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JXB Advance Access originally published online on June 27, 2005
Journal of Experimental Botany 2005 56(418):2163-2172; doi:10.1093/jxb/eri216
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© The Author [2005]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved. For Permissions, please e-mail: journals.permissions@oupjournals.org

RESEARCH PAPER

Impact of deficit irrigation on water use efficiency and carbon isotope composition ({delta}13C) of field-grown grapevines under Mediterranean climate

Claudia R. de Souza1,3, João P. Maroco2, Tiago P. dos Santos3, M. Lucília Rodrigues3, Carlos M. Lopes3, João S. Pereira3 and M. Manuela Chaves1,3,*

1Laboratório de Ecofisiologia Molecular, Instituto de Tecnologia Química e Biológica, Avenida da República, Apartado 127, 2780-901 Oeiras, Portugal
2Instituto Superior de Psicologia Aplicada, Rua Jardim do Tabaco, 34, 1149-047 Lisboa, Portugal
3Instituto Superior de Agronomia, Tapada da Ajuda, 1349-017 Lisboa, Portugal

* To whom correspondence should be addressed. Fax: +351 213 6533415. E-mail: mchaves{at}isa.utl.pt

Received 15 July 2004; Accepted 29 April 2005


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
The objective of this study was to evaluate the effect of deficit irrigation on intrinsic water use efficiency (A/gs) and carbon isotope composition ({delta}13C) of two grapevine cultivars (Moscatel and Castelão), growing in a commercial vineyard in SW Portugal. The study was done in two consecutive years (2001 and 2002). The treatments were full irrigation (FI), corresponding to 100% of crop evapotranspiration (ETc), rain-fed (no irrigation, NI), and two types of deficit irrigation (50% ETc): (i) by supplying the water either to one side of the root system or to the other, which is partial rootzone drying (PRD), or (ii) dividing the same amount of water by the two sides of the root system, the normal deficit irrigation (DI). The water supplied to the PRD treatment alternated sides approximately every 15 d. The values of predawn leaf water potential ({Psi}pd) and the cumulative integral of {Psi}pd (S{Psi}) during the season were lower in 2001 than in the 2002 growing season. Whereas differences in {Psi}pd and S{Psi} between PRD and DI were not significantly different in 2001, in 2002 (a dryer year) both cultivars showed lower values of S{Psi} in the PRD treatment as compared with the DI treatment. This suggests that partial rootzone drying may have a positive effect on water use under dryer conditions, either as a result of better stomatal control and/or reduced vigour. The effects of the water treatments on {delta}13C were more pronounced in whole grape berries and pulp than in leaves. The {delta}13C of pulp showed the best correlation with intrinsic water use efficiency (A/gs) as well as with S{Psi}. In spite of the better water status observed in PRD compared with DI in the two cultivars in 2002, no statistical differences between the two treatments were observed in A/gs and {delta}13C. On the other hand, they showed a higher {delta}13C compared with FI. In conclusion, it is apparent that the response to deficit irrigation varies with the environmental conditions of the particular year, the driest conditions exacerbating the differences among treatments. The highest values of {delta}13C found in the pulp of NI vines in Castelão compared with Moscatel suggest different sensitivities to water deficits in the two cultivars, as was empirically observed.

Key words: Carbon isotope composition, deficit irrigation, grapevines, partial rootzone drying, water use efficiency


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 Conclusions
 References
 
Drought is one of the most important factors limiting crop yield and quality worldwide, especially in regions with a climate of the Mediterranean type. In many regions, viticulturists rely on irrigation water during drought periods. However, there is considerable controversy concerning the positive and negative effects of grapevine irrigation on growth as well as on must and wine quality. The amount of irrigation water has to be regulated since excess irrigation may contribute to increased vegetative growth leading to excessive water loss, fungal diseases, and shading of grape clusters. By contrast, a mild water stress imposed through deficit irrigation may reduce vine vigour and competition for carbohydrates by the growing tips, and may increase the berry and wine quality (Matthews and Anderson, 1988Go, 1989Go). Furthermore, the lack of available water resources in some areas is leading to a significant pressure on viticulturists to bring about improvements in the efficiency of water use as traditional irrigation practices are unsustainable.

