ULTRAVIOLET-B (UV-B) RADIATION
UV-B RADIATION REACHING THE EARTH
Accounting for less than 0.5%, UV-B radiation (280-315 nm) is a minor fraction of the solar radiation reaching the earth surface. This is due to the action of the stratospheric ozone layer. The extinction coefficient of ozone increases several orders of magnitude as wavelength decreases. This property confers to the stratospheric ozone layer the role of protecting earth’s life form solar most-harming UV radiation wavelengths. Stratospheric ozone sustains a cycle in which three forms of oxygen are involved. Molecular oxygen (O2) photodissociates into two oxygen radicals (O) in response to solar radiation around 240 nm.
Oxygen radicals react spontaneously with oxygen molecules to produce ozone (O3), which absorbs UV radiation between 200 and 315 nm. Such reaction leads to the splitting of ozone into a molecule of oxygen and an atom of oxygen. This atom of oxygen may form ozone newly, or react with another oxygen radical to produce molecular oxygen and close the cycle (Harrison and Hester, 2000).
Substances, such as chlorofluorocarbon compounds (CFCs), are dissociated by UV radiation, releasing chlorine atoms (Fig. 8). These act as catalyst, consuming several ozone molecules before they exit the stratosphere. Due to the anthropic release of ozone-depleting substances, stratospheric ozone layer has been thinned at a rate of 4% per decade up to the late 1980s (McKenzie et al., 2007). The successfull implementation of the Montreal Protocol (1987) contributed to reduce to a great extent the emission of ozone depleting substances.
However, due to the resilience of ozone depleting substances, ozone levels may not recover until 2050 (UNEP, 2012). Ozone depletion simulations reveal the relevance of the success of Montreal Protocol (Fig. 9). Under an scenario of constant CFCs emissions, UV levels would have increased several times before the end of the present century, which would have been devastating for plant life (Newman et al., 2009).
PLANT RESPONSES TO UV-B RADIATION
Plant responses to UV-B can be classified into stress responses and photomorphogenic responses, usually coinciding with expositions to high and low fluent rates of UV-B radiation, respectively (Fig. 10). The plants genotypic characteristics (e.g. constitutive levels of UV shielding compounds) and their previous acclimation are important factors determining whether a plant is UV-B stressed or not. Usually plants growing within their natural distribution are acclimated to UV-B (Jansen et al., 2012).
UV-B radiation has been traditionally considered as an environmental stress factor, able to induce damaging effects on plants. This “narrow perspective” resulted from the use of high UV-B fluent rates in experiments associated to ozone depletion (reviewed by Kakani et al., 2003); and it dominated the field of UV-B research for decades (Jansen and Bornman, 2012). Plant UV-B stress response is characterised by the activation of non-specific signalling pathways also triggered by other stresses such as DNA damage, pathogen defence or wounding (Jenkins 2009) (Fig. 9). Although UV-B radiation may target mainly nucleic acids and proteins, the oxidizing side of photosystem II (i.e. degradation of D1 and D2 proteins) and reaction centres may get significantly damaged (Bornman, 1989; Jansen et al., 1998). Non-specific pathways may be mediated by cellular damage sub-products such as ROS, leading antioxidant enzyme up-regulation (Jansen et al., 2012), but also, hydrogen peroxide, nitric oxide, abscisic acid, jasmonic acid, ethylene and salicylic acid participate in the activation of defence mechanisms (reviewed by Bandurska et al., 2013). In addition, UVB radiation may also trigger antioxidant enzyme genes expression, through signal cascades downwards of UV-B photoreception without the stimulus of oxidative stress (Hideg et al., 2013). Therefore, plant responses to UV-B are often the result of a combination of specific and non-specific pathways (Fig. 10).
UV-B is now regarded as a specific modulator, not so much as a generic stress factor, and current research is mainly driven to investigate the regulatory effects of UV-B radiation within the natural light environment (Jansen and Bornman, 2012).The recent elucidation of the role of ULTRAVIOLET RESISTANCE LOCUS 8 (UVR8; Rizzini et al., 2011) in UV-B signalling is one of the major contributions to consider UV-B as a specific modulator. UVR8 protein forms homodimers maintained by salt-bridges interactions between charged amino acids at the dimeric interface. Interception of UVR8 dimers by UV-B radiation causes the neutralization of salt bridges leading to monomerization (Christie et al., 2012). UVR8 monomers interact directly with CONSTITUTIVE PHOTOMORPHOGENIC1 (COP1, Fig. 11) for the transcriptional activation of the ELONGATED HYPOCOTYL 5 (HY5). This transcription factor controls the regulation of around 20% of the light-regulated genes and triggers photomorfogenic responses under a wide range of wavelengths (i.e. far red, red and blue light) (Koornneef et al., 1980; Oyama, et al., 1997; Osterlund et al.,2000), including those required for UV-B acclimation (Heijde and Ulm, 2012). REPRESSORS OF UV-B PHOTOMORPHOGENESIS proteins (RUP1 and RUP2) mediate in the re-dimerization of UVR8. This terminates COP1-UVR8 interaction (inactivates the signalling pathway) and regenerates UVR8 dimers pool for a UV-B perception (Tilbrook et al., 2013).
