Changes in Organic Acid Contents and Related Metabolic Enzyme Activities at Different Stages of Growth of Two Tangerine Cultivars

Citric acid accumulation and decline and its related enzymes were studied at different stages of growth of two tangerine cultivars, ‘Sai Num Phueng’ and ‘See Thong’. Citric acidcontent increased to the peak at early growth stage (16-24 weeks after full bloom, WAF), then decreased rapidly for 28-34 WAFand slightly decreased during maturation (35-37 WAF). ‘Sai Num Phueng’ had higher citric acid content than ‘See Thong’. The activities of enzymes citrate synthase in mitochondrial fraction, aconitase in both mitochondrial and cytosolic fractions and NADP-isocitrate dehydrogenase (IDH) in cytosolic fraction were determined. The changes in these three enzyme activities followed similar patterns in both cultivars. For 16-24 WAF, citrate synthase activity increased and aconitase activity decreased in mitochondrial fraction, which accelerated the increasingin citric acid content of both cultivars. Therefore, citrate synthase and aconitase enzymes in mitochondria were essential forcitric acid accumulation at early growth stage. During 28-34 WAF, aconitase and NADP-IDH activities in cytosolic fraction were induced with the concomitant decrease of citric acid content. Thus, aconitase in cytosol played a key role in the reduction of citric acid. NADP-IDH could be consisted with aconitase for enhancement the reduction of citric acid in cytosol. Aconitase activity could account for the difference in citric acid contentbetween ‘Sai Num Phueng’ and ‘See Thong’. It was also found that the level of citrate synthase activity was quite similar between the two cultivars but aconitase activity was lower in ‘Sai Num Phueng’ than in ‘See Thong’.