Deliberate withholding of irrigation water may, therefore, be a useful management strategy to manipulate crop water use and this is embodied in the technique known as regulated deficit irrigation (RDI) (Boland et al., 1993Go; Alegre et al., 1999Go; Dry et al., 2001Go). In RDI, the control of growth is made by imposing water stress at key stages of fruit development. However, this technique requires the precise control of water application, which is difficult to achieve in practice, and the reduction in irrigation has often been at the expense of yield. An alternative approach to RDI is deficit irrigation (DI) that gives a constant, but partial, amount of transpired water throughout the growing season. A special kind of deficit irrigation is the so-called partial rootzone drying (PRD) where the same amount of water is only given to one side of the root system, while the remainder is left to dry; this was proposed as a way to standardize grapevine yield and quality (Loveys et al., 2000Go). PRD functioning relies on hormonal signals, possibly ABA, originated from the roots in response to the low soil water potential within the dry zone and transported to the leaves in the transpiration stream, leading to the reduction of growth and stomatal conductance (Loveys, 1984Go; Davies and Zhang, 1991Go; Loveys et al., 2000Go; Souza et al., 2003Go). Partial stomatal closure caused by roots signalling may lead to a decrease in transpiration and, possibly, to an increase in water use efficiency (WUE) (During et al., 1997Go; Turner, 1997Go; Loveys et al., 2000Go).

In previous work by this laboratory, when the amount of irrigation was halved in both PRD and DI, as compared to full irrigation (FI), differences between the two deficit irrigation treatments were more pronounced in growth than in gas exchange, implying that the regulation of stomatal conductance in PRD was very subtle (dos Santos et al., 2003Go; Souza et al., 2005Go). In contrast to gas exchange techniques that provide measurements of photosynthesis rates at a single point in time, leaf carbon isotopic composition ({delta}13C) integrates the ratio of intercellular (pi) to air CO2 concentration (pa) for longer periods. The basis of the biochemical discrimination against 13C in C3 plants lies with the primary carboxylating enzyme, ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) which discriminates against 13C because of the intrinsically lower reactivity of 13C compared with 12C (Farquhar et al., 1982Go; Brugnoli and Farquhar, 2000Go). Thus the isotopic composition reflects the effect of the plant water status on photosynthesis throughout the growing season (Farquhar and Richards, 1984Go). In the present study it was used to evaluate the intrinsic plant water use efficiency as it was for many plant species in response to different environmental conditions (Zhang et al., 1994Go; Le Roux et al., 1996Go; Osório et al., 1998Go; Martin et al., 1999Go; Robinson et al., 2000Go; Araus et al., 2003Go), including grapevines (Gaudillère et al., 2002Go).

Although the whole-plant {delta}13C is dominated by CO2 assimilation and diffusion into leaves, internal partitioning and metabolism of primary assimilates may produce differences in {delta}13C among plant organs (Leavitt and Long, 1985Go; Gleixner et al., 1993Go; Le Roux-Swarthout et al., 2001Go; Ghashghaie et al., 2001Go; Brugnoli and Farquhar, 2000Go). This influence is particularly important in deciduous woody species, such as grapevine, where stored organic compounds are the dominant carbon sources for leaf growth in the early spring. In such instances, {delta}13C of leaf tissue not only represents pi/pa and water use efficiency of the growing season, but also reflects the previous year's carbon assimilation and allocation. For grapevines, sugar {delta}13C in the grape berries can be used to characterize soil water availability in the vineyard, since the sugars of mature berries integrate leaf photosynthetic isotopic discrimination of carbon during berry ripening. (Gaudillère et al., 2002Go). However, the effect of grapevine water status on fractionation of {delta}13C in different organs/tissues of the plant has not been studied.

The aim of this study was, therefore, to evaluate grapevine water use efficiency when subjected to different water regimes, namely the two deficit irrigations treatments with and without partial rootzone drying, compared with fully irrigated and rain-fed grapevines. Water use efficiency, evaluated through the measurements of {delta}13C, provides an estimate of stomatal closure integrated over time that will help to understand differences in stomatal conductance and/or sink/source balance between treatments that are not easily perceived with point measurements. Furthermore, the effects of watering treatments on the fractionation of 13C were explored in different tissues (leaves and in different parts of the berries) and in two grapevine varieties. The inter-annual variability was evaluated in one of the varieties.


