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OXIDIZED POLYMERIC PHENOLICS: COULD THEY BE CONSIDERED PHOTOPROTECTORS? Laura Rustioni J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.7b03704 • Publication Date (Web): 18 Aug 2017 Downloaded from http://pubs.acs.org on August 18, 2017
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Journal of Agricultural and Food Chemistry
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Oxidized Polymeric Phenolics: Could They Be
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Considered Photoprotectors?
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Laura Rustioni*
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DISAA – Dipartimento di Scienze Agrarie e Ambientali - Università degli Studi di Milano; via Celoria 2, 20133
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Milano – Italia
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*Corresponding author (Tel: 0039 02 50316556; Fax: 0039 02 50316553; E-mail:
[email protected])
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ABSTRACT
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Photooxidative sunburn is the consequence of photosystem overexcitations. It results in tissue color
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changes due to chlorophyll degradation and accumulation of oxidized polymeric phenolics (OPPs) resulting
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from scavenging of reactive oxygen species (ROS). From a productive point of view, OPPs should be
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considered as damages, decreasing the economical and esthetical values of plants and crops. However,
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from a physiological perspective, OPPs could be also play a screening role against excessive
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photosynthetically active radiation (PAR), as they follow the criteria proposed for the identification of
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photoprotectors, as follows: i) Due to the complex conjugated double bond systems, OPPs absorb and,
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thus, screen the visible photosynthetically active radiation; ii) The accumulation of brown OPPs is well-
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known to be stimulated by light exposure resulting in sunburn symptoms; iii) OPPs induce PAR resistance:
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for example, the sunburned brown skin allows the fruit ripening to proceed without further interferences;
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iv) the screen provided by the accumulated OPPs in death cells protect underlying tissues, demonstrating
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an increased resistance to radiation when other physiological processes are not functioning.
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KEYWORDS: Photooxidation; sunburn; brown pigments; PAR screen; radiative excess
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Journal of Agricultural and Food Chemistry
OXIDIZED POLYMERIC PHENOLICS AND PHOTOOXIDATIVE SUNBURN
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Plants are photoautotrophs and light is the basis of their life. However, excessive radiation, often coupled
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by thermal stresses (due to radiative heating), could produce serious damages to the exposed tissues. In
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their extensive review, Racsko and Schrader1 reported a clear classification of sunburn effects on apples,
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discriminated by characteristic symptoms and originating causes. The first sunburn type, sunburn necrosis,
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is the result of excessively high temperatures (higher than 52 °C for at least 10 min), independent of light
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stimuli. These lethal conditions result in loss of membrane integrity in correspondence of symptomatic
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necrotic areas. 2 The second sunburn type, sunburn browning, results from the combination of excesses in
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temperature (cultivar specific threshold ranging between 46 and 49 °C) and UV-B radiation. 2 These sub-
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lethal conditions do not affect the membrane integrity; however, they produce tan areas on the sun
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exposed side of the fruits. 1 Although the above described symptoms could represent a major injury in
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agricultural productions, they are expected to afflict mainly organs with an inadequate thermal regulation
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due to stomata degeneration. For example, considering grapevines, the berry latent heat flow is considered
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nearly null, due to the extremely limited evapotranspiration, 3 while, in leaves, transpiration significantly
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decreases the organ temperature. 4 In this case, also the water status could affect the plant susceptibility. 4
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This Perspective focuses on the third type of sunburn: photooxidative sunburn. In this case, the triggering
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cause is not heat nor UV exposure, in fact the sole visible radiation is able to produce the symptomatic
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pigmentations in both apples and grapes. 1, 5, 6 Excessive photosynthetically active radiation could produce
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serious damages to photosystems, due to the production of reactive oxygen species (ROS).7 In steady state
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conditions, ROS are scavenged by various anti-oxidative defense mechanisms, but stress factors (such as
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radiative excesses) could perturb the equilibrium between the production and the scavenging of ROS.
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These free radicals can act as damaging, protective or signaling factors. 8-11
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The physiological response to photodamage is preserved among plants (it appears in different species,
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organs and tissues), and it could be experienced by exposing non-acclimated plants to excessive radiation
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(e.g.: an indoor plant, adapted to low photosynthetically active radiation (PAR) input, suddenly moved to
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direct sunlight). The first photooxidation symptoms appear as tissue color changes. The green color
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decreases as a consequence of chlorophyll degradation and brown pigments are accumulated as a results
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of ROS scavenging by phenolic compounds. 5, 6, 12 ROS scavenging requires phenolic hydrogen atoms, and
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the obtained phenolic free radicals are then deactivated by polymerizations which result in brown pigment
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accumulations6, 12-14 (Figure 1). It is interesting to note that the central role of phenolic compounds as
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oxidized polymeric phenolic (OPP) precursors in early plant adaptation to terrestrial stress conditions have
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been proposed.15 Close and McArthur16 demonstrated the main function of plant phenolics as defense
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against photodamage, suggesting their role in plant ecology and evolution related to different risks of
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photodamage. They stated that “differences in phenolic levels at any level (within a plant, between
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individuals within a species, and between species) are a response to oxidative pressure and risk of
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photodamage”. In their review, the authors limited the phenolic role to their antioxidant capacity. It is
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worth to notice that OPPs undergo a phenolic regeneration process during polymerization14 and the
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regenerated phenols are more readily oxidized than their corresponding original ones13 resulting in an
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increased antioxidant capacity of the oxidized compounds. In addition to this evidence, a possible
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photoprotective screening role of oxidized phenolics against PAR excesses is proposed in this Perspective.
