Enhancement of Ethylene Release from Leaf Tissue during Glycolate Decarboxylation

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Abstract
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 mm 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. 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. Glycolate decarboxylation at subsaturating levels of CO2 in the light apparently stimulates C2H4 release by raising the CO2 level in the tissue.