Histological and proteomic approaches to study floral bud induction in relation to biennial bearing in apple

Abstract

Biennial or alternate bearing in fruit trees leads to fluctuating yields, characterized by ‘On’ years with high crop loads followed by ‘Off’ years with little or no yield. To study the mechanism of biennial bearing in apple (Malus domestica Borkh.), the cultivars ‘Royal Gala’ as a regular bearer and ‘Fuji’ as a strongly alternating cultivar were investigated. ‘Off’ trees were completely thinned at full bloom, whereas ‘On’ trees were not thinned to maintain the natural high crop load. The first signs of floral bud initiation were detected for ‘Fuji’ at 70 days after full bloom (DAFB) and 120 DAFB for ‘Off’ and ‘On’ trees, respectively. In ‘Royal Gala’, initiated buds were observed not before 100 DAFB in both treatments. Hence, buds from ‘Fuji’ ‘Off’ trees committed to flowering about one month earlier than those from ‘Royal Gala’ ‘Off’ trees. In contrast, flower initiation of buds from ‘Royal Gala’ ‘On’ trees occurred 20 days earlier when compared to those from ‘Fuji’ ‘On’ trees. Electrospray ionization mass spectrometry (ESI-MS) coupled with ultra-high performance liquid chromatography (UHPLC) allowed the identification of 3011 proteins. A Student’s T-Test revealed 181 differentially expressed proteins between buds from ‘On’ and ‘Off’ trees. Specifically, 81 proteins were significantly upregulated in floral buds and 100 proteins in vegetative buds, respectively. The largest log2 fold change of 3.17 was identified for a zinc metalloprotease in buds from the ‘Fuji’ ‘On’ treatment. The protein with the highest up-regulation in ‘Fuji’ ‘Off’ buds was a nigrin b-like protein. Histological analysis of bud development has proven to be an essential and reliable tool to identify the time point of floral initiation in apple. Moreover, this study demonstrates that especially in biennial bearing cultivars, crop load heavily influences the onset of floral bud initiation and can differ for up to 50 days within the same cultivar depending on crop load. The identification of this critical period enables in-depth analysis of biochemical changes such as differences in protein profiles to further reveal the mechanism of biennial bearing in apple.