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 Conclusions
 References
 
Location and irrigation treatments
The experiments were conducted in a commercial vineyard located 70 km east of Lisbon, at the Centro Experimental de Pegões, during 2001 and 2002. The climate is of the Mediterranean type with hot, dry summers and mild winters. The two years were quite different regarding the amount of precipitation, especially from January to May, but similar in respect to the near absence of rain in July and August (Fig. 1). The soil is derived from podzols, with a sandy surface layer (0.6–1.0 m) and clay at 1 m depth. The two varieties of Vitis vinifera L. studied were Moscatel (syn. Muscat of Alexandria), a vigorous and productive white variety used for wine and table grapes, and Castelão, an early season red wine variety (dos Santos et al., 2003Go). Both were grafted on 1103 Paulsen rootstock in 1997 and 1996, respectively. The grapevines were spaced 2.5 m between rows and 1.0 m within rows and trained on a vertical trellis with a pair of movable foliage wires for upwards shoot positioning. The vines were spur-pruned on a bilateral Royat Cordon (~16 buds per vine).



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  		Fig. 1. Seasonal distribution of rainfall and predawn water potential ({Psi}pd) measured in different water treatments in Moscatel (2001 and 2002) and in Castelão (2002). The water treatments are NI (squares), PRD (circles), DI (diamonds), and FI (triangles). Values of {Psi}pd are means ±SE.

 
Irrigation water was applied with drip emitters (4.0 l h–1 for FI and PRD and 2.0 l h–1 for DI), two per vine, positioned 30 cm from the vine trunk (out to both sides of the rows), and distributed to both sides of the root system. The water was supplied according to the crop evapotranspiration (ETc) calculated from the evaporation of a Class A pan and corrected with the crop coefficients (Kc) proposed by Prichard (1992)Go. The irrigation treatments were rain-fed, non-irrigated (NI); partial rootzone drying (PRD, 50% of the ETc was supplied to one side of the root system only, alternating sides approximately each 15 d); deficit irrigation (DI, 50% of the ETc was supplied to both sides of the vine, 25% to each side); full irrigation (FI, 100% of the ETc was supplied to both the sides of the root system, 50% in each side). Water was supplied twice per week from the beginning of berry development (June) until harvest (September). Cumulative rainfall during the experimental period (mid-June until the end of August) was 5.2 mm in 2001 and 0.5 mm in the 2002 growing season. The total amount of water supplied to FI plants was 168 mm (420 l per vine) and 197 mm (493 l per vine) for 2001 and 2002, respectively. The PRD and DI vines received half of that quantity.

Water relations and gas exchange
Predawn ({Psi}pd) leaf water potential was measured weekly with a Scholander-type pressure chamber (Model 1000; PMS Instrument Co., Corvallis, OR, USA), from the beginning of berry development until harvest. The measurements were taken on six fully expanded leaves per treatment on five dates from June to August (Fig. 1), just prior to irrigation. The {Psi}pd was used to calculate a water stress integral (S{Psi}) as proposed by Meyers (1988)Go. The S{Psi} expresses the stress intensity experienced by each treatment by integrating the duration of water status below a maximum value of {Psi}pd. It was estimated from t measurements of {Psi}pd at intervals of n days by means the formula:

where {Psi}pd i, i+1 is the mean {Psi}pd for any interval i, i+1, and c is the maximum {Psi}pd measured during the study. In this case, for the c value –0.13 MPa was used as the maximum {Psi}pd value measured in the FI treatment.

Net CO2 assimilation rate (A) and stomatal conductance (gs) were measured at midday on sun-exposed fully mature leaves (from primary shoots) using a portable Li-6400 IRGA (Li-Cor, Lincoln, Nebraska, USA). All measurements were replicated 4–8 times. Measurements were taken at 2 week intervals from the beginning of berry development until harvest. The A and gs values were used to calculate the instantaneous intrinsic water use efficiency (A/gs). The seasonally integrated values of A/gs were obtained by calculating the area of the curve of A/gs seasonal evolution over time for each year and cultivar.

