CO₂-enhanced C₂H₄ Production in Tissues of Imbibed Cocklebur Seeds

Abstract

Both endogenous C₂H₄ production and the conversion of 1-aminocyclopropane-1-carboxylic acid (ACC) to C₂H₄ in the axial tissues of upper cocklebur (Xanthium pennsylvanicum Wallr.) seeds were enhanced by CO₂, and this enhancement was accompanied by increased axial growth. However, in the seed-coat (endosperm layer), which had no growth potential, the endogenous C₂H₄ production was preferentially promoted by CO₂. CO₂ increased the content of ACC in both tissues. The maximal ACC-C₂H₄ conversion in the axial tissue occurred at CO₂ concentrations that varied from 1 to 10%, depending upon the O₂ tension of the ambient atmosphere. Similar to the inhibition of seed respiration by NaN₃, the ACC-C₂H₄ conversion was more strongly inhibited by KCN or NaN₃ in the presence of CO₂ than in its absence. This suggests an increased involvement of the cytochrome path in cocklebur seed respiration in the presence of CO₂. This suggestion was supported by data showing that CO₂ elevated the ATP level in the axial tissues. Little CO₂-enhancement of the ACC-C₂H₂ conversion occurred when the ATP level of seeds had been reduced by KCN pretreatment and maintained low in a hypoxia during a subsequent incubation period. These results suggested that CO₂ enhances C2H4 production of imbibed cocklebur seeds in two different ways: through both increases of ACC supply and of ACC-C₂H₂ conversion, probably resulting from the CO₂-induced increase in the cytochrome path flux or in the resultant ATP level.