Elucidating the Genetics of Absorption and Translocation of Macro- and Micronutrients by Apple Rootstocks in the Context of Breeding Populations

Apple rootstocks are the foundation of a productive apple orchard by providing anchorage, access to water and nutrients, precocity, dwarfing, and disease protection to the scion.  During the process of breeding new apple rootstocks, very little attention has been devoted to studying the absorption and translocation of macro- and micronutrients into the scion leaves and fruit.  Recent research in our lab has uncovered abundant genetic variation in the apple rootstock germplasm pool for traits relating to plant nutrition. We grafted replicates of an apple rootstock breeding population 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. Quantitative trait analysis in this breeding population uncovered quantitative trait loci (QTLs) for leaf mineral concentrations of leaves for potassium (K), sodium (Na), phosphorous (P), calcium (Ca), zinc (Zn), magnesium (Mg), and molybdenum (Mo). Correlation analysis of the relationships between different nutrients uncovered significant positive linear correlations between Ca and Cu, Mg, P, and S.  A significant correlation was also detected between Cu and K, Cu and P, and also between K and P and between S and P.  Segregation of a major QTL for leaf K concentration in certain rootstocks had strong effects on the concentrations of other nutrients in the leaves.  We are attempting to understand the physiological influence of these QTLs on other measurable traits in apple rootstocks and scions.  It is possible that even subtle changes in plant nutrients caused by variable gene combinations in the rootstocks can affect productivity and disease resistance of apple trees.  Of the elite and commercial apple rootstocks released by our program, G.890, G.210, G.935, G.214, CG.5087, and CG.4011; all inherited the high K leaf concentration QTL, while only G.969 inherited the low K allele.  The preponderance of the high allele in elite rootstocks might be a sign of its importance for field performance.  The discovery, breeding and selection of efficient absorbers and translocators of mineral nutrients should increase efficiency and predictability for developing better rootstocks and have a positive impact on sustainable apple production worldwide.