Effects of defoliation and water restriction on total phenols and antioxidant activities in grapes during ripening

摘要

Aim: To compare the effect of water restriction and defoliation on the phenol contents and oxidant and antioxidant activities of Tempranillo grapes grown in the region of Extremadura, Spain.Methods and results: The results showed that at harvest, the water restriction treatment altered total foliar area, pH, and total soluble solid (TSS) and phenol contents but not polyphenol oxidase (PPO) and superoxide dismutase (SOD) activities. By contrast, the defoliation treatment did not affect TSS and pH, however, titratable acidity and phenol compounds were enhanced and PPO and SOD activities were decreased. Moreover, defoliation resulted in advanced grape ripening and reduced total foliar area.?Conclusion: At harvest, water restriction led to alterations in the levels of peroxidation and peroxidase (POX) activity, without any effect on the other parameters. Defoliation induced a lowering of PPO and SOD activities and an increase in phenol content.Significance and impact of the study: Given the environmental characteristics of the Extremadura region, defoliation may be suitable and efficient for regulating Tempranillo grape PPO activity, which is ?nologically relevant since it could lead to lower levels of phenol oxidation and hence prevent wine discolouration. IntroductionTraditional indices of grape ripening are based on the content of sugars and acids in the must. However, in the case of red wine grapes, these indices alone are not reliable markers of quality (Saint-Criq de Gaulejac et al., 1998). It is thus essential to understand how the content of phenolic compounds evolves during ripening in order to control the quality of the final product, as these compounds are responsable for the color and astringency of the resulting red wine (Ribéreau-Gayon and Glories, 1987).Reactive oxygen species (ROS) are produced as a result of normal metabolism and play an important role in the plant life cycle and stress response (Dangl et al., 1996; Kawano, 2003). It has been demonstrated that the plasma membrane NADPH oxidases (RBOH-NOX) and peroxidases are involved in the ROS production (Grant et al., 2000; Bolwell et al., 2002). ROS are potentially dangerous and their overproduction, called “oxidative burst”, is part of the plant response to environmental stresses (Miller et al., 2010; Apel and Hirt, 2004). Plants respond to stress by developing a series of oxidative and antioxidative reactions specific to each stress (Mittler et al., 2011). Moreover, the mechanisms involved in the production and scavenging of ROS in these stress responses are also key agents in other physiological processes such as ripening (Jiménez et al., 2003). The antioxidant system plays an important role in ROS homeostasis (Apel and Hirt, 2004) and includes enzymes such as peroxidase (POX), catalase (CAT) and superoxide dismutase (SOD). Increased synthesis of phenolic compounds, including flavonoids and phenylpropanoid glycosides, is a common plant response to stresses (Dixon and Paiva, 1995) and ripening (Fortes et al., 2011). Phenylpropanoid glycosides, in particular, enhance the antioxidant capacity of cells (Grace and Logan, 2000). Many studies have demonstrated the ROS scavenging capacity of flavonoids and phenolic acids (Rice-Evans et al., 1996). This suggests that phenylpropanoid glycosides and flavonoids are involved in protection against oxidative stress (Grace, 2005).How a plant's total antioxidant capacity, phenol content, and lipid peroxidation level evolve may be used as an indicator of stress response (Dixon and Paiva, 1995; Ma et al., 2007). For grapes, ripening involves changes in the composition and accumulation of phenols that will subsequently be extracted and transferred to the wine. Antioxidant activity in grapes is positively correlated with the concentration of phenolic compounds (Landbo and Meyer, 2001). Phenolic composition may vary during ripening (Jordao et al., 2001) due to enzymatic alterations, as is the case for the oxidation of phenols by phenol oxidases (Ryan et al., 2002). Doshi et al. (2006) reported reduced antioxidant capacity during ripening, as reflected in phenolic compounds. Dr?ghici et al. (2011) described a steady increase in total polyphenol content during ripening until maturity, followed by a slight decline. This evolution in production and accumulation depends, however, on a great variety of factors, including (i) weather conditions, (ii) farming practices (Conde et al., 2007; Singh et al., 2010), such as the input of N and K fertilizers (Delgado et al., 2004), and (iii) water availability at different grapevine phenological stages (Valdés et al., 2008; Yuste et al., 2012). Also, polyphenol synthesis depends partly on the activity of the enzyme phenylalanine ammonia lyase, whose activity is temperature and light dependent (Roubelakis-Angelakis and Kliewer, 1986). This, of course, implies the involvement of a large number of factors (exposure of the clusters to light, the temperature reached by the grapes