The green pseudobulbs of epiphytic orchids function as storage organs for water, mineral and carbohydrates. They are responsible for the ultimate re-distribution of assimilated carbon from the leaves to other plant organs. Our research suggested that they play crucial roles in regulation of leaf photosynthesis, growth, flower initiation and survival of orchids. When epiphytic orchids are subjected to environmental stress such as water deficit, we have also found that the pseudobulbs play an important role in maintaining the water level of leaves and thus protecting epiphytic orchids against drought stress.

Keywords: epiphytic orchids, growth, photosynthesis, pseudobulbs, drought stress

Epiphytic orchids are typically found in tropical environments. Evolution of epiphytes has been postulated to result in the formation of the green pseudobulbs,^1,2 which is characterized by the presence of very thick cuticle, absence of stomata and the abundance of water-storing cells.³ These 'false bulbs' are known to function as storage organs for water, mineral and carbohydrates, playing crucial roles in the growth and survival of epiphytic orchids.^4,5 In addition, photosynthesis of green pseudobulbs can contribute positively to carbon balance by recycling respiratory carbon that would otherwise be lost.⁶ Although the leaves are main sources that supply carbon for inflorescence development, the green pseudobulbs is responsible for the ultimate re-distribution of assimilated carbon from the leaves to other plant organs.⁷ As such, the green pseudobulbs can be envisioned to play an active regulatory role in assimilate partitioning as well as a passive role as a storage organ. Furthermore, the green pseudobulbs may serve as a reserve for water, minerals and carbohydrates when epiphytic orchids are subjected to environmental stress such as water deficit.⁸ The main objective of mini review was to discuss the roles of green psedobulbs in growth and development of epiphytic orchids. Photosynthetic capacity of the green pseudobulb and its roles in regulating leaf photosynthesis and protecting epiphytic orchids against drought stress were also addressed.

During new shoot and floral development, the developing new organs demand a large proportion of the newly fixed carbon for their growth.⁹ The volume of the green pseudobulbs affects the rate of growth and final size of the new shoot were observed in epiphytic orchid hybrid of Cattleya forbesii Lindl. X Laelia tenebrosa Rolfe.¹⁰ The volume of the green pseudobulbs was greatly reduced at certain stage of the young shoot development, suggesting that some storage compounds of the green pseudobulbs might have been transferred to developing shoot. The volume of the green pseudobulbs increased again at the later stage of the young shoot development. However, shading of the green pseudobulbs affected the translocation of the nutrients to the young shoots, indicating that this relationship is light dependent.¹⁰

The size and maturity of the green pseudobulbs were also critical for the orchid plants to attain the capacity of flowering.

For instances, flower initiation in Miltoniopsis was most rapid, uniform and complete when green pseudobulbs had reached their maturation stage.¹¹ Although green pseudobulbs do not show any gaseous exchange in light or in darkness, photosynthesis do take place in green pseudobulbs when exposed to light, which is involved primarily in the refixation of respiratory CO₂ produced by the underlying massive parenchymatous tissues.¹² This contributes to an increase amount of carbon assimilates and hence more food can be translocated to the developing inflorescence.¹² In the study with Oncidium Goldiana, the amount of ¹⁴C assimilates contributed by the leaves on different shoots showed a decreasing trend with increasing distance from the inflorescence of the current shoot.⁶ It is logical that the green pseudobulbs received and accumulated more ¹⁴C during the short period (8hours) after ¹⁴CO₂ feeding from the source leaves. It was also found that the green pseudobulbs of Spathoglottis unguiculata accumulated higher percentage of ¹⁴C assimilate during the vegetative stage than during the flowering stage.⁶ With the present of new organs such as inflorescences, more photo assimilates are channeled to the developing inflorescences than to other parts of the plants. Hence, there was a substantial interaction between the developing inflorescences emerging from the current back shoot and photo assimilates exported from the green pseudobulbs.

The partitioning of assimilates between sources and sinks results in phloem translocation. A source may be defined as an exporter of sugars to the phloem and a sink is an importer of sugar from the phloem. Sink organs utilize the imported assimilates for growth, maintenance and storage.¹³ In our previous study, we investigated source-to-sink relationship between leaves and green flower petals of CAM orchid Dendrobium cv. Burana Jade. It was found that green flower petals function as sinks and depend on carbohydrates exported from leaves for their development and growth.¹⁴ In their natural habitat, would the leaves and green pseudobulbs function efficiently as source if the photosynthetic capacity of either part is removed? How does the changing of source/sink ratio regulate the photosynthesis of leaves or green pseudobulbs? That the accumulation of photo assimilate in leaves has a role in regulation of photosynthetic rate was hypothesized as early as 1868 by Bouddinhault.¹⁵ In plants, changes in the photosynthetic source/sink balance are important in regulating leaf photosynthetic rate through effects on the leaf carbohydrate status.^16–18 In our study with C₃ orchid Oncidium Golden Wish, it was found that light saturation for photosynthesis and maximum photosynthetic rates were significantly higher in leaves than in green pseudobulbs. The green pseudobulbs also had lower light utilization than that of leaves.

