This study aimed to use different nitrogen fertilizer regimes to produce Butterhead lettuce with such large differences in nitrate content that they could be used as treatment and placebo to study the effect of inorganic nitrate on human health. Plants were grown under controlled conditions at 27/23 °C day/night with a relatively low photosynthetically active radiation (PAR) of 150 μmol m-2 s-1 for 14 h day-1 and nitrogen supplies ranging from 26 to 154 ppm of N as ammonium nitrate in the fertigation solution. This resulted in contrasting high (∼1078 mg nitrate 100 g-1 FW) or low (∼6 mg 100 g-1) nitrate contents in the leaves. Contents of carotenoids and chlorophylls in fresh weight did not differ significantly between the highest and the lowest N-supply levels. However, increased nitrogen supply reduced contents of phenolic compounds from 154 to 22 mg 100 g-1 FW, dry matter content from 8.9% to 4.6%, and fresh weight per plant from 108.52 to 47.57 g/plant FW (all P < 0.001). Thus, while fertilizer treatments can provide lettuce with substantially different nitrate contents, maintaining similar pigment contents (color), they also strongly influence the contents of phenolic acids and flavones.
Keywords: Lactuca sativa var. capitata; bioactive compounds; light intensity; plant secondary metabolites; quality management.