QTL Mapping of Apple Rootstock Mediated Uptake and Transport of Sodium and Potassium Ions, Monitored in Apple Scion Leaves

Nutrient and micronutrient absorption and translocation are important apple rootstock traits that have been largely ignored in breeding programs due to the high cost connected to testing tissue samples for mineral concentration. Developing a methodology that would allow the discovery, breeding and selection of efficient absorbers and translocators of mineral nutrients would greatly improve the overall impact on sustainable apple production worldwide. Furthermore, it could improve the concentration of essential dietary minerals for humans.  For this experiment we grafted an apple rootstock F1 pseudo test cross mapping population (from a cross between Ottawa 3 and Robusta 5) with the same scion (Gala) and grew these finished trees in a replicated potted trial with uniform soil media and fertilizers.  At the end of the growing season we measured leaf mineral concentrations of the leaves (dry weight basis), and used these data to test for genetic influence on the uptake of macro- and micronutrients.  Here we report on a major QTL (LOD 5.4) discovered on Linkage Group 5 of the apple genome that explains 25% of the genetic variation for potassium concentration in leaves, where one of the alleles increases the mean potassium concentration in leaves by 18% over other segregating alleles.  We also report on another major QTL (LOD 4.75) discovered on Linkage Group 17 that explains 22% of the genetic variation for sodium concentration in leaves, where one of the alleles increases the mean sodium concentration in leaves by 25% over other segregating alleles.  While the physiological influence of these alleles remains largely unknown, the discovery of heritable genetic variation for mineral transport and translocation to the scion is a significant discovery that may lead to better understanding of genes and pathways that improve plant nutrition.  Furthermore, the development of apple rootstock lines with increased efficiency in the uptake and translocation of important nutrients may lead to decreased waste of fertilizers used in apple production.  Long term plans include incorporating such knowledge into the Marker Assisted Breeding scheme of our breeding program and improving our phenotypic selection pipeline.