Light Microclimate

Among the climatic components, light is the one that suffers higher attenuation from the canopy. From the total solar radiation that reaches the surface of the hearth only the visible fraction of the spectrum is photosynthetically effective. This radiation is called Photosynthetic Active Radiation (PAR) and its intensity can be monitored measuring the photosynthetic photon flux density (PPFD).

Under cloudy sky conditions, the ambient PPFD values vary from 300 to 1000 μmol m-2 _s-1 depending on the cloudiness degree, and in clear sky conditions and high irradiance there have been recorded maximum PPFD values around 2500 μmol m-2 _s-1 _{(Smart et al., 1985).}

Canopy light interception depends upon the interaction of leaves geometry, dimension and orientation with solar angle and cloudiness. In vineyards, maximum interception is achieved with tall canopies, low row distance, without causing long shading periods between row, and North- South row orientation (Smart, 1985).

During the day, the intercepted PPFD values vary with the beams incidence angles, and in the canopy side directly exposed to light the values can be double from those recorded at the top of the canopy (ca. 1400 and 700 μmol m-2 _s-1_{, respectively). The shaded side which intercepts most} exclusively diffuse radiation can receive nearly 3 to 6% of the PPFD measured on the illuminated side.

There is also an important effect of leaf angle on light interception, particularly in the solar exposed side, since the orientation of leaves toward beam direction varies greatly.

Generally, literature indicates that the well exposed leaves are the major source of plant overall photosynthesis. Smart et al. (1990) indicated an optimum value of 1.5 leaf layers to avoid excessive shading inside the canopy and for a good PAR interception and photosynthetic performance, berry sugar and colour accumulation.

The centre of a dense canopy is very shaded and frequently PPFD values of less than 10 μmol m-2 _s-1 _{can be recorded. Inside such a canopy PAR intensity is very low and the near infrared} fraction of the spectrum is relatively higher, which reduced the Red/Far red ratio (R:FR), and may affect the reactions at phytochrome level, a photoreceptor of plants that regulates plants physiological response to red light. On the canopy surface the ratio R:FR is near 1:1, both in sunny

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 50 of 234 and overcast days but inside the canopy the values decrease to 0.15 – 0.75 and in deep shadow they can fall to less than 0.1 (Dry, 2000).

Plants manifest physiological responses to radiation in a range of wave length from 280 to 800 nm, which includes visible but also UV-A, UV-B and near Far red radiation. These wave lengths are involved on the regulation of several important phenomena like flowering induction, stomatal opening, chloroplasts syntheses and development, anthocyanins and polyphenols synthesis.

Several authors observed that when compared to shaded clusters, berries ripened under high light conditions presented higher sugar, anthocyanins and total phenols contents and lower acidity, malic acid, pH and berry weight (Dokoozlian and Kliewer, 1996, Haselgrove et al., 2000; Bergqvist

et al., 2001; Santos et al., 2007). Gaudillere et al. (2001) also observed that the light received by the

clusters had a positive effect on total polyphenols although, in hot climates, high cluster exposure to radiation, near 80% of ambient PPFD may not by desirable for anthocyanin production on berries. In fact, from the grape exposure and metabolism point of view, such canopy density value may not be ideal in many wine grape terroirs.

The response of grape anthocyanins to the variation of light exposure has not been shown to be consistent throughout berry development. For instance, Haselgrove et al., 2000 observed significantly higher concentration on anthocyanins in sun-exposed grapes of cv. Shiraz shortly after veraison, but the difference was not maintained until harvest. It seems that, as light intensity increases, there is a threshold over which anthocyanin concentration in the berries declines (Mahbrouk and Sinoquet, 1998). This may be caused by the elevated temperatures at high light intensities, which can limit anthocyanin accumulation for both a decrease in synthesis and increase of degradation within the biosynthetic pathway (Bergqvist et al., 2001; Spayd et al., 2002, Mori et al., 2007).

In the field it is difficult to separate the effects of light intensity and temperature but experiments like those of Downey et al. (2004) and Ristic et al. (2007) have cast some doubts about the importance of light exposure on grape composition. In their studies, when Shyraz clusters where totally shaded after fruit set under constant temperature, these authors did not observe a decrease in total anthocyanins per gram of berry weight nor even in the concentration and proportion of the sum of acylated and non-acylated glucosides between shaded and control fruit.

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 51 of 234 However, the treatment caused a reduction on flavonol level (quercetin and quercetin-3-glucoside) and in deoxygenated anthocyanidins by more than 30%, namely cyaniding and peonidin, and an increase in trioxigenated ones (namely dephinidin, petunidin and malvidin derivates).

The application of irrigation to grapevines has been generally found to cause an increase of vegetative growth, thus an increase in canopy density and shading, and a delay in berry ripening. Although, as mentioned before, some studies suggest that DI strategies allow improving berry composition due to a better light and thermal environment within the fruit zone (Santos et al., 2007).

Generally, moderate water stress imposed during grape development reduces canopy density, optimising the cluster microclimate, thus affecting positively the synthesis and accumulation of sugars, anthocyanins and phenols (Bergqvist et al., 2001, Downey et al., 2006). This effect is potentially promoted by PRD, since the reduction of lateral shoot growth and leaf area improves light penetration into the fruit zone.

During a study of the effects of deficit irrigation strategies on cluster microclimate in cv. Moscatel, Santos et al. (2007) observed that full irrigated vines (FI) had the highest LLN and, consequently, displayed the lowest incident PPFD values during ripening at the cluster zone. On the contrary, non irrigated (NI) plants presented the highest cluster exposure. The authors also verified that, within the irrigated treatments, the reduction in vegetative growth observed in PRD resulted in a more open canopy, with significantly higher values of PPFD (10.2 ± 0.9%) received by the clusters when compared to DI (4.2 ± 0.5%) and FI (2.7 ± 0.3%). In this study higher incident light and temperatures measured during ripening in NI and PRD plants were associated with higher concentrations in anthocyanins and total phenols. However, Keller and Hrazdina (1998) showed that in cv. Cabernet Sauvignon, the anthocyanin concentration in berries was similar at 20 % and 100 % sunlight interception. It seems that if light conditions within the canopy are above a given threshold, light might not be necessarily a limiting factor for anthocyanin synthesis.

Research on grapevine aromatic compounds generally links environmental conditions with the metabolism of aromatic precursors as carotenoids. There is evidence that the increase of sunlight incidence on developing clusters mediates the accelerated decrease of carotenoids after veraison (Marais et al., 1991; Bureau et al., 1998; Razungles et al., 1998). In particular, high exposure to UV radiation as been associated with a reduction in carotenoids of berry skin at maturity

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 52 of 234 (Schultz et al., 1998). Full sunlight increases the concentration of a range of C13-norisoprenoids in berries compared to shaded conditions (Bureau et al., 2000). However, comparison between sun- exposed and natural shaded clusters showed that the accumulation of son C13-norisoprenoids is more strongly affected by sunlight than others. The 1,1,6-trimethyl-1,2-dihydronaphthalene (TDN) was increased up to 52% whereas β-damascenone concentration was unaffected (Marais et al., 1992).