Broccoli yield and nitrogen composition in response to different management regimes

A three‐year study was undertaken to investigate the response of field‐grown broccoli (Brassica oleracea var. italica) to different management regimes (i.e. various NH+ 4 :NO− 3 ratios, the addition of a nitrification inhibitor known as nitrapyrin, and boron nutrition). None of the treatments influenced the harvestable yield and the composition of N in xylem and phloem saps and plant tissue. The total N (NH+ 4, NO− 3 and amino acids) and the mol % NH+ 4 in the xylem sap decreased over the growing season. The NH+ 4 constituted up to 20% of the xylem sap‐N, suggesting that this N form, as well as NO− 3, is being absorbed from the soil. The substantial recovery of NO− 3 in xylem sap (30–70% N) indicates that with an adequate N supply, at least a portion of the incoming NO− 3 is reduced in the shoot. The concentrations of NH+ 4, NO− 3, amino acids and proteins in the tissues declined during the growing season and from the bottom to the top of the shoot. The concentrations of NO− 3 in the florets, which ranged from 1 to 154 mg N kg‐1 fresh weight, are considered to be under permissible levels by European standards, whereas some of those in the foliage exceeded the permissible levels. Cultivar differences in NO− 3 accumulation were evident within and between study sites.