2018 ASHS Annual Conference

‘Honeycrisp’ has rapidly increased in production in Washington State over the last 20 years. This cultivar is predisposed to develop bitter pit during storage and produces oversized fruit. Previous work has identified a link between plant water status, fruit size, and bitter pit when crop load was controlled indicating that irrigation could be used as a tool to control fruit size and bitter pit. However, accurate indicators of the physiological water status of ‘Honeycrisp’ need to be developed. Currently, there is little known about how ‘Honeycrisp’ responds to developmentally-timed water limitations. Here, three-year-old ‘Honeycrisp’ were subject to four different irrigation treatments where soil moisture was reduced to approximately 30-40% of field capacity for 30-day increments. Early water limitations were imposed from 15-45 days after full bloom (DAFB), mid-season water limitations were imposed from 45-75 DAFB, and late-season water limitations were imposed from 75-105 DAFB. The well-watered control was maintained at 80-90% of field capacity for the entire season. Physiological measurements were made every 15 days from 30 to 105 DAFB. Measurements included leaf gas exchange, plant water status, stomatal conductance, chlorophyll fluorescence, and leaf reflectance to assess how ‘Honeycrisp’ apple responded to water limitations. At the end of the season, vegetative growth and return bloom was measured to assess the impact of water deficits on growth and productivity. Stomatal conductance was significantly influenced by irrigation deficit. During the early water limitation, stomatal conductance was nearly 40% and 70% lower during middle and late-season water limitations compared to the control. Photosynthesis was approximately 50% lower during all water limited periods compared to the control. Stem water potential was also influenced by deficit irrigation and decreased by 30-50% throughout the growing season and were -1.69 MPa, -2.33 MPa, and -2.70 MPa at the end of the early, middle, and late-season water limitations, respectively, compared to -1.23 MPa for the well-watered control. Stem water potential and stomatal conductance were the most responsive during water limitations carried out later during the growing season when temperatures and vapor pressure deficit (VPD) were greater. Early season water limitations have a lower impact on plant response to abiotic stress compared to late-season deficits. Stem water potential, an integrator of soil moisture availability and water demand, and stomatal conductance measurements have the potential to guide in the making of irrigation decisions for fruit size control in ‘Honeycrisp’ apple to better meet market targets and reduce bitter pit.