Mitochondrial respiration dynamics in grape exocarp: electron partitioning to AOX pathway during pre-veraison development

Fruit respiration varies throughout development, reflecting dynamic changes in energy demand and metabolic activity. Grapes grow more rapidly during the preveraison stage, as cell division and expansion drive berry enlargement before the onset of ripening. The plant mitochondrial electron transport chain contains two terminal oxidases; alternative oxidase (AOX) and cytochrome c oxidase (COX), the activities of which are central to predicting patterns of growth in plant organs. However, no previous studies have tested their activities in fruits when growth respiration predominates. Studies about the effect of the genetic variability in grape respiration during phase I of development are scarce. We characterized respiratory parameters in vivo (AOX and COX), and modelled ATP production, at five consecutive weeks during preveraison stage in grape exocarps of two varieties of Vitis vinifera (Merlot and Callet) under well-watered conditions. We also evaluated physical and chemical parameters in fruits (weight, diameter, malic acid and total soluble solids). We identified relationships between respiration via COX activity and carbon accumulation that indicated that ATP production is crucial for the growth of grape berries before the lag phase. After this, AOX activity was increased in parallel to malic acid content in fruit, to lower the reduction level of ETC components when ATP is less required for carbon accumulation. Further studies in more fruit layers, and during ripening, are needed to identify the metabolic role of AOX in grapes.