Changes in Sugar, Titratable Acidity, and Ascorbic Acid Content during Fruit Development in Sea Buckthorn (Hippophae rhamnoides L.)

Abstract

We compared the changes of sugar content and titratable acidity throughout fruit development in two cultivars, ‘Russian Orange’ from subspecies (ssp.) mongolica and ‘Hergo’ from ssp. rhamnoides of sea buckthorn (Hippophae rhamnoides L.). In ‘Russian Orange’ fruit, glucose content increased remarkably from mid-July to early August, whereas the contents of fructose, sucrose and ethylglucose did not increase. In ‘Hergo’ fruit, the content of each sugar did not increase during fruit development. Fruit titratable acidity was low during early fruit development, then remarkably increased from mid-July to early August and decreased at harvesting time in both cultivars. The sugar/acid ratio in ripe fruit was 1.14 in ‘Russian Orange’ and 0.34 in ‘Hergo’, indicating that ‘Russian Orange’ fruit has better flavor. In tracer experiments using 14CO2, 14C-sucrose was found to be the major sugar in the leaves and the stems, suggesting that sucrose is the primary product of photosynthate and a translocatable sugar in sea buckthorn plants. Ascorbic acid content in ‘Russian Orange’ fruit decreased during fruit development, while the content in ‘Hergo’ fruit was roughly constant. ‘Russian Orange’ fruit showed high L-galactose dehydrogenase (L-GDH) activity. In tracer experiments, 14C-ascorbic acid was synthesized from L-14C-galactose and D-14C-mannose. These results suggest that the mannose/L-galactose pathway plays a role in the synthesis of ascorbic acid in sea buckthorn fruit.