Varietal Differences in Thermal Response of Passion Fruit Pollen Germination

Abstract

The thermal response of pollen germination and varietal differences in heat tolerance were examined in 14 passion fruit (Passiflora spp.) cultivars and varieties including 7 hybrid cultivars (P. edulis × P. edulis f. flavicarpa) and 2 different species. Pollen germination was tested on agar medium between 20°C and 42°C at 2°C increments. Thermal responses varied markedly among the hybrid cultivars. ‘Ruby Star’ and ‘Tai-nung #1’ pollen germinated well at 30°C–34°C, ‘Summer Queen’ and ‘Suico’ at higher temperatures of 32°C–36°C, and ‘King Ruby’ and ‘Sunny Shine’ at yet higher temperatures of 34°C–36°C. Germination was highest in ‘Minami-jujisei’ and was highest at 36°C, but decreased at lower and higher temperatures, indicating a limited favorable temperature range. In purple passion fruit (P. edulis), the favorable temperature range was lower than that of the hybrids and the thermal response varied between varieties. Kagoshima strain pollen germinated well at 28°C–32°C, and Amami wild strain at 28°C–34°C. Purple passion fruit germination tended to decrease slightly at lower temperatures. In yellow passion fruit (P. edulis f. flavicarpa), pollen of each cultivar and variety germinated well at 36°C, indicating a limited favorable range similar to that of ‘Minami-jujisei’. Sweet passion fruit (P. alata) pollen germinated well at 28°C–32°C, similar to the purple passion fruit. Water lemon (P. laurifolia) pollen germinated well at cooler temperatures of 26°C–28°C and remained generally high. Our results indicate that heat tolerance of pollen germination varies by approximately 2°C among hybrid passion fruit cultivars.