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The Occurrence of Plants with Crassulacean Acid Metabolism (CAM) Photosynthesis in Tropical and Subtropical Rain Forests with Very High Rainfall

景天酸代謝(CAM)植物竟出現在高雨量熱帶及亞熱帶雨林

摘要


景天酸代謝(Crassulacean acid metabolism, CAM)是一種將氣孔開啟限制在夜間以減少水份散失的光合作用途徑,因此在乾燥氣候區有眾多的CAM植物。除了乾燥地區,熱帶和亞熱帶水份充足的地區也有許多附生的CAM植物。雖然雨量充足,但有證據顯示相對乾燥的時期以及附生植物特殊的型態與微棲地足以造成逆壓而使CAM成為相當有利的適應方式。 然而要解釋在年雨量極高(超過4000毫米)的熱帶亞熱帶雨林,CAM在適應上的意義是有相當難度的。本文回顧檢視多個用以解釋此一看似難解現象的假說,這些假說包括: 1.景天酸代謝使附生的CAM植物能利用夜間宿主林木冠層內空氣中高濃度的CO2。 2.即使在雨林無雨的時期仍夠長或者夠頻繁讓景天酸代謝成為有利的保水適應。 3.許多CAM植物能在白天和夜晚均能固定CO2在這種環境是有利的。 4.CAM代謝組織內白天CO2濃度升高可以避免光抑制而減少CO2的損失。 5.至少在白晝的初期CAM代謝組織內高的酸含量可以減少植食。 6.至少在一種附生CAM植物曾有研究指出葉面潮濕可刺激CO2利用在多雨的環境中是有利的。 7.至少在一種CAM植物發現其對必需元素的需求低可提高其營養利用效率,這在多雨因而葉組織淋溶旺盛的的雨林中是有利的適應。 8.滲透引起的高組織水含量所造成的泌液作用對CAM植物有多重好處。 9.附生CAM植物對遮蔽的適應良好,這在高雨量且多雨的森林而言是極具價值的適應能力。 10.CAM在此環境中並非有利的適應,而是因為其它原因而在此環境出現,其可能原因至少有3個。 過去的研究對以上所有假說的探討都非常少,因此極需進一步的研究加以深入探討檢測。

關鍵字

適應 CO2濃度 CO2吸取 CAM景天酸代謝 乾旱

並列摘要


Crassulacean acid metabolism (CAM) is a photosynthetic pathway that conserves water by restricting stomatal opening, hence water loss, to the night. Thus, it is not surprising to find that floras of arid regions include a large number of CAM plants. In addition, CAM is common among epiphytes in tropical and subtropical environments, where water is plentiful. Despite the latter, evidence exists that comparatively short periods of drought, coupled with the unusual morphology and/or microhabitat of such epiphytes, comprise stress substantial enough that the CAM pathway proves highly adaptive. It is more difficult to explain the adaptive significance of CAM in tropical and subtropical rain forests with exceedingly high annual rainfall, i.e., in excess of 4 meters per year. This review explores a variety of hypotheses proffered as explanations for this apparent conundrum. Such hypotheses include: 1. CAM allows an epiphytic CAM plant to capitalize on the high concentrations of atmospheric CO2 at night in the canopies of its host trees; 2. even in such rain forests, rainless periods are long enough and/or frequent enough that CAM proves advantageous as a water conservation adaptation. 3. The ability of many CAM plants to assimilate CO2 during both the day and night is advantageous in such environments. 4. The increase in daytime CO2 levels in CAM photosynthetic tissue prevents photoinhibition and minimizes CO2 losses during the day. 5. The high acid content of CAM photosynthetic tissue, at least early in the day, deters herbivory. 6. The stimulation of CO2 uptake following wetting of the leaves reported for at least one epiphytic CAM plant is advantageous in such high-rainfall environments. 7. The low demand for essential elements observed in at least one CAM plant increases the nutrient efficiency of such plants, which is adaptive in rain forests with excessive rainfall and, hence, high levels of nutrient leaching from the leaf tissues. 8. Guttation resulting from osmotically-driven high tissue water contents benefits CAM plants several ways. 9. Epiphytic CAM plants are well-adapted to shady microhabitats, comprising a valuable adaptation to the cloudy environment of high-rainfall rain forests. 10. CAM is not beneficial for such plants in these environments for at least 3 possible reasons. All of the hypotheses have only rarely been examined in past studies. Thus, all are in need of further investigation.

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