Effects of Various High Tunnel Coverings on Color and Phenolic Compounds of Red and Green Leaf Lettuce (Lactuca Sativa)

*Eleni D Pliakoni
Department Of Horticulture And Natural Resources, Kansas State University, 22201 West Innovation Drive, Olathe, KS 66061, United States

*Corresponding Author:
Eleni D Pliakoni
Department Of Horticulture And Natural Resources, Kansas State University, 22201 West Innovation Drive, Olathe, KS 66061, United States
Email:epliakoni@ksu.edu

Published on: 2022-12-05

Abstract

High tunnel (HT) coverings can reduce light intensity and spectral quality, thus negatively affecting pigmentation and accumulation of several important phytochemicals in lettuce. The objectives of this study were to evaluate the effect of six HT coverings and two growing seasons (spring vs fall) on the red ‘New Red Fire’ and green ‘Two Star’ lettuce at harvest and after 5 days of storage with respect to leaf color and phenolic compound accumulation. Total phenolic content, anthocyanin accumulation, major individual phenolic acids and flavonoid compounds were measured. Chlorogenic and chicoric acid were the most prevalent phenolic compounds in both green and red lettuce. The phenolic compound accumulation in the red lettuce was significantly greater in the spring than in the fall for all measured compounds other than caffeic acid and anthocyanin. In the spring, the flavonoid accumulation was higher under the movable covering for both red and green lettuce. Red lettuce grown under clear poly (clear) and standard poly removed 2 to 3 weeks prior to initial harvest (movable) had darker leaves during both seasons (P <.001), as well as greater anthocyanin accumulation compared to the shade poly (shade) during the spring. During that season, the isoquercetin concentration of the red lettuce was 72% higher under the movable coverings compared to the shade covering (P <0.01). The effect of covering and season on phenolic compound accumulation after 5 days of storage was inconsistent. This study indicates that secondary metabolites can be manipulated due to solar light with the use of various HT coverings.

Keywords

Phytochemicals; Spectral Quality; Light Intensity; UPLC-MS

Introduction

Phenolic compounds are available to humans through plant consumption and play a diverse role in both plants and humans. As antioxidants in humans, phenolic compounds have shown antiinflammatory and antitumor activity in the prevention of coronary heart disease and cancer [1-3]. In plants, phenolic compounds also act as antioxidants [4,5], provide pigmentation [6-9], and have been shown to reduce electrolyte leakage in response to membrane oxidation [10].
Lettuce (Lactuca sativa), especially red leaf lettuce, is a good source of phenolic acids and flavonoids, including quercetin glucosides, anthocyanin conjugates, and caffeic acid derivatives [11-16]. More specifically, phenolic acids such as chicoric acid (dicaffeoyltartaric acid), chlorogenic acid (5-caffeoylquinic acid), caffeic acid, and ferulic acid have shown antidiabetic effects, prevented the formation of mutagenic compounds, and can inhibit lipid peroxidation [17-19]. Quercetin derivatives in lettuce include quercetin 3-O-(6”-O-malonyl)-glucoside, isoquercetin (quercetin-3-O-glucoside), rutin (quercetin- 3-O-rutinoside), and quercetin 3-O-glucoronide, which have shown anti-inflammatory effects [20]. The flavonols, isoquercetin, and rutin, contain a catechol group on the B ring which makes them highly active antioxidants [21].
Plants exposed to various environmental stresses during production will generate reactive oxygen species (ROS). Nonenzymatic antioxidants, such as phenolic compounds, are upregulated with increased stress to scavenge ROS and prevent further oxidative stress. Thus, it has been shown that phenylalanine ammonia-lyase (PAL), a key gateway enzyme of the phenylpropanoid pathway, is typically upregulated in response to environmental and biotic stresses (light, temperature, pest damage) [22-24]. Ultraviolet light (UV-B) is known to increase PAL activity in lettuce and cucumber [25,26]. Krizek DT, et al. (1998) found that ambient UV-B increased the PAL content by 27-83% in lettuce, which resulted in increased accumulations of anthocyanins and other flavonoids [26].

scroll up