BACKGROUND To counteract its dramatic species endangerment caused by extensive loss of habitat, Singapore is currently re-introducing into nature some of the native orchids to conserve and improve their germplasm. A main challenge of re-introduction is growing and establishing these plants under natural conditions, which are semi-arid with periodic drought. In this study, six native species were examined, of which three, Bulbophyllum vaginatum, Dendrobium leonis and Phalaenopsis cornu-cervi, are viewed as CAM species while the other three, Coelogyne rochussenii, Coelogyne mayeriana, and Bulbophyllum membranaceum are usually characterized as C3 species. We aimed to compare their physiological responses to drought under two different light conditions: (1) moderate light (photosynthetic photon flux density, PPFD of 900 μmol m-2 s-1) and (2) low light (PPFD < 100 μmol m-2 s-1). RESULTS After 7 weeks of drought under moderate light (DRML), photosynthetic light utilization was reduced in all six species, and relative water content (RWC) in leaves decreased to < 50% in CAM orchids, compared to > 50% in C3 species, while RWC in pseudobulbs (produced by 4 of the species) fell to < 50%. Both effects were reversed after 14 weeks of re-watering. Proline concentration in leaves increased in the CAM orchids and B. membranaceum (60-130 µmol g-1 FW), and CAM acidity increased (0.2 to 0.8 mmol H+/g fresh weight) in leaves and pseudobulbs of most species including C3 orchids after 7 weeks of DRML, but to lesser extent in B. membranaceum. CONCLUSION In the six native orchid species tested, osmoregulation by proline and CAM expression were adaptive responses to maintain photosynthesis under drought stress. Expression of CAM is a significant adaptive mechanism to drought in both C3 and CAM orchids. For C3 B. membranaceum, this CAM activity is best described as 'CAM-idling'. We propose that any future work in understanding adaptive responses in Singapore's native epiphytic orchids to periodic water deficit should also analyse the significance of CAM plasticity on water conservation within the plant and the regulation of CAM by prevailing water status and light intensity.

中文翻译：

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附生热带兰花物种响应环境胁迫的 CAM 可塑性。

背景为了应对因栖息地广泛丧失而造成的严重物种濒危，新加坡目前正在将一些本土兰花重新引入大自然，以保护和改善其种质。重新引入的主要挑战是在半干旱且周期性干旱的自然条件下种植和建立这些植物。在这项研究中，检查了六种本地物种，其中三种，Bulbophyllum vaginatum、Dendrobium leonis 和 Phalaenopsis cornu-cervi，被视为 CAM 物种，而其他三种，Coelogyne rochussenii、Coelogyne mayeriana 和 Bulbophyllum membranaceum 通常被定性为 C3 物种。我们的目的是比较它们在两种不同光照条件下对干旱的生理反应：（1）中等光照（光合光子通量密度，PPFD为900 μmol m-2 s-1）和（2）弱光（PPFD < 100 μmol m-1） 2秒-1)。结果 中光干旱 (DRML) 7 周后，所有 6 个物种的光合光利用率均下降，CAM 兰花叶片相对含水量 (RWC) 降至 < 50%，而 C3 物种的叶片相对含水量 (RWC) 降至 < 50%。而假鳞茎（由 4 个物种产生）中的 RWC 则降至 < 50%。重新浇水 14 周后，这两种影响都逆转了。经过处理后，CAM 兰花和 B. membranaceum 叶片中的脯氨酸浓度增加（60-130 µmol g-1 FW），大多数物种（包括 C3 兰花）的叶片和假鳞茎中 CAM 酸度增加（0.2 至 0.8 mmol H+/g 鲜重）。 7 周的 DRML，但在 B. membranaceum 中程度较轻。结论 在测试的六种本地兰花中，脯氨酸和 CAM 表达的渗透调节是干旱胁迫下维持光合作用的适应性反应。CAM 的表达是 C3 和 CAM 兰花对干旱的重要适应机制。对于 C3 B. membranaceum，这种 CAM 活性最好描述为“CAM 空闲”。我们建议，未来在了解新加坡本土附生兰花对周期性水分亏缺的适应性反应方面的任何工作，还应分析 CAM 可塑性对植物内水分保存的重要性，以及对 CAM 的调节作用。