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Title:   Author  item
                                            Tworkoski, Thomasitem
                                            Fazio, Gennaroitem
                                            Glenn, David
  Submitted to: Scientia Horticulturae
                                    Publication Type: Peer Reviewed Journal
                                    Publication Acceptance Date: 1/4/2016
                                    Publication  Date: 4/8/2016
                                    Citation: Tworkoski, T., Fazio, G., Glenn, D.M. 2016. Apple rootstock resistance to drought. Scientia Horticulturae. 204:70-78.Interpretive Summary: Water for irrigation will likely be less available in apple-growing regions due to climate change and competition with human needs other than agriculture.  Two experiments were conducted to improve understanding of the biological basis for apple cultivar and rootstock differences in water use that is necessary for acceptable fruit quality and yield.  Both rootstocks provided drought resistance but by mechanisms which differ and can provide benefits to apple trees grown with reduced irrigation or with water stress.
                                Technical Abstract: 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 one week without irrigation in 2014, the potting soil moisture and leaf water potential (WP) were reduced more in trees on MM.111 than M.9 rootstocks and by ‘Gala’ than ‘Fuji’ scions.  Abscisic acid (ABA) and associated metabolites ABAGE, PA, and DPA generally were greater for both scions on M.9 than MM.111.  ABA metabolites were greater in ‘Gala’ than ‘Fuji’ suggesting significant metabolism in leaves.  As noon WP decreased between negative 1.5 and negative 2.0 Mega Pascals (MPa), leaf ABA levels increased exponentially to concentrations above 500 ng / g dry weight and stomatal conductance (SC) decreased to less than 50 mmole water/square meter/sec.  At reduced SC, carbon assimilation (A) was low but greater in trees on M.9 than on MM.111 rootstocks.  Trees grown on M.9 partitioned less dry weight to roots than MM.111.  However, the percent dry weight 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 three weeks to maintain a stable WP between negative 1.5 and negative 2.5 MPa.  This longer-term water stress in 2015 resulted in greater SC and A in trees on MM.111 rootstock and in larger pots but water use efficiency was still greater in trees grafted to M.9 rootstock in three of the five 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.


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