Variations in the Predawn Leaf Water Potential and Photosynthetic Rate during the Dry Season and Drought-Tolerance Mechanisms of Coastal Tree Species

海岸林樹種黎明前葉部水勢及光合作用率在乾季的變化及其耐旱機制

郭耀綸(Yau-Lun Kuo) ； 楊雅萍(Ya-Ping Yang) ； 彭世賢(Shih-Hsien Peng)

《台灣林業科學》 32卷2期 (2017/06) Pp. 131-144

https://doi.org/10.7075/TJFS.201706_32(2).0005

摘要

An approximately 6-mon prolonged drought occurs yearly in the Hengchun Peninsula of southern Taiwan. Species capable of establishing themselves in this drought environment should be tolerant to drought either through dehydration postponement or dehydration tolerance. To explore drought adaptations of tree species, we investigated predawn leaf water potentials (PWPs) and values of the net photosynthetic rate (P_n) of 9 native tree species which were established in a secondary forest on the west coast of the Hengchun Peninsula. Results showed that both the PWP and P_n in each species significantly declined during the dry season. The PWP during the dry season was maintained at ＞ -0.9 MPa in Pittosporum pentandrum, ＞ -1.5 MPa in Planchonella obovata, Gelonium aequoreum, and Melanolepis multiglandulosa, but ＜ -3.0 MPa in Aglaia formosana. The P_n during the dry season was maintained at ＞ 70% of the level during the rainy season in Hibiscus tiliaceus and Pit. pentandrum, while they were ＜ 50% in Pla. obovata, Gel. aequoreum, and Agl. formosana. A regression analysis was applied to estimate the PWP value of each species when the P_n decreased to 0. It was found that PWP values of Hib. tiliaceus, Allophylus timorensis, and Agl. formosana, were as low as -7.24, -4.57, and -4.40 MPa, respectively, indicating that these 3 species possess high physiological tolerance abilities against drought through dehydration tolerance. During the dry season, few leaves were retained on saplings of Broussonetia papyrifera, All. timorensis, Mel. multiglandulosa, and Ehretia resinosa. These species adopted a dehydrationpostponement mechanism by shedding most of their leaves. Pittosporum pentandrum, Pla. obovata, and Gel. aequoreum, 3 evergreen species, on the other hand, had extended root systems to enhance soil water absorption, which is also a dehydration-postponement mechanism. The 2 mechanisms are not mutually exclusive, and each species could incorporate various approaches for drought tolerance. Hibiscus tiliaceus might also have extended root systems in addition to adopting a dehydration-tolerance mechanism. Planchonella obovata, Bro. papyrifera, All. timorensis, and Ehr. resinosa also possessed physiological tolerance abilities in addition to adopting a dehydration-postponement mechanism.

關鍵字

dehydration-postponement mechanism ； dehydration-tolerance mechanism ； drought tolerance ； leaf shedding ； physiological tolerance

並列摘要

台灣南部恆春半島每年有長達半年的乾季，能在此環境建立的樹種應具備耐旱能力。然而，不同樹種的耐旱機制可能各不相同。林木耐旱機制可分為延遲脫水及忍受脫水兩類，分別可藉由不同方式達成。本研究選取已在恆春西海岸次生林建立的九種原生樹種，測定其黎明前葉部水勢與光合作用率的變化，藉以判斷各樹種可能的耐旱機制。結果發現各樹種在乾季期間，黎明前水勢及光合作用率都顯著較雨季時降低。台灣海桐乾季全期黎明前水勢保持在-0.9 MPa以上，樹青、白樹仔及蟲屎則多在-1.5 MPa以上，但紅柴會降至-3.0 MPa以下。黃槿及台灣海桐在乾季光合作用率可維持雨季時的70%以上，而樹青、白樹仔及紅柴則不到雨季時的一半。藉直線迴歸分析，估算各樹種光合作用率降至零的黎明前水勢。黃槿、止宮樹及紅柴該數值分別低至-7.24、4.57及4.40 MPa，顯示此3樹種有極高的生理耐旱性，具忍受脫水機制。構樹、止宮樹、蟲屎、恒春厚殼樹植株在乾季期間只留存部分葉片，顯示是藉掉落一部分葉片來延遲脫水。常綠樹種台灣海桐、樹青及白樹仔有旺盛的根系，具有延遲脫水機制。此兩類耐旱機制並非互斥，各樹種可兼有不同的耐旱方式。黃槿除了具忍受脫水機制外，可能兼有旺盛的根系。構樹、樹青、止宮樹及恒春厚殼樹除了延遲脫水機制外，另具生理耐旱能力。

並列關鍵字

延遲脫水機制 ； 忍受脫水機制 ； 耐旱性 ； 落葉 ； 生理耐性