Carbon isotope composition
Samples for the determination of carbon isotope composition from mature leaves were collected from six plants per treatment at harvest. The leaves were harvested from primary and lateral shoots (primary and lateral leaves, respectively). The lateral leaves were previously selected at the beginning of their growth during the veraison period and only collected at harvest time. The objective was to investigate if the {delta}13C of new leaves formed during the highest water stress period could be more affected by water treatments than primary leaves formed at the beginning of the season. Berry samples consisted of 30 berries per replicate (six replicates per treatment) taken randomly from exposed clusters. Berry samples were divided in whole berries, pulp, skin, and seeds. The skins were peeled from frozen berries to avoid mixing with pulp. The dried leaves and berry samples were ground into a fine homogeneous powder and 1 mg subsamples were analysed for {delta}13C using a Europa Scientific ANCA-SL Stable Isotope Analysis System (Europa Scientific Ltd. Crewe UK). Carbon isotopic composition was expressed as

where Rs is the ratio 13C/12C of the sample and Rb is the 13C/12C of the PDB (Pee Dee Belemnite) standard.

Vegetative growth
Leaf area per shoot (8 shoots per treatment) was assessed periodically in count shoots from bud break onwards in a non-destructive way, using the methodologies proposed by Lopes and Pinto (2000)Go. Primary leaf area was estimated using a mathematical model with four variables: shoot length, leaf number, and area of the largest and the smallest leaf. Lateral leaf area estimation was done by another model that uses the same variables with the exception of lateral shoot length. The area of single leaves was estimated using an empirical model based on the relationship between the length of the two main lateral leaf veins and leaf area on 1645 leaves of all sizes, using a leaf area meter (LI-3000; Li-Cor, Lincoln, Nebraska, USA). Leaf area per plant was calculated by multiplying the leaf area average by the mean shoot number.

Statistical analyses
Three-factorial analyses of variance (ANOVA), with year, sampling time, and treatments as the main factors, were used to test the main effects and factor interactions on {Psi}pd and A/gs. Two-factorial ANOVA with year and treatments was used to test the main effects and factor interaction on S{Psi}, SA/gs, and {delta}13C. Statistically significant differences between factor groups were evaluated with Tukey's HSD for {alpha}=0.05 using the ‘Statistica’ software (version 5.0 StatSoft, Tulsa, OK, USA). All measurements shown are the means ±SE. Linear regressions were obtained using Sigma Plot software (version 7.0, SPSS Science, Chicago).


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 Conclusions
 References
 
Environmental conditions, plant water status, and vegetative growth
There were large differences in rainfall distribution during the growing seasons of 2001 and 2002 (Fig. 1). The higher rainfall in spring and summer (248 mm) in 2001 led to higher values of predawn water potential ({Psi}pd) for all watering regimes, compared with 2002 (Fig. 1). In Moscatel, a significant interaction was observed for {Psi}pd between years, sampling date, and treatments (P <0.05). In both years, FI vines showed almost constant {Psi}pd throughout the growing season, c. –0.13 MPa in 2001 and slightly lower in 2002 (–0.2 MPa). By contrast, NI vines showed a progressive decline in {Psi}pd from July onwards, with lower values observed in 2002 compared with 2001. The two deficit irrigation treatments (PRD and DI) had {Psi}pd values intermediate between FI and NI, but generally closer to FI. In Castelão, measured only in 2002, a similar pattern among treatments was observed. However, in this cultivar, {Psi}pd of PRD was significantly higher than DI vines (Fig. 1) and the {Psi}pd of NI vines at the harvest time reached values (c. –0.8 MPa) which were lower than those of NI in Moscatel (c. –0.6 MPa).

The intensity and duration of water stress, as indicated by the cumulative integral of predawn leaf water potential (S{Psi}, MPa d–1), was significantly different among watering treatments in cv. Moscatel (Table 1). In both years, NI vines showed the highest values of S{Psi} compared with other treatments. The values of S{Psi} in all treatments were lower in 2001 than in 2002, and there was a significant interaction between year and treatment (Table 1). However, only in the driest year, 2002, was there a significant difference between PRD and DI, with PRD showing a lower S{Psi}. A similar response of S{Psi} to the treatments was also observed in Castelão.


