在不同溫度與光度下桃實百日青之光合速率與葉綠素螢光特性

Chlorophyll Fluorescence and Photosynthetic Rate of Podocarpus nakaii at Different Light Intensities and Temperatures

桃實百日青（Podocarpus nakaii）爲台灣特有種，僅侷限在低海拔300-1,000m之闊葉樹林中，已被列爲瀕臨絕滅級。爲探討其對環境變遷之適應能力，本研究以培育於10%、50%及100%日照環境下之小苗爲材料，在10℃及25℃之溫度下，各以100、200、500及1,000 μmol m^（-2）s^（-1） PPFD之光照照射30 min，再暗處理30 min，期間以葉綠素螢光儀測定其光系統II （PSII）效能之變化情形。同時也以高光合能力之台灣赤楊（Alnus formosana）作爲對照，探討兩種植物在光照下之光合速率與PSII效能之關係。結果顯示，無論在光合作用尚未達穩定狀態之照光初期，或已達穩定狀態之光照後期，光合速率與PSII效能間均呈極顯著之正相關。此外，PSII效能之下降幅度會隨光度增加及溫度降低而增大，此下降又以於遮蔭程度越大處培育者越劇。在適溫及低光下，光抑制引起之PSII效能降低百分率只有1-2%左右。隨著溫度降低及光度上升，光抑制引起之PSII效能降低百分率會隨PSII效能之降低呈上彎之指數曲線增加。其中，於100%日照培育之桃實百日青個體，在10℃及1,000 μmol m^（-2）s^（-1） PPFD下，其光抑制引起之PSII效能降低百分率更可達28%左右。此現象或爲桃實百日青只分布於低海拔之原因。由於葉綠素螢光能反映植物之光合速率及生態習性，測定簡便迅速，有利於生態生理研究。

Podocarpus nakaii is endemic to Taiwan, and grows strictly in broad-leaf forests at 300-1,000m above the sea level. It has been listed in the extinction-class by IUCN (International Union for Conservation of Nature) as an endangered species. For interpreting its adaptation to environmental changes, seedlings of P. nakaii were incubated for 30 minutes at different intensities of sunlight (10%, 50% and 100%), artificial illumination (100, 200, 500 and 1,000 μmol m^(-2) s^(-1)), and temperature (10 and 25℃), and then, followed by a 30 minutes dark recovery period. The efficiency of photosystem II (PSII) was assessed with the measurement of chlorophyll fluorescence. Also, the photsynthetic rate (Pn) and PSII were measured for Alnus formosana at 25℃ and 1,000 μmol m^(-2) s^(-1) photosynthetic photon flux density (PPFD) as the references. The results indicated that the efficiency of PSII assessed with chlorophyll fluorescence was closely related to Pn. With the decreasing temperature and increasing PPFD, the efficiency of PSII declined drastically. The decline was more drastic for the seedlings incubated in shade than in full sunlight. At same temperature and light intensity of 25℃ and 100 μmol m^(-2) s^(-1) PPFD, a portion of the PSII efficiency, due to photoinhibition, declined about 1-2%. However, the declined portion increased exponentially with the decrease in temperature and the increase in PPFD. At 25℃ and 1,000 μmol m^(-2) s^(-1) PPFD, the portion rose to about 28% for P. nakaii under the full sunlight. The results explained the reason why the distribution of P. nakaii is limited at low elevations. Also, the chlorophyll fluorescence measurement was found to be a fast and simple method to estimate photosynthetic rate and ecological behavior of plants for ecological researches.