Thermal Microclimate

The thermal conditions and energetic requirements during vegetative growth period, berry development and ripening stages determine the intensity of physiologic activity in grapevine, namely as far as growth rate and photosynthesis are concerned. Temperature exerts also a marked influence on berry ripening dynamics, thus determining, to a great extent, must and wine composition (Champagnol, 1984; Fernaud et al., 2001; Zufferey and Murisier, 2002).

Photosynthetic activity experiences a stimulating effect from temperatures between 10 and 25°C and over 30°C there is a depressive effect of temperature over photosynthesis, with progressive inhibition of enzymatic activity (Carbonneau et al., 1992; Lissarrague et al., 1999).

From the beginning of flowering until veraison, the optimum temperature of adult leaves for maximum Pn, varies between 27.5 and 30°C (Baeza, 1994). Generally, in growing leaves the maximum Pn is reached for optimum values slightly lower (of 1 to 1.5°C) compared to those of the adult ones. These temperature values can decrease even 6 to 7°C during the season (Zufferey and Murisier, 2000), depending on the varieties and growing conditions (Rogiers et al., 2009).

In the field, berry temperature evolves mainly in response to the radiation directly absorbed and convective heat losses, increasing linearly over air temperature in well exposed berries with increasing incident radiation but considerably less in interior shaded berries (Smart, 1976; Zufferey and Murisier, 2002).

As a matter of fact, several authors have reported that clusters exposure to sunlight varies drastically with cluster location in the canopy (Smart, 1985; Dokoozlian, 1990; Bergqvist et al., 2001; Santos et al., 2007; Chorti et al., 2010). In cv. Thompson seedless, Kliewer and Lider (1968) observed that, in directly exposed berries, temperature was 3 to 8°C higher compared to the shaded ones. Also Berqvist et al. (2001), working with cvs. Cabernet Sauvignon and Grenache, found that

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 53 of 234 at midday berries had 3 to 4°C less on the north side of the canopy than those in the south side. In this study, the fully exposed berries had 9 to 10°C more than the exposed ones.

Canopy management techniques as leaf removal or shoot thinning, together with the timing of these interventions, play an important role on berry temperature regime (Fernaud et al., 2001; Downey at al., 2004; Chorti et al., 2010). For instance, in North-South oriented rows, in the thin canopies obtained by leaf removal at the fruit zone, Fernaud et al. (2001) observed that berry temperature patterns during the day and along the season became symmetric on opposite sides of the canopy.

The differences between clusters are not only a function of their exposure, but also a result of the heterogeneity on their structure. Moreover, during ripening, berry thermal regime changes due to the decrease in the albedo as they change colour at veraison. After this stage, there is an increase in berry temperature, as colour is intensified and radiation absorption increases.

During maturation cluster compactness increases as berries get thicker and larger, leading to cluster closure, which reduces cluster thermal sensitivity to wind. As a result, berry flesh maximum temperature tends to increase, and the difference between air and berry flesh temperature during daytime is amplified. These effects were observed by Igounet et al. (1995) during a study conducted in cv. Syrah, where the difference between berry and air temperature didn‟t exceed 2°C in the beginning of July, archived 10°C at the end of the month and reached 50°C at the end of August. Berry minimum temperature remained close to air temperature or even slightly inferior due to the emission of long wave radiation during the night.

As far as temperature effect on berry growth is concerned, Dokoozlian (1990), working on cv. Cabernet Sauvignon, observed that it augmented with the increase of clusters exposure to sunlight, but they also found that no increase of temperature resulted of the enhanced sunlight.

Conversely, Bergqvist et al. (2001) reported a gradual decline on berry mass with higher exposition to light, which was probably a result of lower cell division and elongation, higher transpiration and dehydration, caused by excessively elevated temperatures on the berries.

