Selenium as stressor and antioxidant affects pollen performance in Olea europaea

Selenium (Se) as an antioxidant is a trace element essential to wellness and the maintenance of human health. Although it has not been confirmed to be an essential micronutrient in higher plants, there is increasing evidence of its benefits in plants in which it inhibits the detrimental effects of environmental stressors, while only few studies refer to its action on pollen germination. Selenium enhances the stress tolerance regulating the production and quenching of reactive oxygen species (ROS); however, the endogenous ROS are essential to the cross-talk between pollen and stigma and promote pollen tube growth. The action of Se has many mechanisms, not all yet fully clarified. In order to deepen the knowledge and fill the gaps in the role of Se as an inhibitor of ROS and, at the same time, a promoter of pollen germination, we attempted this research, enriching olive trees growing in pots and in the field with Se. The plants in pots were kept at a controlled water regime in order to induce drought stress. To test the effect of antioxidant on pollen performance, a single application of Se was supplied to the plants at the beginning of pollen development. Two olive cultivars (Arbequina and Maurino) were used in three different experiments in which Se enrichment was carried out through i) endo-xylematic drip injection, ii) foliar spray, iii) soil application. The pollen performance was assessed at anthesis. The results showed that Se enrichment in non-stressed plants induced a higher rate of pollen viability and germination, but it did not always stimulate their reproductive performance. Different responses were obtained in drought stressed plants, in which Se induced pollen germination, obtaining a performance similar to non-stressed plants. The ROS detection by a quantitative method, applied on hydrated pollen, verified the results just discussed.