Our results also revealed that low levels of carbohydrate in leaves after reducing photosynthetic source/sink balance adversely affected the photosynthetic rate of leaves for C₃ orchid Oncidium Golden Wish.¹⁹ Due to their lower photosynthetic capacities, green pseudobulbs function mainly as sinks. As pseudobulbs store a large amount of carbohydrates, leaves could depend on pseudobulbs carbohydrates to regulate their photosynthesis when their source capacity was removed.¹⁹ Some researchers failed to relate photosynthetic rate to source-sink relationships, suggesting that photosynthesis per se does not allow to arrive at specific conclusions on source or sink limitation.^20,21 However, the large pseudobulbs of Oncidium Golden Wish, constitutes almost two-third of the total biomass, which clearly plays an important role in regulation of leaf photosynthesis and thus, the carbon balance of the plant.¹⁹ Furthermore, the green pseudobulbs had significantly higher concentration of insoluble sugar than that of leaves.¹⁹ The pseudobulbs could switch from acting as a strong carbohydrate sink when leaves were actively photosynthesizing to become a strong source when leaves were subjected to unfavorable conditions such as long period of cloudy days and drought.

Although the amount of imported assimilates was lower and reasons for mature leaves acting as a sink was unclear, mature orchid leaves that imported assimilates from other non-foliar photosynthetic organs were reported by a number of researchers.^13,22–24 Functioning primarily as sinks but with additional source capacity, pseudobulbs points to the dynamism and plasticity in plant growth and development. For horticultural practice, it would be useful to further understand how environmental conditions would ultimately modulate the pattern of growth and development through their effects on source/sink relations between leaves and green pseudobulbs.

Epiphytic orchids are directly or indirectly exposed to natural air currents and solar radiation and receive only intermittent rains.^10,25,26 Among the many abiotic factors involved in the survival of epiphytes, water availability is probably the most important environmental factor limiting growth and survival of epiphytes.^27–29 Thus, tolerance to water deficit is a decisive factor in their survival. Water stored in the pseudobulbs of the epiphytic orchids may facilitate a slow reduction in the leaf water content and decline in water potential during a period of drought for protecting those plants against the effects of water deficits.^1,4,5,10 Although large amounts of water and carbohydrates are stored in the pseudobulbs, many epiphytic orchids are sensitive to prolonged water deficit. It was reported that relative water content (RWC) decreased continuously in leaves and pseudobulbs of epiphytic CAM orchid (Cattleya forbesii Lindl×Laelia tenebrosa Rolfe) after they were subjected to drought stress up to 45days.¹⁰ Yang et al.³⁰ examined the anatomical traits, water loss rates, and physiology of leaves and pseudobulbs of four Dendrobium species with different pseudobulb morphologies. Their results indicate that Dendrobium species with thin cuticles tend to have pseudobulbs with high water storage capacity that compensates for their faster rates of water loss. In our study, the decrease of water content in CAM orchid Cattleya laeliocattleya Aloha Case was much greater in pseudobulbs than in leaves after subjecting to drought stress, indicating that pseudobulbs facilitated a slow reduction in the water content of leaves. This finding was supported by the result of RWC of leaves, which started to decrease only after 3weeks of drought stress, Our results also showed that decreases in total chlorophyll content, photosynthetic light use efficiency and CAM acidity were much less in leaves than in pseudobulbs.³¹ Compared to leaves, although pseudobulbs were more sensitive to drought stress, to a certain extent pseudobulbs played an important role in maintaining the water level of leaves. Thus, the green pseudobulbs play an important role in protecting epiphytic orchids against drought stress.

By understanding the physiological roles of green pseudobulb in tropical epiphytic orchids, it will be able to provide scientific basis for a large scale of gene expression analysis involved in the mechanisms responsible for water, mineral and carbohydrate storage and re-distribution. For horticultural practice, it would be useful to understand how environmental conditions would ultimately modulate the pattern of growth and development through its effects on source/sink relations between green leaves and green pseudobulb. Therefore, more attention would be pay to the growth management of the pseudobulb. By evaluating the physiological role of the pseudobulb in response to drought stress, a better understanding how abiotic factors are limiting epiphyte growth and survival which, in turn, should affect epiphyte community composition.

Our project on “Physiology of green pseudobulbs” was supported by Academic Research Fund (RI5/07HJ), Ministry of Education, Singapore.

The author declares there is no conflict of interest regarding the publication of this article.

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