Effects of different nitrogen supply level on the phenolic content and antioxidant activity in broccoli head

The effects of different nitrogen supply level (5, 10, 15 and 20 mmol/L) on the head growth, total phenolic, the flavonoids contents and antioxidant activity were investigated in two broccoli (Brassica olerecea var. italica) cultivars (middle late maturity cv. Malasong and middle early maturity cv. Lüyou No.2 ) by inorganic medium culture. The results showed that plant height, head diameter, head fresh and dry weight increased significantly with increasing nitrogen applied. However, total phenolic content decreased with increasing nitrogen supply, especially when applied nitrogen level was higher than 10 mmol/L. Quercetin and kaempferol were identified in broccoli head, and kaempferol content was 1.48 times higher than quercetin content, and their contents reached highest level when applied nitrogen concentration was 5 mmol/L. Increasing nitrogen supply decreased DPPH radical scavenging and FRAP value, and markedly inhibited the antioxidant acitivity in broccoli head when nitrogen supply level was 20 mmol/L. A significantly positive correlation was found between FRAP value and the phenolic content in two cultivars. Compared with two cultivars, the head growth of cv. Malasong was better than that of cv. Lüyou No.2, but was contrary to total phenolic and antioxidant activity. These results clearly demonstrated the phenolic substances and antioxidant activity in broccoli were strongly affected by fertilizer regimes especially nitrogen application level.