Enhancement of Ethylene Release from Leaf Tissue during Glycolate Decarboxylation : A Possible Role for Photorespiration

When leaf discs of Xanthium strumarium L. and Salvia splendens L. are incubated in sealed flasks in the light, more C₂H₄ gas is released in the presence of added CO₂ (30-200 millimolar NaHCO₃) than without CO₂. In Salvia, the maximum rate of C₂H₄ release occurs when sufficient CO₂ (above 125 millimolar NaHCO₃) is added to saturate photosynthesis confirming previous studies. The maximum rate of C₂H₄ release from illuminated discs is similar to the rate in the dark with or without CO₂ in both species. Glycolate enhances a CO₂-dependent C₂H₄ evolution from illuminated leaf discs. However, the maximum rate of C₂H₄ release with glycolate is the same as that observed with saturating CO₂. When photosynthesis is inhibited by darkness or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, glycolate has no effect.

Studies with 2,3-¹⁴C]-1-aminocyclopropane-1-carboxylic acid (ACC) show that the pattern of C₂H₄ release and the specific activity of the ¹⁴C₂H₄ in the presence and absence of glycolate is similar to that described above, indicating that glycolate does not alter uptake of the exogenously supplied precursor (ACC) or stimulate C₂H₄ release from an endogenous source at appreciable rates. Glycolate oxidase in vitro generates H₂O₂ which stimulates a slow breakdown of ACC to C₂H₄, but since exogenous glycolate is oxidized to CO₂ in both the light and the dark it is argued that the glycolate-dependent increase in C₂H₄ release from illuminated leaf discs is not mediated directly by the action of enzymes of glycolate catabolism. The effects of glycolate and CO₂ are not easily explained by changes in stomatal resistance. The data support the view that glycolate decarboxylation at subsaturating levels of CO₂ in the light stimulates C₂H₄ release by raising the CO₂ level in the tissue.