INTERACTIONS BETWEEN ROOTSTOCK GENOTYPE AND SOIL ENVIRONMENT AFFECT SCION PHYSIOLOGY AND MINERAL NUTRITION OF APPLE

Thesis (Ph.D.), Horticulture, Washington State UniversityIrrigation is essential for many apple production regions, which elevates the risk in the future where water shortages will likely occur. Soil environment is a critical factor contributing to tree growth and development. Apples are composite woody perennials composed of a genetically distinct rootstock and scion. Rootstock genotypes can strongly vary in vigor and productivity. However, the interactions between rootstock genotypes and soil moisture and temperature have not been extensively studied. The objective of this research was to evaluate the influence of rootstock genotype on scion physiological and nutritional responses under different soil environmental conditions. In the greenhouse, ‘Honeycrisp’ and ‘Gala’ apple cultivars were grafted onto G41, G890, M9, and B9 rootstocks. In the field, ‘Honeycrisp’ was grafted onto the same rootstock genotypes. Two irrigation treatments were established: a water-limited and a well-watered control for both experiments. Physiological measurements such as leaf gas exchange, stem water potential, shoot growth, and quantum yield of photosystem II (ΦII) were made every two weeks since the onset of the experiments. At the end of each experiment, tree growth was assessed, and nutrient concentration and carbon isotope composition (δ13C) was measured in roots, stem, and leaves. In both experiments, water limitations reduced aboveground biomass and, to a lesser extent, root biomass. When G890 was used as a rootstock, growth, and mineral nutrient accumulation was more plastic to water limitations. ‘Gala’ on all rootstocks and both scions on G890 had elevated mineral nutrient uptake. Water-limited conditions increased the nutrient concentration in roots and stems but had no effect on leaves. ‘Honeycrisp’ grafted onto G890 was the most responsive to drought indicated by decreasing stomatal conductance, reducing net CO2 exchange rates, ΦII, and ultimately, shoot growth. In contrast, B9 maintained growth and stomatal conductance when water-limited and had the highest δ13C and lowest stem water potential. These findings demonstrate differential responses of rootstock genotypes to soil environment and indicate opportunities for selection of rootstocks that are more suitable in water limited regions.Washington State University, Horticultur