Effect of Temperature and Duration of Root CHILLING on the Balance between Antioxidant Activity and Oxidative Stress in Spinach

Abstract

Plant factories are controlled-environment agricultural facilities that enable stable vegetable production yearround regardless of the weather conditions. Many researchers have investigated the production of high value-added vegetables in plant factories by controlling environmental conditions or giving certain stimuli, such as air temperature, root-zone temperature, water availability, salinity, ozone, and ultraviolet irradiation (Gazula et al., 2005; Chaves et al., 2009; Hikosaka et al., 2010;Bettaieb et al., 2011; Ito et al., 2013; Sudheer et al., 2016). Recent studies have shown that cold stress applied to the root area has a positive effect on the nutritional quality and produces a significant increase in the levels of highly functional plant constituents. For example, Chadirin et al. (2011a; 2011b; 2012) reported that spinach root chilling induced a significant increase in the levels of beneficial substances (such as sugars, ascorbic acid, and Fe) and a decrease in those of harmful substances (such as NO3 and oxalic acid). Furthermore, Ogawa et al. (2018) indicated that root chilling at 10°C for 6 days increased the levels of antioxidants, such as rosmarinic acid and luteolin, in red perilla. Several studies have reported that environmental stress accelerates the production of reactive oxygen species (ROS) in the plant body, which induces oxidative stress and triggers antioxidant pathways to manage them with the production of antioxidant molecules (Asada, 2006). Under severe stress conditions, oxidative stress markers, including H2O2, lipid peroxides (LOOH), as well as lipid peroxidation-derived aldehydes, and oxidized proteins might accumulate in the plant body when ROS generation overcomes antioxidant capacity. For instance, Sakamoto and Suzuki (2015) reported that although root chilling increases the levels of beneficial substances, such as anthocyanin, phenols, and ascorbic acid, it also increases the levels of harmful substances, such as hydrogen peroxide (H2O2) and malondialdehyde (MDA), which are highly reactive molecules formed under oxidative stress. In that regard, several studies have reported that products of the oxidation of biomolecules cause cancer and liver disease (Nair et al., 2007; Li et al., 2015). Thus, it is necessary to consider the effects of environmental stress on the levels of oxidation byproducts, and not only antioxidant content, for the proper evaluation of the quality and functionality of stress-exposed vegetables. However, changes in antioxidant capacity and oxidative stress markers in vegetables under different root chilling temperatures and time frames have not been investigated yet. Here, we measured ascorbic acid content, superoxide dismutase activity, 2,2-diphenyl-1-picrylhydrazyl (DPPH) scavenging activity, as proxies of antioxidant activity, and MDA content, as an oxidative stress marker, in spinach. This study aimed to investigate the change in antioxidants and oxidized molecules in spinach under different levels of cold stress applied to the root area to produce high quality vegetables under artificial environment.