海岸樹種臺灣海桐、樹青及紅柴耐旱性的比較

臺灣海桐、樹青及紅柴能生長在每年長達6個月乾旱少雨的恆春半島西海岸，應具備適應乾旱環境的能力。本研究目的為藉由測定此三樹種膨壓喪失時的葉部水勢來得知其耐旱性。臺灣海桐、樹青及紅柴的形態及遭遇缺水的生理表現，是否可解釋此三樹種能夠適應恆春氣候的原因？本研究分別進行野外監測與苗圃盆栽斷水試驗。野外監測試驗測定石珠海岸此三樹種從雨季至乾季期間，黎明前葉部水勢及葉部耐旱性的變化。苗圃盆栽斷水試驗分為二項。第一項試驗測定此三樹種盆栽在不同葉部萎凋狀態的黎明前水勢及葉綠素螢光Fv/Fm值降低情形。第二項試驗將此三樹種培育在60 cm深的管狀容器內，並分別進行斷水與充分供水處理，在斷水期間測定植株的生理表現變化，並在試驗結束後測定充分供水植株的根數量。結果顯示，恆春西海岸的臺灣海桐、樹青及紅柴在2017年乾季後期的黎明前水勢分別達到-0.36、-1.00及-1.50 MPa，紅柴受乾季的影響較大，樹青次之，而臺灣海桐並未遭遇缺水。此三樹種在乾季後期的葉部膨壓喪失時水勢顯示樹青與紅柴的葉部耐旱性較高，而臺灣海桐較低。苗圃盆栽斷水試驗中，未遭遇缺水逆境的臺灣海桐的氣孔導度及蒸散速率顯著高於樹青及紅柴，而臺灣海桐苗也具有較高的總葉面積，使其斷水初期的耗水量分別為樹青及紅柴的3.8及5.1倍，臺灣海桐因而較快消耗容器內的土壤水分。在斷水50天時，臺灣海桐的光合作用率、氣孔導度及蒸散速率僅維持斷水初期的20%以下，樹青及紅柴的此3項生理表現皆維持在40%以上。臺灣海桐、樹青及紅柴在光合作用率到0的斷水日數分別為72、126及140天，顯示臺灣海桐較早遭受缺水而造成嚴重生理抑制。經過108天斷水處理後，臺灣海桐、樹青及紅柴的膨壓喪失時水勢分別為-2.66、-2.78及-3.03 MPa，均有高的葉部耐旱性。紅柴的光合作用率達0的黎明前水勢為-8.30 MPa，顯示該樹種極高的生理耐旱性。臺灣海桐充分供水的盆栽苗在土表40 cm以下的根數量多達412條，顯著高於樹青(125條)與紅柴(181條)，多量根系可能有利於該樹種持續吸收土壤水分。本研究結果顯示臺灣海桐、樹青及紅柴均具有相當高的耐旱性，可解釋此三樹種適存於石珠海岸的原因。

關鍵字

葉部耐旱性；生理耐性；根數量；斷水處理；缺水逆境

並列摘要

Pittosporum pentandrum, Planchonella obovata and Aglaia formosana can grow on the west coast of the Hengchun Peninsula where is dry and rainless for up to six months every year, and they have the adaptability to arid environments. The purpose of this study is to find out their drought tolerance by determining leaf water potential at turgor loss point of these three types of trees. Can the forms of P. pentandrum, P. obovata, and A. formosana and their physiological performances when facing water shortage explain the reasons why these three types of tress can adapt to the weather in Hengchun? Field monitoring and water shut-off tests on potted plants in nursery gardens were conducted respectively. Field monitoring aimed to test and determine the changes of these three trees in predawn leaf water potential and leaf drought tolerance between the rainy season and the dry season on the coast of Shihchu. Water shut-off tests on potted plants in nursery gardens were divided into two parts. The first test determined the predawn water potential of these three potted plants with different leaf wilt states, and the decrease in chlorophyll fluorescence (Fv/Fm). The second test was to grow these three types of trees in a 60-cm deep tube container, and then cutting off the water or providing ample water was conducted respectively. During the water shut-off period, the physiological performances of plants were determined, and after the test, the numbers of roots of the plants with ample water supply were determined. Results showed that the predawn water potential of P. pentandrum, P. obovata and A. formosana on the west coast of the Hengchun Peninsula in the later period of the dry season in 2017 was -0.36, -1.00 and -1.50 MPa respectively. A. formosana was influenced the most by the dry season, followed by P. obovata, while P. pentandrum did not encounter the water shortage. As to their leaf water potential at turgor loss point in the later period of the dry season, the results indicated that P. obovata and A. formosana had higher leaf drought tolerance, while that of P. pentandrum was lower. In the water shut-off tests on potted plants in nursery gardens, the stomatal conductance and transpiration rate of P. pentandrum that had not encountered water shortage were significantly higher than those of P. obovata and A. formosana. In addition, seedlings of P. pentandrum also had higher total leaf areas, which made its water consumption 3.8 times and 5.1 times higher than those of P. obovata and A. formosana respectively at the initial stage of cutting off water. As a result, P. pentandrum had faster consumption of water in the soil inside the container. During the 50 days of cutting off water, the photosynthesis rate, stomatal conductance and transpiration rate of P. pentandrum only remained below 20% at the initial stage of cutting off water, while the three physiological performances of P. obovata and A. formosana both remained over 40%. The period of cutting off water resulting in the zero photosynthesis rate for P. pentandrum, P. obovata and A. formosana was 72, 126 and 140 days respectively. This result showed that P. pentandrum suffering from earlier water shortage had serious physiological inhibition. After the water had been cut off for 108 days, water potential at turgor loss point of P. pentandrum, P. obovata and A. formosana was -2.66, -2.78 and -3.03 MPa respectively, which all showed high leaf drought tolerance. The predawn water potential of A. formosana with the zero photosynthesis rate was -8.30 MPa, which indicated that this type of tree had extremely high physiological drought tolerance. The potted seedlings of P. pentandrum with ample water supply had up to 412 roots at the 40 cm depth under the soil surface, which was obviously much higher than those of P. obovata (125) and A. formosana (181). Large quantities of root systems were advantageous to its continuous absorption of water in the soil. Results of this study showed that P. pentandrum, P. obovata and A. formosana all had very high drought tolerance, which could explain why they were adapted to grow on the coast of Shihchu.