Differences in in vitro pollen germination and pollen tube growth of coconut (Cocos nucifera L.) cultivars in response to high temperature stress

Temperature is a primary factor affecting the rate of plant development. Warmer temperatures expected with climate change and the potential for more extreme temperature events will impact plant productivity. Pollination is one of the most sensitive phenological stages to temperature extremes and in this study an in vitro pollen germination technique is used to screen coconut genotypes tolerant to high temperature. The pollen of twelve coconut genotypes comprising five talls (CCNT, FMST, LCT, PHOT and WCT), five dwarfs (CGD, COD, CRD, GBGD and MYD) and two hybrids (COD X WCT and MYD X WCT) were screened at different temperature levels from 10 to 50 °C at an interval of 2.5 °C. Cultivar variation existed for cardinal temperatures (Tmin, Topt and Tmax) of pollen germination percentage and pollen tube growth. Mean cardinal temperatures calculated from the bilinear model for the 12 genotypes ranged from 23.5 °C to 29.5 °C, 9.7 °C to 16.5 °C and 40.1 °C to 43.9 °C for Topt, Tmin and Tmax, respectively. In general tall, cultivars FMST, LCT, WCT, dwarf cultivar COD and hybrids showed better adaptability to high temperature while dwarf MYD was the least adaptable. At the metabolic level, high temperatures induced about 20% reduction in soluble protein content compared to optimal temperature in all the studied genotypes. There was an inverse relationship between superoxide dismutase activity and pollen germination percentage. Overall, there was wide variation in coconut cultivars for cardinal temperatures (Tmin, Topt and Tmax) of pollen germination percentage and pollen tube growth. The genotypes with higher Tmax for pollen germination and tube growth may be more tolerant to high temperature stress during flowering. Further studies are required to validate the results under in vivo condition; and also to understand the mechanism and factors that lead to pollen sterility in coconut.