Reduced nitrogen availability during growth improves quality in red oak lettuce leaves by minimizing nitrate content, and increasing antioxidant capacity and leaf mineral content

Abstract

Overuse of N in lettuce production can lead to environmental problems caused by leaching and the accumulation of harmful nitrates in edible tissues. This study investigated the effect of applied nitrogen (N) concentrations between 40 and 2400 mg·L<sup>–1</sup> on growth, nitrate accumulation, mineral leaf content, and antioxidant capacity in Oak Leaf lettuce cv. “Shiraz” grown under hydroponic conditions in Australia. Yield (g FW) increased with nitrogen (N) application rate up to 1200 mg·L<sup>–1</sup>, as did leaf N content, while C:N declined. Nitrogen Utilization Efficiency (NUtE) increased rapidly from 40 to 75 mg·L<sup>–1</sup> applied N, leveling at 150 mg·L<sup>–1</sup> with no subsequent effect of N concentrations between 400 and 2400 mg·L<sup>–1</sup>. Nitrate content rose significantly with increased N, particularly at 1200 and 2400 mg·L<sup>–1</sup>. Leaf total plant phenolic content (TPP) and antioxidant capacity (measured by ferric reducing antioxidant power—FRAP) were both maximal at 75 and 400 mg·L<sup>–1</sup> applied N, while highest oxygen radical absorption capacity (ORAC) values were found in leaves supplied with low N (40 to 400 mg·L<sup>–1</sup>). Applied N as calcium nitrate also significantly affected leaf mineral content as B, Mg, Mn, and Zn significantly decreased with increasing N. These results indicate that N applications of 1200 mg·L<sup>–1</sup> or higher can result in reduced antioxidant capacity and mineral content in lettuce leaves

Interaction of soil moisture and excess of boron and nitrogen on lettuce growth and quality

“…Some quality indices of Crisp-head lettuce grown under high N (200 kg/ha), like dry matter, sugar and vitamin C contents, were declined (Poulsen et al 1995). Furthermore, Butter-head, Romaine and Oak leaf lettuce quality was maximized by as little as 80 kg/ha, significantly less than the normal recommended N application rate for field-grown lettuce (D' Antuono, Neri 2001; Stefanelli et al 2011).…”

“…Lettuce has relatively high water requirements, thus soil moisture and rainfall shortage would seriously stunt growth and head quality. Irrigation greatly reduces risk of crop failure (Stefanelli et al 2011). Nitrogen (N) is an indispensable element for the synthesis of proteins such as enzymes, which are responsible for the production of all the cellular components and secondary metabolites required for the development of the plant, and hence nitrate availability in the soil and plant tissue concentration can limit plant growth (Lawlor 2002). Nitrate reduction catalysed by nitrate reductase (NR), is the primary step in the nitrate assimilation process and it is essential for the production of ammonium to be incorporated into carbon skeletons for amino acid biosynthesis (Matas et al 2009).…”

“…Minimizing N availability in lettuce, while maintaining yield and quality is a subject of recent studies (Stefanelli et al 2010(Stefanelli et al , 2011. Fortification of leafy vegetables could be a successful strategy for improving human diets, resulting in an increase in the value and quality of the production.…”

Nitrate Content in Minimally Processed Lettuce (Lactuca sativa L.) as Affected by Fluorescent Light Exposure During Storage

“…Accumulation of nitrates in lettuce also has been observed to be affected by light intensity. Lettuce grown in the lower light intensity accumulates the higher level of nitrate [1,5,6]. The increased nitrate at low light intensity is due to reduced activity of nitrate reductase [7].…”

Reduction in Nitrogen Prior to Harvest Increases Phenolic Content of Baby Red Lettuce Leaves

“…Nutrition during production can have a significant effect on antioxidant content in lettuce with low N availability tending to increase antioxidant activity which was attributed to increased phenolic content [9]. Limited N resulted in higher total plant phenolic (TPP) content in basil leaves [10] while, conversely, increased N via foliar urea application resulted in an increase in free radical scavenging activity in lettuce [11].…”