Major nitrogen compounds transported in xylem vessels from roots to top in Citrus trees [Citrus unshiu, satsuma mandarin, nitrogen transport, xylem sap, nitrate, asparagine, glutamine]
1981
Kato, T. (Shikoku National Agricultural Experiment Station, Kagawa-ken, Zentsuji (Japan). Plant Nutrition Lab.)
Four-year-old citrus tree (Citrus unshiu Marcovitch) were fed via the roots with (('15)NH(,4))(,2)SO(,4) or K('15)NO(,3) as a nitrogen source. Nitrogenous compounds and their isotopic abundances in fine roots and xylem sap from trunks were assayed in order to obtain information on the species of nitrogen released by the root system into the ascending xylem stream. Arginine, asparagine, nitrate and proline in xylem sap accounted for 48, 21, 12 13 and 10 per cent, respectively, of the total nitrogenous constituents in the sap. However, in the trees fed with labelled ammonium the main nitrogenous compound labelled with ('15)N in the xylem sap was asparagine and glutamine, which accounted for 79 per cent and 18 per cent, respectively, of total labelled nitrogen. In the xylem sap of trees fed with labelled nitrate, nitrate accounted for 94 per cent of total labelled nitrogen. Nitrate and asparagine followed by glutamine showed the highest ratios of isotopic abundance in xylem sap as compared to fine roots. Proline and arginine had much lower ratios. These results indicate that nitrate, asparagine and glutamine are the main nitrogenous compounds released by the roots to the xylem stream, whereas arginine and proline are released into the xylem vessels by the trunk tissues. Furthermore, nitrate and asparagine are probably in steady movement upward in the trunk xylem, whereas glutamine is more easily taken up by the trunk tissues than nitrate and asparagine.
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