Relationships Among the Rootstock, Crop Load, and Sugar Hormone Signaling of Apple Tree, and Their Effects on Biennial Bearing

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.

Inheritance of Yield Components and Morphological Traits in Avocado cv. Hass From “Criollo” “Elite Trees” via Half-Sib Seedling Rootstocks

“…Hass plantations may confer a beneficial horticultural quality to the tree across a wide spectrum of traits ( Reyes-Herrera et al, 2020 ), such as increased fruit yield ( Herrera-González et al, 2013 ), postharvest performance ( Willingham et al, 2001 ), vegetative vigor ( Mickelbart and Arpaia, 2002 ), salt tolerance ( Bernstein et al, 2001 ), and disease resistance ( Smith et al, 2011 ; Sánchez-González et al, 2019 ). These reports are in line with previous research that have shown how rootstocks might also induce less trivial scion morphological changes, such as dwarfing, and even alter yield traits and fruit quality ( Egea et al, 2004 ; Picolotto et al, 2010 ; Madam et al, 2011 ; Expósito et al, 2020 ; Kviklys and Samuoliene, 2020 ). For instance, rootstock effects may even influence properties typically attributed to the clonal Hass scion, such as fruit sensorial and nutritional quality, e.g., texture, sugar content, acidity, pH, flavor, and color ( Giorgi et al, 2005 ; Gullo et al, 2014 ; Balducci et al, 2019 ), cold tolerance, and shoot pest and pathogen resistance ( Rubio et al, 2005 ; Goldschmidt, 2014 ).…”

Metabolic Pathways for Observed Impacts of Crop Load on Floral Induction in Apple

“…Histological analysis has shown that the initial development of a flower bud coincides with a developing embryo or fruitlet [ 6 ]. The developing seed in young fruitlets is believed to repress flower induction in vegetative buds via phytohormones and removal of fruitlets at the 3–18 mm fruit size, through thinning practices have been shown to lead to a more consistent flower formation (return bloom) for the following season [ 3 , 7 , 8 ]. There have been investigations of other factors that influence flower formation in apple such as stress associated with temperature, photoperiod [ 9 , 10 ], water deficit as well as internal factors associated with carbon-nitrogen ratio, hormones and interaction with other organs (leaves, terminal shoot growth, and fruit) [ 3 ].…”

Inheritance of Rootstock Effects in Avocado (Persea americana Mill.) cv. Hass

“…The root phenotype may confer direct resilience to root pest and pathogens ( Cháves-Gómez et al, 2020 ) as well as to abiotic stresses ( He et al, 2020 ; Martínez-García et al, 2020 ) such as drought, flooding, and salt soil conditions ( Gautier et al, 2019 ). Rootstocks can also induce less trivial scion morphological changes such as dwarfing and precocity, and even alter yield traits ( Egea et al, 2004 ; Picolotto et al, 2010 ; Madam et al, 2011 ; Expósito et al, 2020 ; Kviklys and Samuolienė, 2020 ). Rootstock effects can go further and influence properties typically attributed to the clonal scion such as fruit sensorial and nutritional quality—e.g., texture, sugar content, acidity, pH, flavor, and color ( Giorgi et al, 2005 ; Gullo et al, 2014 ; Balducci et al, 2019 ), cold tolerance and shoot pest and pathogen resistance ( Rubio et al, 2005 ; Goldschmidt, 2014 ).…”