Abstract
Adjustable crop load primarily involves bud manipulation, and usually switches from vegetative to reproductive buds. While this switch is not fully understood, it is still controlled by the ratio of hormones, which promote or inhibit bud formation. To determine the reasons for biennial bearing, the effect of apple rootstock, scion cultivar, crop load, as well as metabolic changes of endogenous phytohormones [zeatin, jasmonic acid, indole-3 acetic acid (IAA), abscisic acid (ABA), and gibberellins 1, 3, and 7 (GAs)], and soluble sugars (glucose, fructose, and sorbitol) were evaluated, and their connections with return bloom and yield of apple tree buds were analyzed. Cultivars “Ligol” and “Auksis” were tested on five rootstocks contrasting in induced vigor: semi-dwarfing M.26; dwarfing M.9, B.396, and P 67; and super-dwarfing P 22. Crop load levels were adjusted before flowering, leaving 75, 113, and 150 fruits per tree. Principal component analysis (PCA) scatter plot of the metabolic response of phytohormones and sugars indicated that the effect of the semi-dwarfing M.26 rootstock was significantly different from that of the dwarfing M.9 and P 67, as well as the super-dwarfing P 22 rootstocks in both varieties. The most intensive crop load (150 fruits per tree) produced a significantly different response compared to less intensive crop loads (113 and 75) in both varieties. In contrast to soluble sugar accumulation, increased crop load resulted in an increased accumulation of phytohormones, except for ABA. Dwarfing rootstocks M.9, B.396, and P 67, as well as super-dwarf P 22 produced an altered accumulation of promoter phytohormones, while the more vigorous semi-dwarfing M.26 rootstock induced a higher content of glucose and inhibitory phytohormones, by increasing content of IAA, ABA, and GAs. The most significant decrease in return bloom resulted from the highest crop load in “Auksis” grafted on M.9 and P 22 rootstocks. Average difference in flower number between crop loads of 75 and 150 fruits per tree in “Ligol” was 68%, while this difference reached ~ 90% for P 22, and ~ 75% for M.9 and M.26 rootstocks. Return bloom was dependent on the previous year’s crop load, cultivar, and rootstock.
Highlights
Apple rootstocks are categorized according to tree vigor, i.e., dwarfing, semi-dwarfing, semi-vigorous, and vigorous
Experiments investigating the effect of apple rootstock, scion cultivar, and crop load on apple return bloom were performed in Lithuania (55°60′ N, 23°48′ E) in 2014–2016
Phytohormone analysis was performed in buds collected in mid-September, according to the procedure described by Šimura et al (2018) with modifications, using ultra performance liquid chromatography (UPLC; Waters) combined with mass spectrometry (LC-MS/MS)
Summary
Apple rootstocks are categorized according to tree vigor, i.e., dwarfing, semi-dwarfing, semi-vigorous, and vigorous. The proposed role of GAs in the dwarfing response (Van Hooijdonk et al, 2010) has been unclear since Van Hooijdonk et al (2011) reported that concentrations of GA19 were similar in the xylem sap collected throughout the growing season from scions grafted on to M.9 or MM.106 rootstocks, and suggested that GA19 may be a precursor of bioactive GA1, required for shoot extension growth. There is evidence of metabolite dynamics, especially of raffinose family oligosaccharides, being correlated with many cold-related gene expression changes during transition to flowering (Peace et al, 2019) It is unclear whether the reduced sugar concentrations in dwarfing rootstocks are the cause or result of altered hormone levels or signaling, these metabolic changes have a significant effect on grafted apple tree growth and development. The aim of the present study was to evaluate interactions between apple rootstocks, scions and different crop load levels that could influence return bloom and identify the differences in physiological status between dwarfing and vigorous rootstocks
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