TRANSLOCATION OF N TO AND FROM BARLEY ROOTS - ITS DEPENDENCE ON LOCALNITRATE SUPPLY IN SPLIT-ROOT CULTURE

The relationship between availability of external nitrate and N transl ocation between root and shoot was studied in N-limited barley (Hordeu m 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 0.09 day-1, and d istributed between the subroots at ratios of 50:50 or 80:20. The plant s 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 nitrate-N doses thus experienced by individual su broots ranged from 2 to 11 mg N g-1 root dry weight day-1. N-15-Nitrat e 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 al l 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 b een supplied (cycled) via the shoot. Labelling of xylem N at the end o f 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 in creased translocation of N from the shoot contributed to this effect. Thus, the results indicate that increased external availability of N a lso increased the sink strength of the root for cycling N.