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Physiological responses of three tropical epiphytic orchid species with different morphologies to drought stress and re-watering
Author
Ho, Qian Yi
Supervisor
He, Jie
Abstract
Epiphytic orchids are predisposed to environmental changes due to their direct interface with microclimatic conditions in forest canopies, which are subjected to water deficit conditions, prompting them to evolve morphologically and physiologically to deal with water stress challenges. Many have pseudobulbs to store water and nutrients to support the plants during drought stress and have evolved CAM pathways to photosynthesise during stomatal closure due to environmental water deficiency. As climate change intensifies, there is increasing concern about its effect on threatened orchid populations worldwide. Three native species, Acriopsis liliifolia, Bulbophyllum pulchellum and Callostylis pulchella have been identified as threatened. Reintroduction of these species into the natural habitat requires an understanding of how they respond to drought stress to plan strategies to boost repopulation success rates. In this study, susceptibility of the three species were hypothesised to be dependent on both morphological and physiological traits. To investigate this, a range of morphological and physiological traits were studied—pseudobulb volume to leaf area ratio, Fv/Fm ratios, photosynthetic pigments, Chlorophyll (Chl) fluorescence parameters such as Electron Transport Rate (ETR), effective photochemical quantum yield (Y(II)), photochemical quenching (qP) and non-photochemical quenching (NPQ), water relations measured by water content and relative water content, proline concentration and CAM acidity. The three species were found to be morphologically adapted to drought stress through strategies such as CAM photosynthetic pathway and using large pseudobulbs as a reservoir. There was evidence of stress in the plants as observed in dips of the Fv/Fm ratios during drought stress and recovery after re-watering. Chl readings showed the same pattern of decrease as Fv/Fm ratios only under severe drought stress (SS) conditions. It was postulated that C. pulchella was most tolerant to drought stress because its Fv/Fm ratio did not drop as much as the other two species did during mild drought stress (MS) and SS treatments. While Chl fluorescence values did not correspond with Fv/Fm ratios, it was observed that pseudobulbs had less photosynthetic capacity than leaves in the three species. This was also reflected in the low chlorophyll concentration in pseudobulbs as compared to leaves, except for B. pulchellum. There was a decrease in water content of MS-treated pseudobulbs and leaves of A. liliifolia and B. pulchellum but this was not seen in C. pulchella, further indicating its tolerance to drought stress. Significantly lower MS- and SS-treated pseudobulbs than well-watered (WW)-treated pseudobulbs compared to their leaf counterparts suggested that pseudobulbs dispensed water to the more photosynthetically efficient leaves during drought stress. A. liliifolia underwent significant changes in proline concentration during drought stress, indicating it used osmoregulation as a drought tolerance strategy. After re-watering, the three species recovered to WW-treatment Fv/Fm ratio values, but their physiological responses differed. High proline concentrations in B. pulchellum and C. pulchella for re-watered treatments pointed towards drought-hardening strategies. High CAM acidity after re-watering in the same two species was observed and possibly improved drought recovery. It was concluded that morphological traits were equally important as physiological traits in enabling the orchids to survive drought stress.
Date Issued
2021
Call Number
QK495.O64 Ho
Date Submitted
2021