Implicaciones de algunos problemas técnicos del modelo

Most studies on grapevine irrigation have demonstrated that water deficits affect vegetative growth to a greater degree than they affect reproductive growth (Williams and Matthews, 1990; Lopes, 1994; Williams, 2000). For example, studying the effect of various amounts of applied water on vine productivity and vegetative growth in cv. Thompson Seedless, Williams (1990) observed that, as applied water decreased from 100% to 80% to 60% of vineyard evapotranspiration (ET), pruning weights decreased 15% and 39% for the latter two irrigation treatments, respectively,

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 42 of 234 compared to the 100% treatment. In fact, shoot growth is very sensitive to water deficit, responding rapidly to a decrease in water potential. Baeza et al. (2007) observed a 96% decrease in shoot growth in Cabernet Sauvignon grapevines when Ψleaf decreased -1.0 to -1.1 MPa, while Pn

was reduced only 13.5%, and in Riesling, Schultz and Matthews (1988) observed that shoot growth ceased at Ψleaf ≤ -1.2 MPa.

Moderate water deficits usually decrease the rate of shoot elongation, along with inter-node length and radial expansion, but when water stress is severe or when it is onset too rapidly, the stress can kill the shoot tip.

Under water deficit conditions, less leaf area per vine is formed, due to reduced shoot length and to smaller leaves. Frequently, the growth of lateral shoots is reduced more by water stress than that of primary shoots. In fact, Lopes (1994) observed that higher leaf area per shoot was formed in irrigated plants, mainly due to lateral leaf area growth. This may be because, from bud-break until bloom/fruit set stages, soil water content is normally enough, since the rain events during the dormancy period have refilled soil water resources. Water deficits do not develop until later in the growing season, after the primary shoots have already grow considerable, while most lateral shoots have only initiated their growth (Williams, 2000).

In an irrigation trail where the irrigation treatments consisted on applying water amounts at various fractions of lysimeter water use (from 20 to 140% of ETc), Williams et al. (2010) also observed higher leaf area formation in deficit irrigated „Thompson Seedless‟ plants, as water applied increased. The authors suggested that it could have been due to the high frequency with which the vines were irrigated.

Schultz and Matthews (1988) have demonstrated that there is a linear reduction in the growth rate of leaves and shoot apices when tissue Ψ decreases from -0.4 to -1.0 MPa and that growth of leaves, internodes and tendrils can be reduced 100, 60 and 50% at a tissue Ψ of -1.0 MPa with a complete inhibition of internode growth at a tissue Ψ of -1.2 MPa. Nevertheless, Williams et

al. (2010) observed tha,t prior to May, shoot lengths of the 20%ETc irrigation treatment were

already significantly less than those of the 100% ETc irrigation treatment despite the fact that midday Ψleaf at that time was higher than -1.0 MPa (approximately -0.7 MPa). Theses authors

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 43 of 234 signals, originating in the roots (abscisic acid, ABA), proved to be responsible for shoot growth inhibition (Dry and Loveys 1999; Dry et al. 2000).

Furthermore, in very dry years, lack of precipitation may lead to lower soil water budget during the first development stages, which may be responsible for an irregular budbreak, lower shoot growth rate and vigour. In such situations yield, berry ripening and carbohydrate storage in the permanent structures might be limited due to lacking leaf area per vine (Rodrigues, 2011; Gómez-del-Campo et al., 2007). However, Kennedy et al. (2002) reported that when applied during the first month after anthesis, water stress may benefit vigour control and increase bud fruitfulness. In the studies of Williams et al. (2010), deficit irrigation advanced the date of budbreak in Tompson Seedless grapevines as compared to the 100% ETc and 140% ETc irrigated vines, even though year effect had affected the absolute number of days separating treatments. However, these authors also reported that a 7 to 10 days of difference in budbreak date did not result in comparable differences in anthesis and/or veraison date.

Vineyard water stress also affects the amount of carbon partitioned to the permanent structures of the vine but few studies have examined the effects of water deficits on grapevine permanent structures growth. Mullins et al. (1992) observed that root, trunk, and cordon biomass was reduced 31%, 17%, and 26%, respectively, for vines irrigated at 52% of vineyard ET compared to those at 100% ET after 5 years. In this study, the concentration of non-structural carbohydrates in those organs differed only slightly for the two treatments. Total carbohydrate content in those organs decreased on a per vine basis as a result of reduced growth brought about by water stress. In such situations, the concentration of storage sugars in the root system and trunk were not affected by water deficits, but since biomass is reduced, total sugars was less in water-stressed vines than in non-stressed vines (Williams, 2000).

Although at ripening berry growth is not so sensitive to water stress as main shoot growing stages, different ranges of berry size have been obtained varying irrigation supply (Baeza et al., 2007; Thomas et al., 2006).

Several authors have studied the effects of irrigation strategy on vegetative growth. Excessive water is known to stimulate shoot growth and to lead to the formation of a wide and dense canopy with low fruit exposure. Consequently, an increase in fruit diseases incidence on leaves and fruits occur and berry quality can be compromised (Dokoozlian and Kliewer, 1996;

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 44 of 234 Keller and Hrazdina, 1998). Lateral shoot growth is particularly promoted by excessive irrigation, and these young shoots increase shading and compete with berries for the photosynthates, thus delaying ripening (Smart, 1985).

On the contrary, when early water deficit limits canopy expansion, restringing shoot elongation and leaf development for long periods of time, yield and berry quality may be affected as well, both by a photosynthetic limitation and scarcity of sugar production (Matthews et al., 1987). Narrow canopies, particularly in hot regions cause excessive berry exposure and an important thermal stress, with consequent reduction on cell division, berry dehydration and shrinkage (Santos

et al., 2007; Chaves et al., 2007).

