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Keywords:

  • Nitrogen translocation;
  • phloem transport;
  • root growth;
  • specific mass transfer

The root system of wheat seedlings (Triticum aestivum L. SUN 9E) was pruned to two seminal roots. One of the roots was supplied with different levels of NO3, the other was deprived of N. Root respiration and the increment of C and N in roots and shoots were measured to determine the C/N ratio of the phloem sap feeding the N-deprived roots. Thus it was possible to determine translocation of N from the shoots to the roots. It was calculated that the C/N ratio of phloem sap feeding roots of plants growing at optimal and suboptimal N supply was ca 54. A supra-optimal N supply reduced, whilst shading increased, the C/N ratio of phloem sap. At optimal N supply 11% of all N transported to the shoots was retranslocated to the roots. Both a supra-optimal and a limiting N supply increased translocation of N back to the roots to 18% of the N translocated to the shoot, whilst shading of the plants decreased the proportion cycled to 7%. At the optimal N supply, 40% more N was translocated to the roots from the shoot than was incorporated by them. At a lower supply of N, 80% more N was imported from the shoots than was incorporated by these roots. It is suggested that the distribution of N between roots and shoots predominantly occurs in the shoots. The specific mass transfer rate in seminal roots was determined. The highest value was found for roots grown with an optimal N supply: 1.1 mg carbohydrate s−1 cm−2 (sieve tube) which is well within the range observed for other plant organs. Roots supplied with NO3 produced more and longer laterals than N-deprived roots. It is suggested that this is due to the effect of NO3 on import of carbon and other components transported in the mass flow with carbon.