2017 ASHS Annual Conference

There has been recent evidence to suggest that differences exist in water stress responses amongst apple cultivars. Apple cultivars can either be isohydric or anisohydric. Isohydric cultivars maintain constant leaf water potential by reducing stomatal conductance under water deficit conditions. Anisohydric cultivars, on the other hand, keep their stomata open much longer under water deficit conditions and allow their leaf water potential to decrease to a specific threshold. The implication of these different strategies is that the use of plant water status as a ecophysiological stress indicator should be done with caution due to variation in leaf water potential caused by isohydric/anisohydric behavior. Here, we conducted an exploratory study looking at differences in tree water responses amongst different cultivars. The trial was done under transient water stress conditions with high vapor pressure deficit. The experiment was conducted on potted second leaf ‘Granny Smith’, ‘Gala’ and ‘Honeycrisp’ trees at the Washington Tree Fruit Research and Extension Center in Wenatchee, WA. The trees we grafted on M9 T337 rootstock, planted 0.6 X 1.5 m apart. Soil moisture status was measured manually in each pot. Leaf water potential (Ψ_l), stomatal conductance and chlorophyll fluorescence measurements were taken at two-hour intervals from 08:00 to 16:00 for each parameter measured. Additionally, predawn leaf water potential (Ψ_pd) was measured at 04:00 to 04:45 HR and midday stem water potential (Ψ_md) at 13:15 to 14:00 HR. Results showed no differences in volumetric soil water content, therefore differences in soil water content can be discounted as having affected our results. Stomatal conductance measured using a steady state porometer was significantly higher in ‘Granny Smith compared to ‘Gala’ and ‘Honeycrisp’ at all sampling times. Ψ_pd was significantly lower in ‘Granny Smith’ (-0.26 MPa) compared to ‘Gala’ (-0.14 MPa) and ‘Honeycrisp’ (-0.12 MPa), similarly Ψ_md was significantly lower in ‘Granny Smith’ (-1.76 MPa) compared to ‘Gala’ (-1.37) and ‘Honeycrisp’ (-1.33 MPa). Expect for 08:00, Ψ_l was more significantly negative in the ‘Granny Smith’ compared to ‘Gala’ and ‘Honeycrisp’. Chlorophyll fluoresce showed no differences in maximum photochemical efficiency of PSII (Fv/Fm) and quantum photosynthetic yield of PSII (Y(II)) between the three cultivars. In conclusion, ‘Gala’ and ‘Honeycrisp’ had a more controlled response to water stress compared to ‘Granny Smith’. The use of plant water status as an ecophysiological stress indicator needs to be revisited in apple since differences exist between cultivars.