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Different photoprotective mechanisms have been described. Mostly of them dedicated to the reparation or
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mitigation of radiative damages. Nevertheless, the importance of photoprotective mechanisms based on
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the attenuation of harmful excessive PAR has also been underlined: 17 plants can respond to excessive
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radiation by the synthesis and accumulation of pigments able to selectively absorb specific regions of the
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visible spectrum. 17
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Cockell and Knowland18 proposed four criteria to define the compounds with a UV-screening role, and in
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accordance with Solovchenko and Merzlyak, 17 these principles will be generalized to excessive PAR-
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screening.
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1. OPPs must screen the PAR radiation. Phenolic compounds are characterized by the presence of one
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or more aromatic rings substituted by hydroxyl group(s). The presence of conjugated double bond
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systems favors the absorption of low energetic radiation. The absorbed radiative energy allows the
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excitation of π-electrons, which make a transition from π orbitals to π* orbitals. 18 A UV absorption
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band peaking around 280 nm appears in all the phenolic spectra, due to the presence of aromatic
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ring(s). A second long-wave band is generally situated in the 300–360 nm range, but it could also
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extend into the blue-green spectral region, and the exact position varies depending on the phenol
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classes and structure (Figure 2).17 Generally, an increase in the molecular complexity (number of
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conjugated bonds and substituents) results in a bathochromic shift of the absorption bands. 18, 19
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The heterogeneous polymerization characteristic of OPP formation, results in an increase of the
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molecular complexity, 13, 14, 20 including the number of conjugated aromatic rings. The absorption
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band obtained shows a maximum around 500 nm,12 overlapping the spectral region of highest
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sunlight irradiance. 21 Furthermore, due to the heterogeneous molecular structures of OPPs, the
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main absorption band of oxidized tissues appears broad, and, thus, able to absorb radiations in a
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wide range of the PAR wavelengths. 12
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2. OPPs accumulation should be induced in the living cell by PAR radiation. Recently, grape sunburn
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symptoms were characterized by reflectance spectroscopy. The accumulation of OPPs in
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consequence of excessive visible radiation was described both in field condition (solar radiation) 12
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and under controlled conditions (LED light sources). 6 Moreover, the biosynthesis of phenolic
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compounds (OPP precursors) is well-known to be stimulated by light exposure. 16, 22, 23 Jakopic et
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al.24 observed significant increases in flavonoid concentrations due to PAR exposure in apple skins.
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Concerning grapes, light exposure anticipates the beginning of berry anthocyanin accumulation at
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veraison, and, thus, in susceptible green berries. 25 Furthermore, an increased proportion of ortho-
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diphenols (highly oxidizable) has been reported in the anthocyanin profile of sunlight exposed
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berries. 25 Finally, it has been observed that grape sunlight exposure obtained by leaf removal,
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results in an increased accumulation of both OPPs (quantified as sunburn symptoms) and flavonols (phenolic precursors) in Raboso Piave and Sangiovese berries. 26
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3. The accumulation of OPPs should induce the resistance to the radiation in the spectral range of their
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absorption. Considering viticulture, defoliation is a common and well-studied practice and, thus,
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grapevines (Vitis vinifera L.) will be considered as a reference plant in this section. Defoliation
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results in an increase of berry radiative exposure and, at least in some days or portion of them, in
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photosystem overexcitations. In some cases, bunches directly exposed to solar radiation could
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produce evident sunburn symptoms. 12, 26, 27 Green berries are more susceptible than ripened fruits,
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due to the higher concentration of chlorophylls.6 Nevertheless, early defoliations (around
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flowering) cause a decrease in the susceptibility to evident sunburn symptoms, when compared to
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leaf removal in late green phenological stages (pea-size, veraison).28,29 These results indicate the
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existence of adaptive mechanisms able to limit the photooxidation susceptibility. Among these, the
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accumulation of photoprotective OPPs should be considered: during ripening, at least in white
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berried grapes, beside the decrease in chlorophyll content, catabolic processes favor the
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accumulation of OPPs, 30 which could have an additional protective effect against radiative
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excesses. Finally, also in concomitance with sunburn evidence, resulting from oxidative stresses in
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the skin (the external and protective fruit tissue), the berry pulp keeps ripening, as well as seeds. In
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fact, the protective brown skin generally allows the berry ripening to proceed without further
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interferences from the increased radiative stress, when compared to fruits shaded during all of the
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ripening phase. The results reported by Mosetti et al.27 show that leaf removal applied after fruit
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set (and the consequent direct exposition to sunlight) did not affect yield and fruit technological
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composition at harvest despite the presence of sunburn symptoms. Also, Pastore et al.26 showed
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that the berry exposure to excessive radiation in concomitance with significant sunburn symptom
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observations, did not prejudice the fruit development of exposed bunches in comparison to shaded
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grapes.
