Water for irrigation will likely be less available in apple-growing regions due to climate change and competition with human needs other than agriculture. Apple cultivars and rootstocks may differ in water use necessary for acceptable cropping. In two greenhouse experiments in 2014 and 2015 rootstocks (M.9 and MM.111) and scions ('Gala' and 'Fuji') with known differences in size control and potential resistance to drought were compared under conditions of reduced water availability. After 1 week without irrigation in 2014 the potting soil moisture and leaf water potential (ΨL) were reduced more in trees on MM.111 than M.9 rootstocks and by 'Gala' than 'Fuji' scions. Abscisic acid (ABA) and associated metabolites dihydrophaseic acid, abscisic acid glucose ester, and phaseic acid generally were greater for both scions on M.9 than MM.111. Concentrations of ABA metabolites were greater in 'Gala' than 'Fuji' suggesting significant metabolic rates in leaves. As noon ΨL decreased between -1.5 and -2.0 MPa leaf ABA levels increased exponentially to concentrations above 500 ng/g dw and stomatal conductance (gs) decreased to less than 50 mmole H2O m-2 s-1. At reduced gs carbon assimilation (A) was low but greater in trees on M.9 than on MM.111. Trees grown on M.9 partitioned less dry weight (dw) to roots than MM.111. However the% dw partitioned to fine roots was greater in M.9 than MM.111 which may have helped trees grow in a small environment such as a pot. In 2015 trees were grown in 45- and 75-L pots and irrigated over 3 weeks to maintain a steady ΨL between -1.5 and -2.5 MPa. This longer-term water stress in 2015 resulted in greater gs and A in trees on MM.111 and in larger pots but WUE was still greater in trees grafted to M.9 in 3 of the 5 drought dates measured. Both rootstocks provided drought resistance, but by mechanisms which appear to differ and which may provide benefits to apple trees grown with reduced irrigation or with water stress. The dwarfing rootstock M.9 produces higher levels of ABA that may regulate stomatal opening and improve short-term drought resistance. The more invigorating rootstock MM.111 may be drought hardy in the longer-term due to development of a more extensive root system.
Tworkoski, T., Fazio, G., & Glenn, D. M. (2016). Apple rootstock resistance to drought. Scientia Horticulturae, 204, 70–78. https://doi.org/10.1016/j.scienta.2016.01.047