Changes in acid-metabolising enzymes during low O2 treatment of satsuma mandarins
Marsh K.B.; Punter M.; Yearsley C.W. and Woolf A.
5th International Postharvest Symposium . Volume of Abstract . Verona, Italy 6-11 June 2004, p.97
2004
บทคัดย่อ
Changes in acid-metabolising enzymes during low O2 treatment of satsuma mandarins
New Zealand is a marginal climate for sweet citrus and we are studying postharvest treatments which alter the sweetness to acidity ratio of our cultivars. In association with this work we have been studying the metabolism of citric acid in order to refine enzyme targets and look for aspects of acidity control. Acid accumulation and release from the vacuole is dependent on relationships between acid metabolism in the mitochondria and the cytoplasm, and may also be controlled by transport functions across the vacuolar membrane. Our previous work has proposed that a shift to anaerobic respiration in mature citrus fruit may induce utilization of pH gradients across the vacuolar membrane and citric acid loss. Establishment of low O
2 atmospheres had little effect on aconitase or isocitrate dehydrogenase activity measured in extracts from juice sac cells 48 and 96 h after treatment establishment. The activity of the vacuolar pyrophatase (V-Ppase) enzyme was maintained under anaerobic conditions but the activity of the vacuolar ATPase (V-ATPase) was increased compared to that of tonoplast enriched fractions from control fruit stored at 20°C. As result the V-Ppase/V-Atpase ratio decreased in membranes extracted from fruit maintained in an anaerobic atmosphere. The total acidity levels increased between 48 and 96 h in anaerobic fruit but were reduced after 96 h for control fruit.
The results are discussed in relation to the role of the V-Ppase:V-ATPase activity ratio in controlling citric acid content of the vacuole.