Heat stress during reproductive development inhibits fertilization in olives

In this study, we investigated the effect of heat exposure during olive inflorescence on reproductive development. We hypothesized that male development processes during this period are particularly sensitive to high temperatures. We characterized the development of flowers and inflorescences under natural conditions in the cultivars 'Arbequina' and 'Koroneiki', and defined eight phenological stages, beginning with the emergence of reproductive buds until anthesis (international standard BBCH scale of the phenological inflorescence development phases 53–60). In addition, we examined the response of olive inflorescence and flowering, to two heat scenarios: prolonged exposure to moderately high temperatures and a two-hour concentrated heat shock treatment. Analysis of our data revealed that both treatments had a detrimental effect on the development and functioning of the tapetum, the innermost layer of the anther, which is essential for the development and functioning of pollen. Additionally, we found that the 'Arbequina' cultivar displayed relative tolerance to heat compared to 'Koroneiki' in terms of pollen germinability after heat stress. In contrast to the response of the above cultivars, the 'Souri' exhibited an exceptionally high sensitivity to heat. An assessment of pollen viability after in vitro heat shock treatment of 54 cultivars revealed that pollen from seven varieties, 'Niedda de Oliana', 'Lechín de Sevilla', 'Tonda Oliana', 'Tamir', 'Sigoise', 'Taggiasca', and 'Carrasquenha', showed exceptional tolerance to heat. These cultivars show a greater degree of suitability for cultivation in warm climates, (or in areas susceptible to extreme hot-spells). This study demonstrated that heat has a negative impact on the male reproductive system in olives, during early flower development and in mature pollen grains, which are relatively resistant to heat. Heat stress impaired pollen tube elongation capacity, and germination rate. Additionally, heat stress affected the ultrastructure of the external wall of the pollen grain, the exine, potentially impairing its ability to adhere to the stigma and fertilize the ovule. The sensitivity of olive reproductive capacity to heat is a genotype-dependent trait, allowing for the identification of heat-resistant cultivars for use as pollen donors and for development of new heat-resistant cultivars.