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  		Table 1. Integral water stress (S{Psi}) of different water treatments in Moscatel during the 2001 and 2002 growing seasons and in Castelão during 2002

 
NI and PRD vines presented the lowest pruning weight per vine, being significantly different from the FI and DI ones in Castelão, in 2002 (Table 2). The number of water shoots was significantly lower in NI and PRD compared with FI and DI, in both the two years and the two cultivars. Total leaf area at veraison presented significantly higher values (P <0.05) in FI than in NI, whereas PRD and DI vines showed values not significantly different from those of NI and FI, respectively (Table 2). The differences of total leaf area observed between treatments were mainly due to differences in the lateral shoot leaf area, as primary shoot leaf area was similar in the different watering treatments.


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  		Table 2. Growth parameters of different water treatments measured at veraison (leaf area, leaf layer number) or at pruning time (the others) in Moscatel during 2001 and 2002 and in Castelão during 2002

 
Intrinsic water use efficiency
The diurnal courses of intrinsic water use efficiency (A/gs) measured in August are shown in Fig. 2. The ANOVA did not reveal a significant interaction between treatment, years and sampling time in A/gs. The statistical analysis showed significant effects of water treatments in 2002, in both cultivars. In Moscatel, there was a trend for higher A/gs in NI and PRD compared with FI vines. However, there were no significant differences compared with DI vines. In Castelão, NI showed the highest values in A/gs throughout the day. At midday, PRD showed values closer to NI vines compared with DI and FI vines. The higher A/gs ratio in NI, and in some cases in PRD and DI, compared with FI, is attributed to a larger reduction in stomatal conductance (gs) than in photosynthesis (A), mainly at the end of the day, as shown in Fig. 3.



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  		Fig. 2. Diurnal course of intrinsic water use efficiency (A/gs) in Moscatel in 2001 and 2002 and Castelão in 2002. The water treatments are NI (squares), PRD (circles), DI (diamonds), and FI (triangles). The measurements were done in August. Values are means ±SE.

 


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  		Fig. 3. The effects of different water treatments on photosynthesis (A) and stomatal conductance (gs) in Moscatel during 2001 and 2002 and Castelão, during 2002. The values are means ±SE and the data are expressed as% of Full-irrigated treatment (FI). The measurements were made in August at the beginning and at the end of the day.

 
The integrated A/gs (SA/gs) at midday throughout the growing period, estimated by the area of the graphic of the seasonal evolution of A/gs measured at midday, showed a similar pattern in both years and cultivars (Fig. 4), with a decline from NI to FI. In both cultivars, the SA/gs values of PRD and DI exhibited intermediate values between NI and FI vines. However, there were no statistically significant differences between the irrigated treatments (PRD, DI, and FI).



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  		Fig. 4. Integrated intrinsic water use efficiency (SA/gs) in Moscatel during 2001 and 2002, and Castelão during 2002 under different water treatments (NI, PRD, DI, and FI). Values are means ±SE. Different letters suffixes show statistically significant differences for P <0.05.

 
Carbon isotopic composition ({delta}13C)
Figure 5 shows the effects of the watering treatments on {delta}13C values in leaves (primary and lateral leaves), whole grape, pulp, skin, and seeds in Moscatel and Castelão varieties. The tissues of NI plants were less depleted in 13C (higher {delta}13C, lowest discrimination against 13C) than the other treatments, and FI vines showed the lowest {delta}13C (higher discrimination against 13C).



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  		Fig. 5. Carbon isotope composition in different parts of the vines subjected to different water treatments (NI, PRD, DI, and FI). The water treatments are NI (squares), PRD (circles), DI (diamonds), and FI (triangles). Value are means ±SE.

 
In the leaves of Moscatel, the only significant differences in {delta}13C were observed in lateral leaves between the NI and FI treatments. In whole berries and pulp, there were also significant differences in {delta}13C as revealed by ANOVA after accounting for the effects of year. In general, NI vines showed the highest values of {delta}13C while FI had the lowest, with PRD and DI providing intermediate values. However, the differences between DI and FI in {delta}13C of whole berries were not statistically significant. There was a significant interaction between treatment and year for {delta}13C in skin berries, with a significant difference between NI and irrigated treatments only observed in 2002. There was no significant difference among water treatments in {delta}13C of seeds in cv. Moscatel.