After veraison, severe deficit conditions usually result in high early basal leaf senescence, and at this stage the thermal stress caused by high light intensities on the fruit zone is known to have negative effects on berry quality, namely on colour and aroma compounds accumulation (Smart and Robinson, 1991; Spayd et al., 2002).

RDI strategies are frequently used to control vegetative growth and berry size by applying a short period of water deficit after fruit set (McCarthy et al., 2002; Keller, 2005). However, Keller et

al. (2008) observed that irrigation treatments that imposed moderate to severe soil and plant water

deficit (30 to 60% of ETc, supplied with early RDI and late RDI, i.e. pre and post veraison,

respectively) decreased vegetative growth only marginally beyond the level achieved with the standard deficit treatment (designed to replenish 70% ETc from fruit set until harvest).

In order to manipulate wine grape quality, water stress may also be imposed after veraison, which may help to enhance anthocyanin accumulation in specific climate conditions (Matthews et

al., 1987; Dry et al., 2001; Kennedy et al., 2002; Santesteban et al., 2007; Santos et al., 2007). Yet, the

success of RDI strategies requires technical expertise on monitoring of plant water status and the detection of the leaf water potential at which re-watering should be done.

Many authors reported that PRD plants exhibit less shoot growth, lateral leaf and even main leaf area than DI vines irrigated with the same amount of water applied (Dry and Loveys, 1998; Stoll et al., 2000; Santos et al., 2007). The effectiveness of PRD depends clearly on the frequency of switching wet and dry root sides, which is determined according to soil type, rainfall, temperature and evaporative demand (Dodd et al., 2006; Soar et al., 2006b; Chaves et al., 2010). In grapevine,

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 45 of 234 PRD cycles of most of the published data were around 10-15 days (Davies et al., 2000; Santos et al., 2003).

Along with the physiological properties of the roots, root distribution and density are also critical on water uptake rates. Similarly, the relative proportion of shoot to root biomass can contribute to plant water status. Vigorous canopies on vines with small root system are likely to experience more water stress if the rate of uptake does not meet transpiration demands (Dry and Loveys, 1998).

Moreover, the rootstocks used in viticulture are known to confer multiple desirable attributes such as tolerance to salinity, nematodes and drought but there is little information on the specific mechanisms underlying drought tolerance of rootstock/scion combinations. Studying the differences in the physiological performances, growth, gas exchanges and water relations of different Shiraz/rootstock combinations in field grown grapevines, Soar et al. (2006a) concluded that the differences in vine performances between rootstocks resulted from different degrees of leaf water stress, which were most likely due to differences in their capacity to extract and supply water to the shoots. The higher concentration on ABA sap of some of the scions does not seem to be connected to higher capacity of synthesising it in the rootstock, but rather to a greater degree of water stress. The correlation between relative ABA concentration and low water use (gs or E) may

nevertheless be a useful method to compare drought tolerance of rootstocks.

There is also some debate over the success of PRD under field conditions (Chalmers et al., 2004; Bravdo, 2005). According to Bravdo (2005) it is impossible to have an absolute control of root drying in the field, and the clear results obtained in potted vines are difficult to achieve due to hydraulic redistribution of water from deeper layers to shallower roots. Nevertheless, Santos et al. (2007) observed an alteration in the root distribution in the soil profile in „Moscatel‟ plants with root systems totally exposed (NI) or partially (PRD) to soil water depletion, expressed by the increased increment of the root biomass in the deeper soil layers. Conversely, in that study full irrigated and DI 50 plants showed a homogeneous root mass density in the different layers of the soil profile.

Besides the root system, leaf characteristics vary greatly among Vitis vinifera varieties, with extensive differences in terms of morphology, architecture, hairiness and venation. These characteristics can influence plants water storage capacity but, until now, no clear correlation was

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 46 of 234 observed between leaf parameters, as for instance specific leaf area and water use efficiency in grapevine (Rogiers et al., 2009).

Irrigation is also an important cultural practice affecting bud fruitfulness in grapevine. In fact, the irrigation practice used during the previous growing season can affect actual season‟s yield by influencing bud fruitfulness and viability. Severe water stress may also reduce flower primordia induction during shoot development (Buttrose, 1974; Chalmers, 1981) but for the soil water contents usually found in the field in this period, flower differentiation in grapevine is not significantly affected (Freeman et al., 1979). For a given variety and terroir it appears to exist an optimum irrigation amount that maximizes cluster differentiation or potential fruitfulness for the following year (Williams, 2000). Keller et al. (2008) observed that neither RDI strategies nor crop- load adjustment, set in „Cabernet Sauvignon‟ over a five-year study period, ever impacted flower numbers, which implies that neither treatment had a pronounced immediate (same season) or carry-over (following season) effect on cluster differentiation and, consequently, inflorescence size. However, these authors emphasized that withholding water supply too early (before fruit set) may reduce the vines‟ yield potential.

The mechanisms by which irrigation management affects cluster differentiation may be indirect. As noted, light environment around the bud affects cluster differentiation, greater amounts of light favour clusters formation over tendrils. An excessive irrigation produces large, dense canopies, which are not conducive to cluster differentiation, while DI strategies resulting in less dense canopies, allow light to reach the developing buds.

The effects of soil water content on flower development and flowering process are not easy to assess as they are difficult to separate from those of temperature and light intensity (Lopes, 1994). Nevertheless, it is known that severe water stress during flowering and fruit set can result in lack of flower retention, poor fruit-set and fruit abortion, both leading to yield reductions (Chalmers, 1981).