Translocation of N to and from barley roots: its dependence on local nitrate supply in split‐root culture

The relationship between availability of external nitrate and N translocation between root and shoot was studied in N‐limited barley (Hordeum vulgare L. cv. Golf). Nitrate‐N was added at a relative rate (i.e. N added per unit time and unit N in plant biomass) of (1.09 da‐1, and distributed between the subroots at ratios of 50:50 or 80:20. The plants were grown for 13 days under these conditions of nitrate nutrition, and for another three days with the nitrate distribution reversed from 80:20 to 20:80. The nitrale‐N doses thus experienced by individual subroots ranged from 2 to 11 mg N g‐1 root dry weight day‐1 . 15N‐Nitrate labellings were performed after 2 to 3 and 12 to 13 days of nitrate nutrition. and 2 to 3 days after reversal of nitrate additions. For all treatments, between 60 and 82% of the absorbed label initially left the root, and between 25 and 55% of the label recovered in roots had been supplied (cycled) via the shoot. Labelling of xylem N at the end of the 24‐h labelling period ranged from to 36 to 46% indicating that a substantial fraction of the N in the xylem had been absorbed by the plant prior to labelling. It is concluded that cycling of N to roots, and cycling of N in the plant as a whole, is substantial also during N‐limited growth. N allocation to roots increased with external nitrate dose. An increased utilization of non‐translocated N as well as an increased translocation of N from the shoot contributed to this effect. Thus, the results indicate that increased external availability of N also increased the sink strength of the root for cycling N.