The energy available for water vaporization, together with the resistances to water transport along the continuum soil-plant-atmosphere, determines the transpiration (Lopes, 1994).

Air relative humidity also influences plant water status, as the increase in vapour pressure deficit (VPD) between the leaf and the air leads to higher transpiration rates and if soil water content is low, water deficit in the plant may by induced (Düring, 1987; Soar et al. 2006a, b; Poni et

al., 2009).

Heat convective transport also affects water loss from the plant to the atmosphere by evaporation, particularly in moderate to strong wind conditions. As the wind gets stronger, the boundary layer resistance decreases and so the evaporative rate in the canopy increases.

Ana Fernandes de Oliveira - Deficit Irrigation Strategies in Grapevine (Vitis vinifera L). Ecophysiologic Responses,

Growth-Yield Balance, Canopy and Cluster Microclimate for Improving Quality under Mediterranean Climate Page 39 of 234 As a result, the transpiration rate of a single leaf varies greatly along the day and during the season.

The most common, practical and rapid way of monitoring plant water status is by measuring water potential using the pressure chamber method (Sholander et al., 1965). However, there is still some debate over which water potential should be monitored – stem or leaf – as well as over the ideal time of the day that best explains vine performance. Carbonneau (1998), Lopes et al. (1999) and Deloire et al. (2004) used predawn leaf water potential to evaluate vine water status at different developmental stages and obtained good correlations between Pn and Ψpd under stress conditions.

In Napa Valley, Williams and Araujo (2002) concluded that Ψpd, midday leaf water potential (Ψmid) and stem water potential (Ψstem) at noon were equally useful methods. In warmer

conditions though, Ψstem measured at midday and Ψmid seem to perform better than Ψpd

(Intrigliolo et al., 2005, Yuste et al., 2004). In fact, all plant water status measurements are correlated to one another, but some are correlated better to vine physiology. Local ambient conditions can have a greater impact on photosynthesis than soil water availability, as they exert major influence on leaf response.

Zufferey et al. (2000) reported a curvilinear relationship between Pn and Ψpd from -0.05 to –

0.6 MPa but for several other authors Pn responds linearly to Ψpd, from -0.1 to -0.8 MPa (Flexas et

al., 1999; Lopes, 1999, Baeza, 2007).

The best time of the day for monitoring leaf water potential depends not only on soil water content, but mostly upon vine performance at a given situation and crop purpose (Baeza, 2007). Vine response to water stress depends on both current situation and previous conditions and the intensity and duration of the stress affects long-term response. Several authors obtained good water stress indexes based on canopy temperature, ambient temperature and VPD to explain variations in yield or even correlations between soil matrix potential and vegetative growth and berry weight but these methods are time-consuming and expensive. The water stress integral (Sψ)

(Myers, 1988) may provide a good explanation of long-term response of vine to water deficit and represents the average water potential during a given period. Baeza et al. (2007) obtained a good correlation between predawn water stress integral and berry weight but no significant correlation was found between this index and must composition.

Ana Fernandes de Oliveira - Deficit Irrigation Strategies in Grapevine (Vitis vinifera L). Ecophysiologic Responses,

Growth-Yield Balance, Canopy and Cluster Microclimate for Improving Quality under Mediterranean Climate Page 40 of 234 In order to quantify growth, yield and berry composition relative dependence of the intensity and duration of water stress in field-grown conditions, Lopes et al. (2001) have studied the relationships between the integral of predawn leaf water potential below -0.2 MPa and the growth, yield and composition parameters in „Tempranillo‟ and found highly significant correlation between Sψ and yield components (berry weight and yield) independently of the period considered (from

bloom until harvest) and a significantly positive correlation of juice pH and the intensity of Sψ

experienced during ripening. Skin concentration of anthocyanins and phenolics also presented significantly positive dependence upon Sψ, from bloom until harvest, the highest correlation

coefficient being observed in the bloom-verasion period. Regarding vegetative growth parameters, these authors found high correlation coefficients between main leaf area at veraison and the percentage of leaf senescence at verasion, which was one of the most important responses of grapevine to water stress, influencing source size and cluster microclimate. The authors concluded that the negative dependence of berry weight and yield from Sψ during the period from bloom to verasion demonstrates the importance of early deficit on berry growth. At the same time, the higher contribution of the Sψ experienced prior to verasion, to explain the variations in

anthocyanins and phenolics concentration, indicates that the degree of water stress during that period plays an important role on berry colour development.

Modern irrigation management is shifting from an emphasis on production per unit of soil toward maximizing water productivity (production per unit of consumed water), considering not only the total seasonal water available but also the timing of water deficit occurrence to better adjust the supply to water resources (Jones, 2004a; Chaves et al., 2010). For this purpose, the deficit irrigation strategies (DI) can represent an important management technique. As mentioned before, DI strategies consist on applying water volumes below the actual crop evapotranspiration (ETc)

during the growing season or in specific phenological stages and thus are of great interest for enhancing yield, stabilising or even improving berry quality.