Amount and quality of pollen grains in four olive ( Olea europaea L.) cultivars as affected by ‘on’ and ‘off’ years

•Olive pollen grains viability was estimated by using three techniques.•Fluorescein diacetate and germination were significantly correlated.•Differences were detected among the cultivars for pollen amount and viability.•Pollen grains per anther were always negatively correlated with germination.•Viability was significantly higher in the ‘off’ year with respect to the ‘on’ one.