The effect of temperature and genotype on pollen performance in olive (Olea europaea L.)

Abstract

The reproductive phase has proven to be one of the most temperature sensitive stages in plants. Relatively high or low temperatures, especially during the progamic phase, are often correlated with low fertilization percentages and low fruit setting in different years and orchards. Here, the olive variety ‘Oblica’ was employed in a two year field experiment to assess the effect of temperature on the duration of the flowering period as well as on pollen germination, pollen tube growth, and fertilization percentage. An increase in temperature in the field was caused by covering trees with polyethylene cages. Cultivars ‘Leccino’ and ‘Levantinka’ were used as pollen donor genotypes in the cross-pollination treatment, and pollen from ‘Oblica’ trees was applied in the self-pollination treatment. Pollen germination in vitro was enhanced when pollen grains were exposed to higher temperatures two days before the start of flowering (29% for covered trees vs. 17% for uncovered trees in 2009; and 12.3% vs. 10% in 2010). Higher temperatures advanced the appearance of full bloom and shortened the duration of flowering period by one day for ‘Oblica’ trees inside the cages. No significant difference in pollen germination on the stigma was observed between the trees exposed to different temperatures. Pollen donor genotype affected pollen germination in vivo. Temperature had significant influence on pollen tube growth in the style and on fertilization percentage, but the reaction was genotype dependent. Higher temperatures reduced pollen tube growth in the style (decrease for 17% in 2009 and for 9% in 2010) and fertilization percentage (decrease for 9% in 2009 and for 8% in 2010) by increasing the time required for pollen tubes to reach the base of the style or the ovule. Due to faster pollen tube growth, the percentages of pollen tube growth and fertilization were significantly higher in cross-pollinated flowers (94.9% in 2009 and 70.9% in 2010 for pollen tube growth; 58.9% in 2009 and 51.4% in 2010 for fertilization) than in self-pollinated flowers (35.4% in 2009 and 59.6% in 2010 for pollen tube growth; 19.3% in 2009 and 39.1% in 2010 for fertilization). The results of our study support the model that the self-incompatibility level in olive is regulated by temperature because differences in self-fertilization percentage were observed between ‘Oblica’ trees outside and inside the polyethylene cages.