THE EFFECT OF NITROGEN SUPPLY TO URTICA-DIOICA L PLANTS ON THE DISTRIBUTION OF ASSIMILATE BETWEEN SHOOT AND ROOTS

In this study the influence of nitrogen nutrition on the patterns of carbon distribution was investigated with Urtica dioica. The nettles were grown in sand culture at 3 levels of NO3-, namely 3 (low), 15 (medium) and 22 (high) mM. These levels encompassed a range within which nitrogen did not affect total biomass production. The ratio of root: shoot biomass of the low nitrogen plants was, however, significantly higher than that of the nettles grown at medium and high N supply. Carbon allocation from one leaf of each pair of leaves was examined after a (CO2)-C-14-pulse and a subsequent C-14 distribution period of one night. Only the youngest two leaf pairs did not export assimilates. Carbon (C-14) export to the shoot apex and to the roots, as measured at the individual nodes responded to the nitrogen status: At medium and high nitrogen supply the 3rd, 4th and 5th leaf pairs exported to the shoot apex, while lower leaves exported to the root. At low nitrogen supply only the 3rd leaf exported towards the shoot apex. The results illustrate the plastic response of carbon distribution patterns to the nitrogen supply, even when net photosynthesis, carbon export from the source leaves and biomass production were not affected by the nitrogen supply to the plant.