REGULATION OF ETHYLENE BIOSYNTHESIS IN CLIMACTERIC APPLE FRUIT BY ELEVATED CO2 AND REDUCED O-2 ATMOSPHERES

Autocatalytic (System II) C2H4 biosynthesis in climacteric 'Golden Del icious' apples (Malus domestica Borkh) was effectively inhibited at 20 degrees C by atmospheres of 20% CO2-enriched air (17% O-2 + 63% N-2) or 0.25% O-2 (balance N-2). In vitro 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (ACC-S) activity of apples held in atmospheres of air + 20% CO2 or 0.25% O-2 was significantly inhibited when compared t o apples kept in air, and correlated well with fruit C2H4 production r ates. In vivo and in vitro ACC oxidase (ACC-O) activity of fruit held in atmospheres of air, air + 20% CO2 or 0.25% O-2 were similar when th e assays were performed under standard assay conditions (i.e., in vivo assay performed in air, in vitro assay performed in air + 6% CO2). Ho wever, if the in vivo or in vitro ACC-O enzyme activity assays were pe rformed in an atmosphere of 0.25% O-2, ACC-O catalytic competency and activity were significantly reduced. When the in vivo or in vitro ACC- O enzyme activity assays were performed in an atmosphere of air + 20% CO2, ACC-O enzyme activity was actually stimulated. These data indicat e that elevated levels of CO2 do not inhibit ACC-O catalytic competenc y. Western blot analysis revealed that ACC-O protein abundance was not significantly affected by any of the treatments tested, and only the 0.25% O-2 atmosphere significantly inhibited ACC-O activity. ACC-S act ivity was significantly reduced by atmospheres of air + 20% CO2 or 0.2 5% O-2 but not via direct inhibition of ACC-S catalytic competency.