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The aim of this study is to understand the physiological and growth responses of six native epiphytic orchid species, found in Singapore, to light and water deficit, as well as the acceleration of orchid growth under LED lighting. The six species selected comprised three C3 and three CAM species. All species were subjected to water deficit with high, moderate and low irradiances, under natural conditions, and in the greenhouse for 7 weeks. The droughted plants in the greenhouse were then re-watered for 14 weeks. The effects of water deficit and re-watering were studied through photosynthetic light utilisation, pigment concentrations, relative water content (RWC) and titratable acidity (TA) in both leaves and pseudobulbs, and photosynthetic gaseous exchange. Proline and sugar accumulations for osmotic adjustment in leaves and pseudobulbs, and the significance of morphological traits in water conservation were also investigated. Water deficit resulted in a decreased photosynthetic light utilisation, as demonstrated in the decreased dark-adapted Fv/Fm ratio, electron transport rate (ETR) and photochemical quenching (qP). Photosynthetic capacity was also decreased with water deficit, as shown in the decreased maximal photosynthetic rate by O2 evolution (Pmax) after 7 weeks of drought and moderate light (DRML). Leaf and pseudobulb RWC also decreased after 7 weeks of DRML, while CAM activity was upregulated in generally all six species, including the C3 orchids, as stomata closed to reduce water loss by transpiration, which demonstrated that CAM was a significant drought tolerance strategy. The induction of CAM in C3 orchids demonstrated the plasticity of CAM and also the significance of CAM expression for maintaining water and carbon economy in C3 orchids, especially when carbon uptake is limited by stomatal closure under water deficit. Proline accumulation increased in Bulbophyllum membranaceum, Bulbophyllum vaginatum, Dendrobium leonis and Phalaenopsis cornu-cervi, could play a role in osmotic adjustment to maintain leaf turgor during drought, while the larger pseudobulbs of Coelogyne rochussenii and Coelogyne mayeriana played important roles in storing water to supply to the leaves to maintain turgor during drought. Overall, the six orchid species studied demonstrated the ability to tolerate drought but after 7 weeks, there was little or no gain in biomass and the orchids were barely surviving. Re-watering reversed the negative effects of water deficit and enabled recovery of photosynthesis in these orchid species. Therefore, it is recommended that deliberate watering be carried out for the orchid species that are planted under natural conditions if the period of water deficit due to dry spells becomes over-extended more than 7 weeks. In addition, the use of LED lighting has been shown to enhance plant growth of young shoots of B. membranaceum and B. vaginatum. Grown under well-watered conditions achieved by an indoor aeroponics system and a combined red-blue LED lighting, B. membranaceum and B. vaginatum exhibited comparable or higher photosynthetic light utilisation, total Chl content, photosynthetic capacity, and sugar content compared to those grown under shade in the greenhouse. The application of LED lighting may help to accelerate the cultivation of orchids which will be of contribution to Singapore’s orchid conservation programme.
|Appears in Collections:||Doctor of Philosophy (Ph.D.)|
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checked on Jan 24, 2021
checked on Jan 24, 2021
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