Effect of Oxygen Concentration on ¹⁴C-Photoassimilate Transport from Leaves of Salvia splendens L.

Abstract

Partitioning and transport of recently fixed photosynthate was examined following "CCO2 pulse-labeling of intact, attached leaves of Salvia spkndens L. maintained in an atmosphere of 300 microliters per liter CO2 and 20, 210, or 500 milliliters per liter 02-Under conditions of increasing 02 (210, 500 milliliters per liter), a smaller percentage of the recently fixed "C in the leaf was allocated to starch, whereas a greater percentage of the fixed "C appeared in amino acids, particularly serine. The increase in "C in amino acids was reflected in material exported from source leaves. A higher percentage of "C in serine, glycine, and glutamate was recovered in petiole extracts when source leaves were maintained under elevated 02 levels. Although pool sizes of these amino acids were increased in both the leaves and petioles with increasing photorespiratory activity, no significant changes in either "C distribution or concentration of transport sugars (i.e. stachyose, sucrose, verbascose) were observed. The data indicate that, in addition to being recycled intracellularly into Calvin cycle intermediates, amino acids produced during photorespiration may also serve as transport metabolites, allowing the mobilization of both carbon and nitrogen from the leaf under conditions of limited photosynthesis.Although considerable interest exists in intracellular recycling of both C and N during photorespiration (24, 26), little importance is attached to the possible role of the key photorespiratory intermediates as intercellular metabolites. For example, amino acids such as serine and glutamate, which are generated during the recycling of photorespiratory C and N, are also ubiquitous components of phloem exudates (30). This observation challenges the view that key photorespiratory intermediates are conservatively cycled within specific organelles or even within the cells where they are synthesized.The effects of varying 02 (3,8,16,24) and CO2 (24,25,28) levels on metabolism in source leaves are well documented.

Photorespiration Redesigned

“…Others have argued that photorespiration might participate in defense reactions by producing H 2 O 2 (Taler et al, 2004) or that Gly and Ser produced during photorespiration are used by the plant for multiple other purposes (e.g. Madore and Grodzinski, 1984). An interesting additional argument comes from a recent study by Bloom et al (2010) on nitrogen assimilation under conditions of low RuBP oxygenation (low O 2 or high CO 2 ).…”

Peroxisomal Malate Dehydrogenase Is Not Essential for Photorespiration in Arabidopsis But Its Absence Causes an Increase in the Stoichiometry of Photorespiratory CO2 Release

“…Additionally, the export of Ser and/or Gly via the vascular system (Madore and Grodzinski, 1984;Wingler et al, 2000) or the involvement of Ser/Gly in one-carbon metabolism and protein synthesis (Hanson and Roje, 2001) would alter the flow of carbon through the photorespiratory cycle. If large quantities of Gly were lost from the photorespiratory cycle, due to alternative metabolism or export to vascular tissue, this would decrease a and thus G, since the amount of CO 2 released per Rubisco oxygenation reaction would approach zero.…”

Patterns of Assimilate Production and Translocation in Muskmelon (Cucumis melo L.)

“…After lyophilized leaf tissue samples were weighed, they were extracted in 80% ethanol and the extracts partitioned by ion exchange chromatography into neutral and basic fractions as described previously (10). The starch content of the extracted residue was determined following digestion with amyloglucosidase (9) by measuring the released glucose using a commercially available glucose detection kit (Sigma HK 10).…”