Genetic variability in macadamia rootstocks and their effects on early vigor traits of ‘HAES 741′

• Grafting scions onto rootstocks is a key propagation method in macadamia. • Rootstocks influences early vigor traits in 4-year-old trees. • The propagation method of rootstocks influences early scion vigor in macadamia. • Vigor of ungrafted rootstocks correlates with scion vigor after grafting. • Trunk circumference could be used as an indicator of vigor in young macadamia trees. Early vigor in tree crops is crucial as it accelerates orchard establishment and enhances resilience to environmental stresses. Rootstocks play an essential role in determining vigor of tree crops. Macadamia is propagated through grafting, however research into rootstocks is rare. This study evaluated a range of macadamia rootstocks including cultivars, standard rootstocks, breeding selections and wild germplasm, focusing on their effects on early vigor (age 4). Phenotypic variability in the scion ‘HAES 741′ was assessed using 24 accessions of seedling and cutting rootstocks over 4 years (2018–2021), analyzing 7 early vigor traits and growth rates. Principal coordinate analysis and cluster analysis revealed high genetic diversity among accessions, clustering into 4 groups. Significant rootstock effects on early vigor were observed. Broad-sense heritability estimates of rootstock effects were moderate for scion height (H 2 =0.48) and total height (H 2 =0.44), holding promise for improvement through selection. Positive correlations among scion growth traits, particularly trunk circumference, (r g = 0.50 to 0.99) suggested their potential as indicators of early vigor. The strong genetic correlations for canopy area (r g = 0.84) and canopy volume (r g = 0.63) between scions of grafted trees and the ungrafted trees of the same rootstock accessions reflect the possibility of trait transmission from rootstock to scion. Among the studied rootstocks, ‘BHI1’, ‘BHY2’ and ‘BHI3’ imparted the highest early vigor. Compared with the current industry standard rootstocks ‘H2’ and ‘Beaumont’, the least vigorous rootstocks ‘Mjan1’ and ‘BDW3’ showed reductions of 18 % and 47% in tree height, 54 % and 74% in trunk circumference and 97 % and 210 % in canopy volume respectively. These findings suggest promising rootstock candidates for high performing orchard systems, although ongoing evaluation until maturity is crucial.