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Pseudobulbs are carbohydrate storage organs in Oncidesa. A current pseudobulb forms on a developing vegetative shoot in each growth cycle and it becomes a back pseudobulb when the next vegetative shoot emerges. Both current and back pseudobulbs store carbohydrates, but their functions might differ because the inflorescence emerges from the new (current) shoot after the shoot has developed to a certain stage. This study investigated carbohydrate storage and use in current and back pseudobulbs. We analyzed carbohydrates in the current pseudobulb at five stages during inflorescence development. Glucose and fructose were the highest in the current pseudobulb in the first two stages, when the inflorescence was 10 to 35 cm tall. Then, both glucose and fructose decreased in the following stages to support inflorescence development, but starch increased at that time. In addition, we used Oncidesa with one or two new vegetative shoots to study the use of carbohydrates in pseudobulbs during growth cycles. In both plants with one or two shoots, glucose and fructose accumulated when current pseudobulbs formed, but plants with two new shoots had smaller current pseudobulbs and lower monosaccharide concentrations. Plants with two shoots also consumed more starch in all back pseudobulbs, whereas in the plants with one new shoot, starch only decreased significantly in the first back pseudobulb, which was closer to the new shoot. In addition, if an inflorescence did not develop in the previous growth cycle, new shoots used the monosaccharides that remained in the youngest back pseudobulb for growth; at the same time, starch accumulated in all back pseudobulbs. The current pseudobulb was the actively growing part. Its main carbohydrates were monosaccharides, which accounted for 25% of dry weight and Oncidesa used these carbohydrates mainly for inflorescence growth. After monosaccharides in the pseudobulb were used, the pseudobulb began to store starch. Back pseudobulbs, in which >50% of dry weight was starch, were the primary storage organs that supported new vegetative shoot growth and partly supported later inflorescence development that emerged from the new (current) shoot.

Oncidesa Gower Ramsey ‘Honey Angel’ is a cut flower crop of high economic value worldwide. The regulation of flowering is important for cut flower production scheduling. However, its flowering transition mechanism is still unclear. Oncidesa usually flowers at the end of the growth cycle for each pseudobulb; this timing is probably related to carbohydrate accumulation. During this study, we investigated the carbohydrates in the pseudobulbs from juvenile plants to adult plants and compared the carbohydrates in flowering and nonflowering adult plants. The current pseudobulb and back pseudobulbs of the plants at 0, 0.5, 1.0, 1.5, and 2.0 years after having been moved out of the tissue culture flask were collected. The first pseudobulb formed at 0.5 years, and plants had fulfilled four growth cycles and flowered at 2.0 years. Each successive current shoot grew larger and the back shoot number progressively increased after each new growth cycle. The concentration of total soluble sugars in the current shoot increased from 5.5% of dry weight at 0.5 years to 20.2% of dry weight at 1.5 years. Conversely, the starch concentration decreased in the current pseudobulb as the plants matured. The starch concentration in the back pseudobulbs did not change when the plant grew a new shoot. The starch concentrations in the back pseudobulbs ranged from 33.2% to 57.5% of dry weight, but the combined content of starch in all of the back pseudobulbs increased significantly from 168 mg at 0.5 years to 4608 mg at 2.0 years because of the increasing number of back shoots. The starch in the first back pseudobulb of the nonflowering adult plants accounted for 18.0% of dry weight, which was lower than that of the flowering plants (48.3%). There was no significant difference in total soluble sugars in the current pseudobulb of the nonflowering and flowering plants. Overall, we revealed that the increase in the back shoot number increased the total amount of reserve carbohydrates as the plant reached reproductive maturity. A low starch level was observed in nonflowering adult plants. In both cases, flowering plants had higher starch storage in the back pseudobulbs, suggesting that carbohydrates might regulate the flowering of Oncidesa Gower Ramsey ‘Honey Angel’.