RESPIRATORY METABOLISM OF PEAR FRUIT AND CULTURED-CELLS EXPOSED TO HYPOXIC ATMOSPHERES - ASSOCIATED CHANGE IN ACTIVITIES OF KEY ENZYMES

'Bartlett' pears (Pyrus communis L.) at two physiological stages, clim acteric minimum or approaching the climacteric peak as achieved via st orage for 2 or 8 weeks in air at 0C, respectively, were either ripened at 20C in air immediately or after exposure to 0.25 % O-2 for 4 days at 20C. Fruit stored for 2 weeks had relatively stable phosphofructoki nase (PFK), pyrophosphate : fru-6-P phosphotransferase (PFP), and pyru vate kinase (PK) activities but decreasing succinate dehydrogenase (SD H) activities during ripening in air. Similar fruit treated with 0.25% O-2 had slightly increased PFK, PFP, and SDH activities and decreased PK activity. Fruit stored for 8 weeks exhibited higher levels of PFK and PFP activity upon transfer to 20C, in accordance with their more a dvanced physiological state. In general, the enzymic changes in these fruit upon exposure to 0.25% O-2 and subsequent ripening in air were s imilar to those observed in the less-mature counterparts, most notable being an increase in mitochondrial SDH. Exposure of suspension-cultur ed pear fruit cells' to hypoxia resulted in an accentuated rise in pho sphoenolpyruvate carboxykinase activity and a dramatic rise in SDH act ivity upon transfer to air. Taken in concert, the enzymic analysis sup ports the hypothesis that the rise in succinate levels observed in hyp oxic fruit tissues is the result of a partial reductive tricarboxylic acid cycle. Cytochrome oxidase activity did not change during hypoxia whereas soluble peroxidase decreased somewhat, perhaps a reflection of their Michaelis constants for O-2.