Translocation Profiles of [11C] and [13N]-Labelled Metabolites after Assimilation of11CO2and [13N]-Labelled Ammonia Gas by Leaves ofHelianthus annuusL. andLupinus albusL.

Publisher Website
Google Scholar
Abstract
Tracer amounts of atmospheric [13N]-Iabelled ammonia gas, were absorbed by leaves of Lupinus albus and Helianthus annuus in both the light and the dark. Exogenous [13N]-ammonia was only absorbed in the dark when the feeding occurred shortly after a period of illumination and the tissue was not depleted of its carbohydrate reserves (e.g. starch). Incorporation of the [13N]-ammonia appeared to occur via the leaf glutamine synthetase/glutamate synthase (GS/GOGAT) cycle since 2.0 mol m−3 MSX, an inhibitor of the GS reduced uptake in both the light and dark. Photosynthetic incorporation of 11CO2 was not affected by this treatment The rate of movement of [13N]-assimilates in the petiole of attached leaves of Helianthus and Lupinus was similar to that of the 11Cl-photo assimilates. Export of both [13N] and [11C]-Iabelled assimilates from the leaf and movement in the petiole in both the light and the dark was inhibited by source leaf anoxia (i.e. nitrogen gas). Translocation was re-established at the same rate when the feed leaf was exposed to gas containing more than 2% O2 which permitted dark respiration to proceed. After an initial feeding of either 11CO2 or [13N]-ammonia at ambient (21%) O2 exposure of the source leaf to 2% O2, or 50% O2 did not alter the rates of translocation, indicating that changes in photosynthetic activity in the source leaf due to photorespiratory activity need not markedly alter, at least during the short period, the loading and translocation of either [11C ] or [13N]-labelled leaf products.