The effect of watering treatments in {delta}13C was more pronounced in Castelão than in Moscatel. In Castelão, the {delta}13C of pulp (the lowest discrimination) ranged from –23{per thousand} to –26{per thousand} in NI and FI, respectively, whereas in Moscatel ranged from –24{per thousand} to –26{per thousand} in the same treatments. The differences among treatments were similar in different parts of the plant. NI vines showed the highest {delta}13C values, PRD and DI, the intermediate, and FI the lowest values. However, discrimination values of lateral leaves in the PRD treatment were similar to the ones measured on primary leaves of the NI treatment.

In general, the water treatments showed a similar pattern of {delta}13C throughout the different parts of the plants, in both years and cultivars. There was a substantial enrichment of 13C in whole berries and pulp relative to leaves, skin, and seeds. The lowest 13C occurred in lateral leaves, mainly in Castelão, where values reached –30{per thousand} in FI vines. The highest values of {delta}13C were shown in pulp tissues of NI vines in Castelão compared with those of Moscatel.

Significant correlations were found between {delta}13C of the whole grape berries and of the pulp and S{Psi} considering all years and cultivars (Fig. 6). However, the {delta}13C and S{Psi} association were clearly higher in the pulp as shown by the determination coefficient (R2=0.60).



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  		Fig. 6. Relationship of {delta}13C in leaves (a), whole berries (b) (y= –25.89+0.05x), and pulp (c) (y= –27.73+0.06x) with water-stress integral (S{Psi}). Values are means ±SE. Both years and cultivars are mixed. The water treatments are NI (squares), PRD (circles), DI (diamonds), and FI (triangles). Moscatel is represented by open symbols in 2001, symbols with crosses inside in 2002 and Castelão in 2002 is represented by closed symbols.

 
The water use efficiency was also better correlated using {delta}13C of the pulp than with whole berries or leaves. There was an enrichment of 13C in pulp tissues with an increase in the A/gs ratio measured in both cultivars during the 2002 growing season (Fig. 7). The same behaviour was observed when SA/gs was used as an integrated parameter to correlate with {delta}13C. The {delta}13C of the pulp showed the best determination coefficient with SA/gs (R2=0.74) compared with {delta}13C of whole grapes (R2=0.62) or leaves (R2=0.17) when both cultivars and years are considered (Fig. 8).



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  		Fig. 7. Relationship of {delta}13C in leaves (a) (y= –27.81+0.02x) and pulp (b) (y= –27.42+0.03x) of Moscatel and leaves (c) (y= –30.30+0.02x) and pulp (d) (y= –28.47+0.04x) of Castelão with intrinsic water use efficiency (A/gs). Each point represents one replicate of the water treatments. The measurements of A/gs were made in August 2002.

 


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  		Fig. 8. Relationship of {delta}13C in leaves (a) (y= –28.96+0.001x), whole berries (b) (y= –29.16+0.001x), and pulp (c) (y= –29.83+0.001x) with integral of intrinsic water use efficiency (SA/gs). Values are means ±SE. Both years and cultivars are mixed. The water treatments are NI (squares), PRD (circles), DI (diamonds), and FI (triangles). Moscatel is represented by opened symbols in 2001, symbols with crosses inside in 2002 and Castelão in 2002 is represented by closed symbols.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
Conclusions
 References
 
Vines subjected to deficit irrigation practices such as PRD and DI, revealed an intermediate degree of water stress between NI and FI vines, as shown by S{Psi} calculated from {Psi}pd values throughout the growing season (Table 1). The cumulative integral of predawn leaf water potential was shown to be a useful parameter to estimate the degree of water stress in different crops (Meyers, 1988Go) including grapevine (Ginestar, 1998aGo, bGo). In 2002, the driest year, PRD vines were significantly less stressed than DI in both varieties, suggesting a higher stomatal closure in PRD vines. The tendency for a lower leaf area and vigour of PRD compared with DI (Table 2) could also contribute to the explanation of the highest values of predawn leaf water potential observed in PRD vines (dos Santos et al., 2003Go).

Soil water availability affected carbon isotopic composition of the tissues, which reflects the seasonal transpiration efficiency, i.e. the ratio of net photosynthesis to water transpired over the growing season (Farquhar and Richards, 1984Go). For both deficit irrigation treatments (PRD and DI), there was an increase in water use efficiency, as estimated by {delta}13C, in both cultivars and all tissues studied, when compared with full-irrigated treatment FI. However, in most instances, the intrinsic water use efficiency (A/gs) was not significantly different between PRD and DI. In addition, in the same experiment, these irrigation treatments did not show statistically significant differences in water use efficiency estimated as the ratio between yield and the amount of water supplied (data not shown). This suggests that a lower vigour/leaf area, rather than differences in gs, was the key factor for a better plant water status in PRD than in DI.

