Study of reproductive behaviour in low-chill apples in warmer zones of Argentina

• The low-chill apples studied displayed a high rate of fruit set by selfing under mild-winter conditions.
• The rate of self-compatibility varied widely between growing seasons, depending on the cultivar.
• Cross-pollination improved fruit set in most of the growing seasons.

The domesticated apple (Malus × domestica Borkh.) shows abnormalities in its flowering biology when it is growing in mild-winter areas. Hence, the flowering phenology and the mating system (e.g., open-, cross- and self-pollination, apomixis and parthenocarpy) of Caricia, Eva, and Princesa apple cultivars were studied in three consecutive growing seasons. The flowering period ranged between August and October with wide variation between years, especially in relation to the overlapping of the full-bloom periods. Fruit set by selfing was significant, reaching a mean value of 20% with a minimum of 4.5% and a maximum of 43.7%, depending on the growing season and the cultivar. The self-incompatibility index showed significant changes between years in Eva and Princesa but not in Caricia. Fruit and seed set were significantly greater in cross- and open-pollination treatments than by self-pollination, apomixis or parthenocarpy. Those variables were affected by a growing season × treatment interaction. The results indicated that Caricia, Eva, and Princesa apples show partial to full self-incompatibility, depending strongly on the growing season. However, better fruit and seed set rates are obtained by cross-fertilization.