The Influence of Temperature on Malic Acid Metabolism in Grape Berries. II. Temperature Responses of Net Dark CO2Fixation and Malic Acid Pools

The influence of temperature on net dark CO₂ fixation and on the turnover of malic acid in excised, immature Carignane grape berries was examined. Between 10° and 40°C, the rate of net dark CO₂ fixation increased without reaching an optimum. Dark fixation of CO₂ was apparently catalyzed into malic acid by phosphoenolpyruvate (PEP) carboxylase and malic dehydrogenase enzymes. The kinetic labeling of malic acid was followed at 10, 20, 30, and 40°C with a continuous ¹⁴CO₂-labeling technique, and metabolic pool sizes were estimated. As temperature increased, the malic acid metabolic pool size decreased, indicating that turnover was favored at higher temperatures, and synthesis at lower temperatures. The amount of net dark CO₂ fixation was related inversely to malic acid concentration in the berry. The effects of temperature on exchange between metabolic and storage pools of malic acid are discussed.