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Pseudobulbs of orchids have been reported as important storage organ in the epiphytic orchids. The aim of this study is to investigate the physiological role of pseudobulbs in the growth of Oncidium flexuosum. It has been hypothesized that shoots of Oncidium flexuosum grown from plants with unshaded pseudobulbs will have the optimal growth. In the present study, the role of pseudobulbs in growth and development of new shoots, regulation of photosynthesis, photoassimilate partitioning, carbohydrate metabolism and mineral nutrition were studied through a period of 16 weeks.
All plants were first grown in the greenhouse under maximum PPFD of 600 to 700 μmol m-2 S-1 for one week. As pseudobulbs are capable of fixing CO2, the pseudobulbs were then either 100% shading by using aluminium foil or 50% shading by using wire gauze. The shading treatments hence allowed the pseudobulbs to just act as storage organs for the plants. New shoot development, biomass of leaf, pseudobulbs and root, and leaf area were negatively affected by 50% and 100% shading of pseudobulbs after 16 weeks of treatments. Macro minerals such as nitrogen (N), phosphorus (P) and potassium (K) were significantly lower than those plants with unshaded pseudobulbs. This promoted the new shoots to develop. Contents of chlorophyll, caroteinoids and leaf soluble protein of new shoots from plants with unshaded pseudobulbs were higher than those of leaves from plants with pseudobulbs 50% and 100% shaded. Pseudobulbs shading also altered 14c photoassimilate partitioning and carbohydrate metabolism. It was observed that new shoots grown from plants with unshaded pseudobulbs accumulated larger amount of carbohydrates. 14c experiments revealed that the pseudobulbs were capable of remobilizing the reserves to the new growing shoots emerging from the current shoot. C assimilation experiments also showed that more I4c assimilate was imported into the new shoots of control plants with unshaded pseudobulbs. According to the above results, there was a high degree of interdependency of connected shoots in terns of minerals and carbon economies. The current shoots with unshaded pseudobulbs are important in supporting the growth and development of the new shoots. The developing new shoots are an active and strong sinks during its vegetative stage obtaining most of its resources from the backshoots.
Since a highly intergrated pattern of resource allocation and partitioning was observed, it was concluded that the pseudobulbs is an important regulatory organ rather than being a mere storage organ for excess assimilates. Its ability in fixing CO2 contributes to a certain extend the plant net photoassimilates. It plays a critical role in providing carbon assimilates and mineral nutrients to the growing organs such as new shoots and hence determine the growth and development of these organs.
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