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

Yoneyama, Tadakatsu, Arai, Kunio, Totsuka, Tsumugu
Plant & cell physiology 1980 v.21 no.8 pp. 1367-1381
Helianthus annuus, carbon, carbon dioxide, flowers, isotope labeling, leaves, nitrogen, nitrogen dioxide, photosynthates, roots, stable isotopes, stems
To investigate the long-distance transport of nitrogen and carbon from mature leaves, two stable isotopes, ¹⁵N and ¹³C, were introduced to a single mature sunflower leaf for less than 2 hr in the forms of NO ₂ and CO ₂, and the fate of ¹⁵N and ¹³C in plants was followed. In the first experiment, about 4 ppm ¹⁵NO ₂ was applied to a mature sunflower leaf for 65 min in light, and the fate of ¹⁵N was followed over 72 hr. ¹⁵NO ₂ 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 ¹⁵N 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 ¹⁵NO ₂ and 300–400 ppm ¹³CO ₂ 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 ¹³C transfer from the fed leaf took place within 1 day, whereas the transfer of ¹⁵N continued gradually during the experimental period after a small rapid transfer within 1 day. Just after isotope feeding, the ratios of transferred ¹³C to ¹⁵N 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, ¹⁵N did not show a significant loss while some ¹³C loss occurred during the experimental period. The transfer of ¹³C and ¹⁵N to the lower leaves was very low. ¹³C Studies showed that carbon of the flower originated from both reserved carbon and current photosynthates.