Nitrogen Limited Red and Green Leaf Lettuce Accumulate Flavonoid Glycosides, Caffeic Acid Derivatives, and Sucrose while Losing Chlorophylls, Β-Carotene and Xanthophylls

Abstract

Reduction of nitrogen application in crop production is desirable for ecological and health-related reasons. Interestingly, nitrogen deficiency can lead to enhanced concentrations of polyphenols in plants. The reason for this is still under discussion. The plants’ response to low nitrogen concentration can interact with other factors, for example radiation intensity. We cultivated red and green leaf lettuce hydroponically in a Mediterranean greenhouse, supplying three different levels of nitrogen (12 mM, 3 mM, 0.75 mM), either in full or reduced (-50%) radiation intensity. In both red and green lettuce, we found clear effects of the nitrogen treatments on growth characteristics, phenolic and photosynthetic compounds, nitrogen, nitrate and carbon concentration of the plants. Interestingly, the concentrations of all main flavonoid glycosides, caffeic acid derivatives, and sucrose increased with decreasing nitrogen concentration, whereas those of chlorophylls, β-carotene, neoxanthin, lactucaxanthin, all trans- and cis-violaxanthin decreased. The constitutive concentrations of polyphenols were lower in the green cultivar, but their relative increase was more pronounced than in the red cultivar. The constitutive concentrations of chlorophylls, β-carotene, neoxanthin, all trans- and cis-violaxanthin were similar in red and green lettuce and with decreasing nitrogen concentration they declined to a similar extent in both cultivars. We only detected little influence of the radiation treatments, e.g. on anthocyanin concentration, and hardly any interaction between radiation and nitrogen concentration. Our results imply a greater physiological plasticity of green compared to the red lettuce regarding its phenolic compounds. They support the photoprotection theory regarding anthocyanins as well as the theory that the deamination activity of phenylalanine ammonia-lyase drives phenylpropanoid synthesis.

Macronutrient deprivation eustress elicits differential secondary metabolites in red and green‐pigmented butterhead lettuce grown in a closed soilless system

“…Analogous increases in the concentrations of chicoric acid and chlorogenic acid in lettuce were noted by Qadir et al . and Becker et al . under reduced N concentration.…”

“…Carotenoids are C 40 isoprenoid polyene plant compounds divided into oxygenated xyanthophylls and hydrocarbon carotenes, which are pigments of the light harvesting complex of lettuce photosystem II, which is also engaged in non‐photochemical quenching (heat dissipation). They are also potent reactive oxygen species (ROS) scavengers . The dietary intake of carotenoids is related to the reduction of degenerative diseases – specifically, chronic eye impairments, including cataract and macular degeneration, while their bioactive value is also studied for the prevention of cancers and cardiovascular diseases …”

“…Certain studies in controlled environments focused on nitrogen levels only, such as Quadir et al . and Becker et al ., or on nitrogen and phosphorus levels, such as Galieni et al . Nevertheless, nothing is known about the interaction between macronutrient deprivation, in the form of reduced macronutrient levels in the nutrient solution, and cultivar, in respect to lettuce PSM profiles.…”

Application of supplemental UV‐B radiation in pre‐harvest to enhance health‐promoting compounds accumulation in green and red lettuce

“…Anthocyanins can shield the photosynthetic apparatus from excess radiation (Steyn, Wand, Holcroft, & Jacobs, ), but they were not detected in green lettuce turning this lettuce potentially more vulnerable to radiation than red cultivars (Becker et al, ).…”

“…According to Becker et al (), lutein, β‐carotene, neoxanthin, and violaxanthin are major xanthophyll pigments of the light harvesting complex of lettuce photosystem II and light harvesting antenna. Apart from being accessory pigments, they are involved in non‐photochemical quenching and are also potent ROS scavengers (Jahns & Holzwarth, ).…”