臺灣夏季生產切花菊常遇高溫而使花期延遲，篩選夏季較不易開花延遲之新品種為重要目標。耐熱的菊花品種不易因為高溫而延後花期，本試驗擬探討以光系統II最大光化學效率（Fv/Fm）作為高溫逆境指標快速篩選耐熱單株之可行性。以臺灣夏開菊流通品種‘小紅娘’、‘芬蘭’與‘卡迪那’頂芽下第3-4片完全展開葉離體於不同溫度與不同時間測量Fv/Fm，以50℃暗處理20-25分鐘最能鑑別個體間的差異。參試實生苗為臺灣夏開菊‘卡迪那’與‘小紅娘’開放授粉所得，分別使用30及15株進行試驗，在種植後42天，取第3-4片完全展開葉於50℃處理20分鐘再量測Fv/Fm值，參試單株重新扦插後於2021年6月2日定植於遮雨棚（日／夜均溫35/25℃）以土耕種植至開花，分別調查各單株之到花日數，並將到花日數與Fv/Fm值進行迴歸分析，結果Fv/Fm值高之單株其到花日數短，‘卡迪那’與‘小紅娘’之實生苗到花日數與Fv/Fm值的決定係數分別為0.74與0.71，此結果和50℃水浴處理30分鐘之電解質滲漏與開花日數之關係相似，顯示Fv/Fm值與電解質滲漏皆可作為快速鑑別夏開菊實生苗夏季早花之指標。

Flowering-heat-delay is a major issue upon field cut chrysanthemum production in Taiwan during summer. Thus, selection for heat-delay insensitive line/cultivar is crucial in chrysanthemum breeding. Heat-tolerant chrysanthemum tend to maintain on schedule flowering under high temperatures. This research explore feasibility to employ maximum quantum yield of PSII photochemistry (Fv/Fm value) as a heat-stress index in screening for heat-tolerant offspring. The third to fourth unfolded leaves below shoot apical of summer-flowering 'Hsiao-hong-niang', 'Fen-lan', and 'Kaa-di-na' were detached and treated for various temperatures and time durations in dark then measured for Fv/Fm value. Results showed that treatments of 50℃ for 20-25 mins could sufficiently distinguish cultivar difference. Fifteen and 30 seedlings of 'Hsiao-hong-niang' and 'Kaa-di-na' from open-pollination were used, respectively. At 42 days after sowing, the third to fourth leaves below the apical were detached and treated at 50℃ for 20 mins then measured for Fv/Fm value. The seedlings were subsequently pinched and harvested for lateral shoots as cuttings. Rooted cuttings were planted in soil under rain shelter condition (day/night mean temperature 35/25℃) on 2 June, 2021. Days to flowering were recorded and regressed on Fv/Fm value. Results showed that offspring with higher Fv/Fm value tend to flower earlier, and coefficients of determination were 0.74 and 0.71 in seedlings of 'Hsiao-hong-niang' and 'Kaa-di-na' offspring, respectively. This result is similar to the relationship between electrolyte leakage and days to anthesis. Overall, leaf Fv/Fm value and electrolyte leakage ratio could serve as an index for screening summer-flowering 'Hsiao-hong-niang' and 'Kaa-di-na' offspring that tend to flower earlier under higher temperatures in summer.