Regarding fruit composition, several authors observed a decrease in titratable acidity in response to an increase in sunlight exposure and concluded that this came as a result of the decrease in malic acid degradation due to the elevated temperature to which fruits were exposed (Kliewer, 1971; Smart, 1982; Wolf et al., 1986). Other authors also indicated that must pH is

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 54 of 234 inferior when clusters are exposed to light, as compared to shaded ones (Bergqvist et al., 2001). The same authors observed that the total soluble solids (TSS) were higher when the intercepted light in the cluster zone was high (31 – 50 μmol m-2 _s-1 _{in cv. Grenache and 51 – 100 μmol m}-2 _s-1 _{in cv.} Cabernet Sauvignon).

In spite of the increment frequently found in must TSS and pH values under higher cluster exposure, Kliewer (1977) works indicate that cluster temperatures over 37°C inhibit sugar accumulation. For anthocyanins and polyphenols as well, temperatures higher than 35°C might inhibit the synthesis of these compounds (Bergqvist et al., 2001).

As mentioned before, berry sensitivity to temperature varies among ripening stages, varieties and with cultural practices (Downey et al., 2000) but, overall, low night temperatures seem to stimulate anthocyanin accumulation and in warm to hot regions, excessively high temperature might even promote anthocyanin degradation (Mori et al., 2007, Yamane et al., 2006). Nevertheless, the varietal response to temperature stress remains unclear and the effects of light and temperature are complex and difficult to separate (Spayd et al., 2002).

The use of irrigation in hot and dry environments arises as means to prevent excessive canopy temperature, to maintain quality in wine production and, in more extreme cases, to guarantee plant survival (Chaves et al., 2010). On the other hand, irrigation may promote excessive vegetative growth with a negative impact on berry pigments and sugar content, and therefore decrease wine quality (Bravdo et al., 1985; Dokoozlian and Kliewer, 1996).

Analysing diurnal pattern of berry temperature at veraison, for similar days of August (clear sky and high air temperature), on exterior clusters of cv. Moscatel, Santos et al. (2007) observed that in all irrigation treatment (Full irrigation, 100% ETc, deficit irrigation, DI 50% ETc, PRD 50%

ETc and non irrigated control, NI) berry temperature progressively increased from dawn, reaching

maximum values at 11:00 h in the east canopy side and at 16:00h in the west side. Regarding the effect of irrigation treatments, these authors noticed that berry temperatures were always higher in NI and PRD than in FI and DI vines, which presented denser canopies. The largest differences between air and berry temperature were reached at the east side around 11:00 h, the berries on NI presenting a temperature 5.5°C higher than the air as compared to 4°C in FI, while in PRD and DI berry temperature exceeded air temperature by 5.8 and 3.4°C, respectively. During the night, differences between berry and air temperature were detected only in DI and FI berries from east

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 55 of 234 canopy side, which presented lower temperatures than the air. In Santos et al. (2007) study, high temperature was not a limiting factor affecting anthocyanin synthesis. These authors suggested that, in climates where temperatures regularly exceed 30ºC during ripening, moderately open canopies as those obtained in their PRD „Moscatel‟ vines might create optimal conditions for anthocyanins and total phenol synthesis.

Meanwhile, in Cabernet Sauvignon berries, Mori et al. (2007) found that increased temperature decreased the accumulation of anthocyanins and also Haselgrove et al. (2000) in „Syrah‟ and Spayd et al. (2002), working with „Merlot‟ vines, reported an inhibition in anthocyanin synthesis or even increasing degradation, caused by elevated temperatures.

In climates where temperature frequently exceed 35°C during ripening anthocyanin accumulation seems to be blocked (Mori et al., 2007). This is partly due to a decrease in transcription of anthocyanin biosynthetic genes, which can potentially result in decreased production of anthocyanin, but was primarily found to be caused by increased degradation of anthocyanins under elevated temperature. In that study, the response of the different anthocyanin types was variable, malvidin-glucosides being more resistant to degradation under elevated temperature than non-malvidin derivatives, where degradation was enhanced (Bindon et al., 2008).

Studying the effects of PRD in the composition and accumulation of anthocyanins in cv. Cabernet Sauvignon Bindon et al. (2008) evidenced that the accumulation of malvidin derivatives in grape berries was similarly more resistant to possible changes in vine physiology or bunch microclimate induced by the PRD treatment than other anthocyanin types.