What else don’t we know about biennial bearing? Phytohormone profile of seeds and seed number per fruit differ between a biennial and a non-biennial apple cultivar

Among other fruit crops, many apple (Malus × domestica Borkh.) cultivars are prone to alternate, or biennial bearing, which can be described as irregular cropping with high yields in “ON”-years and low or no crop load in the subsequent “OFF”-years. Such cropping behavior causes economic losses in commercial apple production and needs to be artificially stabilized by annual removal of excessive flowers or fruit in order to avoid OFF-years. In apple, subtending spur buds, which serve as flowering organs for the following year, grow and develop simultaneously with the young fruit. It has therefore been suggested that the seeds of the growing fruit produce some compound, which moves to the adjacent buds and suppresses flower bud development. This leads to poor flowering and hence reduced crop load in the subsequent OFF-year. In this study, we sampled seeds from the biennial bearing apple cultivar ‘Fuji’ and from the regular bearing cultivar ‘Gala’ 97 and 168 days after full bloom (DAFB) and compared number of seeds per fruit, seed weight, and water content in the seeds. The results showed that ‘Fuji’ apples collected 168 DAFB contained on average 3.7 more seeds than ‘Gala’, whereas the average seed weight did not differ between the cultivars and was around 60 mg per seed. Water contents in the seeds of ‘Fuji’ and ‘Gala’ were 37.3 and 45.0% of seed fresh weight (FW), respectively, and did not show significant difference between the cultivars. Moreover, using mass-spectrometry we analyzed 39 known phytohormones in the seeds collected from both cultivars during flower bud development (68, 83 and 97 DAFB). From all the analyzed phytohormones, 29 were present in apple seeds. Seven out of eight detected cytokinins and three auxin forms were significantly more abundant in ‘Fuji’ seeds compared to the seeds of ‘Gala’. At the same time, nine out of 12 analyzed gibberellic acids were significantly more abundant in ‘Gala’ seeds compared to ‘Fuji’ seeds. The study provides new insights on the phytohormone profile of apple seeds and addresses the question whether some of these compounds may represent diffusible signals involved in biennial bearing.