Assessment of effective pollination period in olive (Olea europaea) by means of fluorescence microscopy and plant response to sequential pollinations: limitations and drawbacks of current methodologies

The olive tree response to sequential pollinations is a more accurate procedure in the assessment of effective pollination period and flower life span than procedure based on the calculation of the components of the pollen-pistil interaction. Effective pollination period (EPP) was defined by Williams (1965) as the period during which pollination can result in fruit set. EPP was proposed to be the time the ovules remain fertile minus the time needed for a pollen tube to reach the ovule, providing that this value does not exceed the length of stigma receptivity. The duration of the EPP in fruit trees can be estimated by the microscopic analyses of these three components, but also by the plant response to sequential pollinations in term of fruit set. Here, we compare the estimation of EPP duration under cross-pollination in the main olive cultivars (Olea europaea L.) grown for oil production in Croatia ('Lastovka', 'Leccino', and 'Levantinka') based on these two procedures to discuss the advantages and disadvantages of each estimation and propose the best methodology to assess EPP duration and the capacity of flowers of different age to set fruit. Our comparison highlights that the different procedures provide a contrasting duration of EPP in all olive cultivars here analyzed. EPP based on initial fruit set and final fruit set was between 2 and 3 days, while that based on the microscopic study of EPP components was between 5 and 12 days, depending on the cultivar. We defend that EPP based on fruit set is more accurate and more relevant for growers, and believe that the longer duration of EPP when the estimation is based on the study of the components could be due to wrong assessments of the duration of particular components. In this sense, pollen germination may be a better parameter than pollen adhesion to estimate stigma receptivity. Ovule longevity assessment based on the fluorescence emitted could be an overestimation of the actual ovule readiness for fertilization. Style suitability, not considered in Williams' equation, plays an important role supporting pollen tube growth toward the ovules and might be also relevant in determining EPP duration.