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4. OPPs must provide an increased resistance to radiation when other physiological processes are not
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functioning. Fruits have developed complex defense mechanisms against excessive sunlight. These
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systems can safely scavenge free radicals to prevent or minimize sunburn damages. However, the
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high incidence of sunburn under field conditions suggests that these mechanisms are often
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inadequate. 1 It is worth noticing that sunburn symptoms are considered as damages in fruit
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production fields. However, from a physiological point of view, the symptomatic brown areas
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related to the accumulation of OPPs could be considered as an extreme defense mechanism: a sort
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of “parasol” for the underlying plant reproductive organs. The photooxidative sunburn in apples is
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caused by excess of photosynthetically active radiation in non-acclimated fruits. The symptoms
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appear first as photobleached areas, that often turn brown. In this case, the accumulation of brown
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pigments could be associated with cell necrosis. 1, 5 In these death cells of damaged tissues, it is
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difficult to imagine other functioning physiological processes except the screen provided by the
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accumulated pigments, nevertheless the ripening of the underlying rest of the fruit (and of seeds) is
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not blocked by these symptoms. Piskolczi et al.31 state that sunburn does not usually cause serious
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damage to the epidermal tissue, and rarely affects the subepidermal tissue at all, despite the
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golden-bronze discolorations on the sunlit side of the fruit, detracting from its appearance.
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A photoprotective screen is considered as a long-term protection against photodamages. It is characterized
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by highly photostable pigments and it also protects the plant independently from environmental stresses
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(e.g. high temperature) which could disadvantage protective mechanisms based on enzymes. However,
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generally, the initial buildup of photoprotective compounds is considered as a high cost and slow plant
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activity. 17 Considering the OPPs, the accumulation of their precursors is cost and time consuming for the
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plant. It is generally stimulated by high irradiance; 16 however, it commonly occurs in all the organs also
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without specific stimuli. 14, 30 Furthermore, phenolic compounds, the OPP precursors, play different roles
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(e.g. antioxidants, biotic and abiotic stress protection, etc.) in plants and, thus, their accumulation costs are
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split up by the numerous functions. Nevertheless, the oxidative burst could occur in a very short time6, 12, 13
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and, thus, the transformation of molecules unable to absorb the visible radiation in pigments able to screen 7 ACS Paragon Plus Environment
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the PAR excesses is extremely fast. Thus, the OPPs could represent the most extreme PAR screen against
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photodamages.
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The accumulation of brown pigments in response to radiative excess in plants is a ubiquitous mechanism.
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This process is generally considered a damage, as it usually causes a decrease in the economic and
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aesthetic values of plants and crops. However, in plants, the sacrifice of the functionality, and, in extreme
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conditions, also of the life of cells, tissues or organs is a common adaptation strategy of biotic and abiotic
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stress response. For example, defense mechanisms of resistant grapevines against Plasmopara viticola
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include the necrosis of stomata and cells surrounding the pathogen invasion point. 32 Considering abiotic
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stresses, the selective abscission of leaves has been demonstrated to participate in the drought adaptation
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mechanisms of Jatropha curcas. 33
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In conclusion, different physiological roles have been proposed for OPPs, 14 and a photoprotective activity
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should also be considered.
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ABBREVIATIONS
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OPP = Oxidized Polymeric Phenolic; PAR = Photosynthetically Active Radiation; ROS = Reactive Oxygen
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Species
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REFERENCES
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25. Rustioni, L.; Rossoni, M.; Cola, G.; Mariani, L.; Failla, O. Bunch exposure to direct solar radiation
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FIGURE CAPTIONS
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Figure 1: Example of possible flavan-3-ol oxidative dimerization mechanism, in consequence of photo-
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oxidative stress and ROS formation (adapted from Waterhouse and Laurie). 20
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Figure 2: Examples of phenolic structures and their approximative longwave absorption regions: (A) flavan-
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3-ols; (B) hydroxycinnamic acids; (C) OPP dimers (structure proposed by Allen); 34 (D) anthocyanins.
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Figure 1: Example of possible flavan-3-ol oxidative dimerization mechanism, in consequence of photooxidative stress and ROS formation (adapted from Waterhouse and Laurie). 20 220x279mm (96 x 96 DPI)
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Figure 2: Examples of phenolic structures and their approximative longwave absorption regions: (A) flavan3-ols; (B) hydroxycinnamic acids; (C) OPP dimers (structure proposed by Allen); 34 (D) anthocyanins. 500x279mm (96 x 96 DPI)
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