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 C(2)H(4) gas is released in the presence of added CO(2) (30-200 millimolar NaHCO(3)) than without CO(2). In Salvia, the maximum rate of C(2)H(4) release occurs when sufficient CO(2) (above 125 millimolar NaHCO(3)) is added to saturate photosynthesis confirming previous studies. The maximum rate of C(2)H(4) release from illuminated discs is similar to the rate in the dark with or without CO(2) in both species. Glycolate enhances a CO(2)-dependent C(2)H(4) evolution from illuminated leaf discs. However, the maximum rate of C(2)H(4) release with glycolate is the same as that observed with saturating CO(2). When photosynthesis is inhibited by darkness or by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, glycolate has no effect.Studies with [2,3-(14)C]-1-aminocyclopropane-1-carboxylic acid (ACC) show that the pattern of C(2)H(4) release and the specific activity of the (14)C(2)H(4) 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 C(2)H(4) release from an endogenous source at appreciable rates. Glycolate oxidase in vitro generates H(2)O(2) which stimulates a slow breakdown of ACC to C(2)H(4), but since exogenous glycolate is oxidized to CO(2) in both the light and the dark it is argued that the glycolate-dependent increase in C(2)H(4) release from illuminated leaf discs is not mediated directly by the action of enzymes of glycolate catabolism. The effects of glycolate and CO(2) are not easily explained by changes in stomatal resistance. The data support the view that glycolate decarboxylation at subsaturating levels of CO(2) in the light stimulates C(2)H(4) release by raising the CO(2) level in the tissue.