Nitrogen partitioning in greenhouse‐grown broccoli in response to varying NH+4:NO−3ratios

Broccoli (Brassica oleracea var. italica) plants were grown in the greenhouse and supplied continuously with 18 mM N in the following NH+ 4:NO− 3 ratios: 100:0, 75:25, 50:50, 25:75, 0:100. At commercial maturity, the plant characteristics and partitioning of nitrogen in xylem and phloem saps and in plant tissue were determined. Plants fed solely with NH+ 4 were stunted, exhibited signs of marginal necrosis on the lower leaves, and accumulated NH+ 4 in the foliage. The maximum yield and shortest harvest time, together with minimal NO− 3 and NH+ 4 accumulation were found at a ratio of 75:25. Ammonium concentrations in xylem sap decreased linearly with decreasing NH+ 4 portion in the nutrient solution, whereas the NO− 3 concentrations reached a maximum when NO− 3 constituted 50% of the N supply. The glutamate family dominated the amino acid composition of both xylem and phloem saps, but did not vary much with NH+ 4:NO− 3 ratio. It is suggested that the NH+ 4 concentration in xylem sap may be used to assess NH+ 4 vs NO− 3 utilization by broccoli grown under field conditions.