Breeding apple rootstocks for modulation of mineral nutrients in scions

A major function of the root system of an apple tree is to gather mineral nutrients from soil and shuttle them into different sinks in the canopy including leaves and fruit. Significant phenotypic and associated genotypic diversity has been discovered in apple rootstocks with regards to modulation of mineral nutrient concentrations in grafted scion varieties. A series of replicated rootstock field experiments were conducted with 'Golden Delicious', 'Gala', 'Honeycrisp' and 'Fuji' scions aimed at unraveling the phenotypic and genotypic potential of apple rootstocks to modulate the concentration of potassium (K), sodium (Na), phosphorous (P), calcium (Ca), copper (Cu), sulfur (S), zinc (Zn), magnesium (Mg), nickel (Ni), and molybdenum (Mo) in fruit and leaves over multiple growing seasons. Several rootstocks in these experiments were genotyped with single nucleotide polymorphism (SNP) and microsatellite markers to discover quantitative trait loci (QTLs) associated with rootstock mineral nutrient traits and to follow the effect of these markers in the general breeding populations. Our work detected significant genotypic mean correlations between many of the measured mineral nutrients and significant mineral nutrient QTLs over multiple years. Where possible, we tested for the presence of strongly associated markers in diverse field experiments to learn if they could be used for marker aided breeding. Preliminary analyses indicate that gain from selection with markers is possible, however because of the multiple correlated mineral nutrient traits, it may not be possible to disentangle selection of a positive effect nutrient on productivity, with a correlated nutrient having negative effects on fruit quality. Breeding and selection for specific nutrient profiles is further complicated by the genetic and physiological characteristics of the grafted scion. While we may have been successful in identifying some rootstocks with preferable nutrient profiles with specific scions, more data are needed to build a genetic roadmap for breeding new rootstocks that improve productivity and fruit quality.