2019 ASHS Annual Conference

Rootstocks are critical for Malus x domestica Borkh. (apple) production to enhance fruit quality and productivity that increases economic returns for apple producers. With new apple cultivars being introduced in the market and increasing environmental concerns, the need of improved rootstocks. While variation in the dwarfing capacity of rootstocks have been widely described, the physiological mechanisms controlling dwarfing have not been identified. Developing a physiological understanding of how rootstocks affect plant water relations may offer a great opportunity to develop better rootstocks in the future. Currently, there is an assumption that the changes to the graft union and xylem conduit structure may increase hydraulic resistance and thus, decrease plant vigor for dwarfing rootstocks. Anatomical traits and physiological manipulation of water status may be correlated, and both may influence rootstock and scion development. Both carbon and oxygen isotope composition have been previously associated with carbon dioxide and water exchange with the atmosphere that has been closely associated with stomatal control. Here, `Honeycrisp` apple trees grafted on G.87, G.210, G.814, G.969, G.935, G.890, CG.5257, CG.4292, M.26 and B.9 and `WA38` apple trees grafted on CG.4010, CG.4213, CG.4288, CG.4292, CG.4809, CG.5030, CG.5087, CG.6040, CG.4213 were used to link leaf-level physiological traits with carbon and oxygen isotope analysis. Measurements included transpiration rate, stem water potential and stomatal conductance and were compared with leaf carbon and oxygen isotope composition and shoot growth. Both stomatal conductance and stem water potential were correlated with the plant growth in both cultivars, `Honeycrisp` and `WA38` (r²= 0.08; P-value 0.003). Furthermore, there was a significant effect of rootstock on leaf carbon and oxygen isotope composition which can indicate changes in stomatal control and/or leaf anatomy. For the same cultivar, it was possible to observe differences in shoot length over the growth in the season. Moreover, rootstocks had a significant impact on stomatal conductance and stem water potential, therefore, these areas should be the focus to understand the specific effects of rootstocks on plant water status and specifically, the plant components of the soil-plant-atmosphere continuum.