Among the photomorphogenic effects caused by UV-B are reduced leaf expansion, increased leaf thickness, and finally, accumulation of leaf phenolic compounds and cuticle waxes, which in turn may result in altered morphology and metabolic cost, which may lead to biomass reduction (reviewed by Wargen and Jordan, 2013). One of the most relevant photomorphogenic responses to UV-B radiation is the up-regulation of flavonoid biosynthesis. UVR8-COP1 conjugate is able to transactivate the transcription factor VvMYBF1 (AtMYB12; Mehrtens et al., 2005), which activates the promoter of CRUY (encoding for a putative lyase), CHS and FLS. CHS and FLS enzymes are directly involved in flavonoid synthesis. While CHS enzyme synthesizes precursors for the general flavonoid biosynthesis, FLS is exclusively committed to the synthesis of flavonols, which is the group of flavonoids that more efficiently absorbs UV-B radiation (Cerovic et al., 2002). In grape berries, UV-B may up-regulate the biosynthesis of several flavonoids (Berli et al., 2011). As described in section 1.5.4., these compounds are well-valued components of wine grapes, but in turn, the phenolic compounds may reduce the plant susceptibility to pathogens (Keller et al., 2003).
CLIMATE CHANGE RELATED FACTORS
ATMOSPHERIC CO2 CONCENTRATION, AIR TEMPERATURE AND WATER AVAILABILITY
Since the beginning of industrial development, gases such as water vapour, carbon dioxide (CO2), methane (CH4), mono-nitrogen oxides (NOx) and CFC compounds have accumulated in the atmosphere. These gases are the so-called heat-trapping gases, due to their absorption of infrared radiation emitted by the sun. As a consequence, present average air temperature is 0.6ºC higher (compared to pre-industrialization record reference) due to the anthropic release of these gases (IPCC, 2013). CO2 is regarded to cause most of this warming, mainly due to its larger concentration, as other compounds may have a higher heat-trapping ability. Even though plants are able to fixate atmospheric CO2 and oceans act as gigantic CO2 traps, the half-life of a CO2 molecule in the atmosphere is higher than any other green-house gas (IPCC, 2007b). Since the preindustrial period the concentration of CO2 has rose from 280 ppm to the present 400 ppm.
Under a “business as usual” modelling of CO2 emissions, atmospheric CO2 levels may eventually rise up to 700 ppm by the end of the 21st century (IPCC, 2007a). Such level of atmospheric CO2 may result in an increase of up to 4.8ºC in average air temperature, depending on future CO2 emissions (Fig. 12) (IPCC, 2013).
Figure 12. Cumulative CO2 emissions versus the predicted temperature increase. Different simulations (colour lines) respond to emission policies. For more details see IPCC (2013) Due to the increase in temperature, global evaporative demand is expected to change, which in turn may alter cloud patterns. Higher evaporation may induce more precipitations, but not necessarily uniformly distributed. For the Mediterranean region, long-term simulations (up to 2100 A.C.) show a decrease in precipitations of up to 30%, in part due to a reduction of cloud coverage, thus leading to an increase in solar radiation reaching the earth surface (IPCC, 2013; Trenberth and Fasullo, 2009). In addition to changes in global CO2, temperature and precipitation trends, extreme weather and climate events will be more likely in the future.
EFFECTS OF WATER DEFICIT, ELEVATED TEMPERATURE AND ELEVATED CO2 ON GRAPEVINE
The effect of water deficit on grapevine has been well documented due to the seasonal drought experienced in the main viticultural regions. Nevertheless, contradictory effects may be found in the literature due to the wide range of situations encompassed by water deficit studies (Chaves et al., 2010). A mild or moderated water deficit may promote abscisic acid biosynthesis, a phytohormone involved in ripening control (Coombe and Hale, 1973). Water deficit also promotes the accumulation of flavonoids, and other wine quality-related traits (Castellarin et al., 2007a; Deluc et al., 2009; Olle et al., 2011; van Leeuwen et al., 2009). The effect of water deficit on berry acidity is not clear. However, some hints suggest that it may promote organic acid (mostly malate) breakdown (Conde et al., 2007 and references therein). As water deficit becomes more severe, grapevine may suffer a dramatic reduction in carbon assimilation and canopy leaf area, which may compromise grape quality and yield; and, in extreme cases, plant survival (Chaves et al. 2010 and references therein).
Table of contents :
1.6.Flavonoid biosynthesis regulation
2.Ultraviolet-B (UV-B) radiation
2.1.UV-B radiation reaching the earth
2.2.Plant responses to UV-B radiation
3.Climate change related factors
3.1.Atmospheric CO2 concentration, air temperature and water availability
3.2.Effects of water deficit, elevated temperature and elevated CO2 on grapevine
4.Research under controlled conditions
4.1.Using greenhouses to simulate climate change conditions
4.2.Fruit-bearing cuttings model
CHAPTER 1. Short- and long-term physiological responses of grapevine leaves to UV-B radiation
CHAPTER 2. Ultraviolet-B radiation modifies the quantitative and qualitative profile of flavonoids and amino acids in grape berries
CHAPTER 3. Characterization of the adaptive response of grapevine (cv. Tempranillo) to UV-B radiation under water deficit conditions
CHAPTER 4. Ultraviolet-B radiation and water deficit interact to alter flavonol and anthocyanin profile in grapevine berries through transcriptomic regulation
CHAPTER 5. Climate change conditions (elevated CO2 and temperature) and UV-B radiation affect grapevine (Vitis vinifera cv. Tempranillo) leaf carbon metabolism, altering fruit ripening rates
CHAPTER 6. UV-B alleviates the uncoupling effect of climate change conditions (elevated CO2-temperature) on grape berry (Vitis vinifera cv. Tempranillo) anthocyanin-sugar accumulation