Assessing the causes of the low productivity of cider apple orchards in the Spanish region of Gipuzkoa

Apple cider-making has a long tradition in many northern Spanish regions, such as Gipuzkoa in the Basque Country. The production in this region is largely based on traditional local cider cultivars for which very little information is available on their pollination requirements, resulting in a generalized low productivity of the orchards. In this work the self-(in)compatibility, S-alleles and pollen quality have been studied for the main cider cultivars grown in Gipuzkoa, which represent a wide genetic diversity. The S-RNase genotype of 25 cultivars is firstly reported here, which allows allocating the cultivars in their corresponding incompatibility group. All the S-alleles have been previously described, being S₂₆, S₃ and S₅ the most frequently found. Regarding pollen quality triploid cultivars, as expected, showed lower viability (<70%) and germination rates (<20%). Whereas most diploid cultivars showed good pollen viability (>80%) and germination (>30%) rates, it is noteworthy the low performance of ‘Urtebete’, very similar to that of triploids. Self-compatibility tests both in vitro and in situ showed low rates of fruit set and, together with an upper position of the stigmas related to stamens, reveal a low degree of self-compatibility for most cultivars. Surprisingly, two cultivars called ‘Moko’ and ‘Txalaka’, showed certain self-compatibility with around 7% fruit set. All in all, the results will enable to improve traditional cultivars orchard design as well as the profitability of apple cider farms in the region.