2018 ASHS Annual Conference

‘Honeycrisp’ and ‘Gala’ apple cultivars are two of the most commonly planted cultivars in Washington State. However, ‘Honeycrisp’ is susceptible to several nutrient-related physiological disorders and Gala is less susceptible. Rootstocks can affect scion growth and nutrient balance and are strongly influenced by the soil environment. Washington State can experience periodic water limitations and elevated soil temperatures. Still, the contribution of rootstocks including rootstock-scion interactions regulating plant responses to soil abiotic stress are poorly understood. The objective of this study was to evaluate the response of potted ‘Gala’ and ‘Honeycrisp’ apple trees grafted to four different rootstocks (G41, M9-337, B9, and G890) to soil abiotic stress (drought and heat). At the WSU Tree Fruit Research and Extension Center, 72 trees (9 of each rootstock-scion combination) were grown in a greenhouse for 60 days. After this period, for three trees each, the water supply was reduced for the drought treatment (50% of field capacity), soil temperature was elevated by approximately 5°C using a thermostat-controlled heating cables for the heat treatment, and the remaining trees were left as an untreated control. Photosynthesis rate (A), transpiration (E), and midday stem water potential (SWP) were collected every two-weeks. After 60 days of treatment, all trees were destructively sampled to determine stem length, total leaf area, and root: shoot ratio. Stem water potential was greater for both cultivars with G890 as a rootstock. SWP was 0.75 MPa and 0.53 MPa higher than the control for Honeycrisp and Gala, respectively. Similarly, photosynthetic rates were 35% less for Gala trees grafted to G890 exposed to drought compared to the control trees while photosynthesis was 46% lower for Honeycrisp on the same rootstock. Stem length of Honeycrisp on B9 was 55% and 30% lower than the control for drought and heat, respectively. Drought reduced leaf area for all rootstock-scion combinations by 50% while the heat treatment moderately reduced leaf area for some scion-rootstock combinations. The root:shoot ratio for B9 rootstock was lower for both scions cultivar under drought compare to control. For G890 rootstock exposed to elevated soil temperatures, the root:shoot ratio was greater for both scions under heat stress compared to the control. We can conclude that the physiological performance and growth of Honeycrisp on G890 was the most affected by drought and M9-337 was the most sensitive to heat stress for both scions.