Transfer of nitrogen and carbon from a mature sunflower leaf-15NO2 and 13CO2 feeding studies

To investigate the long-distance transport of nitrogen and carbon from mature leaves, two stable isotopes, (15)N and (13)C, were introduced to a single mature sunflower leaf for less than 2 hr in the forms of NO2 and CO2, and the fate of (15)N and (13)C in plants was followed. In the first experiment, about 4 ppm (15)NO2 was applied to a mature sunflower leaf for 65 min in light, and the fate of (15)N was followed over 72 hr. (15)NO2 absorbed in sunflower was first incorporated into the ethanol-soluble fraction, then gradually incorporated into the ethanol-insoluble fraction: after 24 hr, only 12% remained in the soluble fraction in the fed leaf. Some (15)N was transferred from the fed leaf, first to the stems and next to the young growing leaves and roots, although negligible transfer to other mature leaves was detected. In the second experiment, 3.1 ppm (15)NO2 and 300-400 ppm (13)CO2 were simultaneously introduced to a single mature leaf for 110 min in light, and the fate of the two isotopes was followed for 28 days. Most of the (13)C transfer from the fed leaf took place within 1 day, whereas the transfer of (15)N continued gradually during the experimental period after a small rapid transfer within 1 day. Just after isotope feeding, the ratios of transferred (13)C to (15)N were high in all parts and remained high in the lower stem and the root, although they decreased very rapidly in the upper leaves and the upper stem. In the root, (15)N did not show a significant loss while some (13)C loss occurred during the experimental period. The transfer of (13)C and (15)N to the lower leaves was very low. (13)C Studies showed that carbon of the flower originated from both reserved carbon and current photosynthates.