PHOSPHOENOLPYRUVATE CARBOXYLASE AND CARBON ECONOMY OF APPLE SEEDLINGS

Seeds of apple cv. Golden Delicious were germinated and cultivated in the greenhouse until the third leaf emerged. Respiration of germinating seeds or photosynthesis of the first leaves was measured by infra-red gas analysis and porometry, respectively. To study the role of phosphoenolpyruvate carboxylase (PEPC), the dominant carboxylase in the carbon economy, its CO2 refixation potential was related to the amount of CO2 lost in respiration. With a range of 0.2 (dry seeds) to 18 (cotyledons) mu-mol CO2 h-1 g-1, PEPC activity resembled or exceeded the amount of CO2 lost in respiration before the third leaf developed. It is concluded that PEPC largely contributes to economize the carbon metabolism of apple seedlings before they become photosynthetically competent.