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

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–1 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–1, as did leaf N content, while C:N declined. Nitrogen Utilization Efficiency (NUtE) increased rapidly from 40 to 75 mg·L–1 applied N, leveling at 150 mg·L–1 with no subsequent effect of N concentrations between 400 and 2400 mg·L–1. Nitrate content rose significantly with increased N, particularly at 1200 and 2400 mg·L–1. 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–1 applied N, while highest oxygen radical absorption capacity (ORAC) values were found in leaves supplied with low N (40 to 400 mg·L–1). 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–1 or higher can result in reduced antioxidant capacity and mineral content in lettuce leaves.

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

This study investigated baby red lettuce leaves grown in an inert media and fertigated with 300 or 1000 mg·L-1 CaNO3-N (19% Ca;15.5% N), and the effects of altering CaNO3-N concentrations in the last 7 days prior to harvest on yield, antioxidant capacity, TPP and vitamin C content. Plants receiving 1000 mg·L-1 CaNO3-N had a significantly higher leaf FW than those fertilized with 300 mg·L-1 CaNO3-N after both 21 and 28 days. Pulsing plants with 1000 mg·L-1 CaNO3-N for 7 days after 21 days at 300 mg·L-1 CaNO3-N resulted in a significant increase in FW of 14%, compared with plants receiving 300 mg·L-1 CaNO3-N. After 28 days plants fertilized with nitrogen at 300 mg·L-1 CaNO3-N were significantly higher in TPP than those fertilized with 1000 mg·L-1 CaNO3-N. While there was no significant effect on TPP of a 7-day 1000 mg·L-1 CaNO3-N pulse added to 300 mg·L-1 CaNO3-N, a 7-day pulse of 300 mg·L-1 CaNO3-N added to 1000 mg·L-1 CaNO3-N resulted in a significant increase in TPP. These results indicate that reducing the N availability for 7 days prior to harvest can result in a significant increase in plant phenolic content with no adverse effect on yield.