Self-incompatibility and pollination relationships for four Greek olive cultivars

•Koroneiki is highly self-fertile cultivar and Kalamata less self-fertile.•Genetic and environmental factors affect fruit set.•S-alleles were attributed in the framework of the sporophytic SI system.•Compatible pollinizers for each cultivar are recommended. The present three-year study aimed to (1) determine the number of flowers per inflorescence, (2) determine the proportion of hermaphrodite flowers and (3) investigate the genetic and environmental components of self-incompatibility through controlled pollination trials, followed by microscopic observation of in vivo pollen tube growth for olive cultivars ‘Koroneiki’, ‘Kalamata’, ‘Mastoidis’ and ‘Amygdalolia’. Significant differences between cultivars and years were observed for number of flowers/inflorescence and hermaphrodite flowers percentage. The highest number of flowers was observed for ‘Kalamata’ panicles followed by ‘Koroneiki’, whereas the lower numbers were for ‘Mastoidis’ and ‘Amygdalolia’. The highest percentage of hermaphrodite flowers was observed for ‘Koroneiki’, followed by ‘Kalamata’ while lower levels were counted for ‘Mastoidis’ and ‘Amygdalolia’. ‘Koroneiki’ showed a noteworthy capability (3.6–8.7% of hermaphrodite flowers) to set fruit through self pollination. Lower fruit set rates were counted for ‘Mastoidis’ (1.7–2.6%) and ‘Amygdalolia’ (0.5–2.4%). The lowest level of fruit set was observed for ‘Kalamata’ (0.4–2%). In the case of cross pollination, the highest fruit set rates were observed for ‘Koroneiki’, intermediate rates were counted for ‘Mastoidis’, followed by ‘Amygdalolia’, and the lowest levels of fruit set were counted for ‘Kalamata’. In the framework of the sporophytic SI system, we attributed R2R4 to ‘Mastoidis’ and ‘Kalamata’, R1R2 to ‘Amygdalolia’ and R4R6 to ‘Koroneiki’. Based on the results of these experiments, recommendations were released on cross pollination requirements and combinations with compatible cultivars for setting up new olive orchards to ensure high fruit sets and adequate yields.