The carbon isotope composition was different in leaves, berries, and seeds, as shown in Fig. 5. There was a relative enrichment of 13C in berries compared with leaves and the {delta}13C of pulp was better correlated with S{Psi} (Fig. 6) and intrinsic water use efficiency (both A/gs and SA/gs) (Figs 7, 8) than was the {delta}13C of the whole grape, seeds, or leaves. These data indicate that 13C composition of the berry pulp can be as good an indicator as berry sugar {delta}13C in integrating water use by grapevines during the period of berry growth and development, as previously described by Gaudillère et al. (2002)Go.

The highest {delta}13C values in berries were probably due to the lower discrimination of 13C in sugar and organic acids present in the pulp. These were imported from leaves after veraison (Di Marco et al., 1977Go), when stress was more severe and stomata were more closed, thus providing photosynthates with higher 13C. Differences among plant parts can also be attributed to differences in lipid composition, fractionation processes during transport, and/or synthesis of metabolites, contributing to changes in the 13C signature of different metabolites and organs (Brugnoli and Farquhar, 2000Go). On the other hand, fractionation during respiration could contribute to the impoverishment of the leaf tissue in 13C. Several authors have shown that discrimination during dark respiration processes can occur, releasing CO2 enriched in 13C compared with several major leaf reserves and whole leaf organic matter (Duranceau et al., 1999Go, 2001Go; Ghashghaie et al., 2001Go; Tcherkez et al., 2003Go).

The tissues of lateral leaves, formed during the period of high water stress, were slightly more depleted in 13C than primary leaves, in both cultivars. Another explanation is that the higher photosynthetic activity in lateral leaves compared with primary shoots during the later stages of the growing season (Shultz et al., 1996Go) might have contributed to the highest discrimination in lateral leaves.

Although there is a linear association between {delta}13C of berries and S{Psi}, other factors, including cultivar sensitivity to water stress, could contribute to the low determination coefficient (Fig. 6). The cultivar Moscatel has been considered to be well adapted to water-stress conditions as shown by Regina and Carbonneau (1996)Go. In this study, it was observed that carbon assimilation and stomatal conductance in Moscatel can be maintained for a large range of predawn leaf water potentials, whereas in Castelão the sensitivity of stomata to water deficits was higher (Souza et al., 2005Go). The highest values of {delta}13C found in the pulp of NI vines confirm that water stress was higher in Castelão as compared to Moscatel (Fig. 5).


   Conclusions
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
References
 
The results show that deficit irrigation treatments promoted an increase in water use efficiency as compared with full irrigation, either in the short-term, as expressed by the A/gs ratio or in the long-term shown by the increase in 13C in the plant tissues, especially the berries. It was also apparent that the response to deficit irrigation varied with the grapevine variety and with the environmental conditions of the particular year, differences between treatments being more marked under drier conditions. In a drier year (2002), PRD induced higher leaf water potentials than DI. This resulted from reduced leaf area and higher midday stomatal closure in PRD than in DI. Nevertheless, no statistically significant differences in WUE between the two treatments were found, as shown by gas exchange and the isotope carbon composition of grape berries. This suggests that stomatal closure in PRD plants had only a marginal effect on plant water status compared with growth reduction. The good correlation observed between {delta}13C in berry pulp and S{Psi} or SA/gs indicates that carbon isotope composition of this particular tissue may be a valuable index for the evaluation of plant water availability during the growing season. Under Mediterranean climates, where severe water deficits are likely to occur by the end of the growing season due to restricted summer rainfall, tissue {delta}13C may be highly correlated with the amount of water given by irrigation.


   Acknowledgements
 
The financial support from EU project IRRISPLIT (IAC3-CT-1999-00008) is acknowledged. CRS was a recipient of a doctoral fellowship granted by Fundação para Ciência e Tecnologia (PRAXIS XXI/BD/21856/99). We also thank A Rodrigues and E Breia for technical support in the laboratory and field experiments.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 Conclusions
 References
 
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