Introduction
Apples (Malus domestica Borkh.), like many horticultural crops, are clonally propagated and are comprised of a rootstock and a scion. Apple production heavily depends on dwarfing rootstocks that enhance yield efficiency, fruit quality, disease resistance, and tolerance to freezing conditions (Sansavini et al, 1981; Palmer and Werthein, 1981; Mika et al, 1981). In the last fifty years, the adoption of dwarfing rootstocks has transformed apple production due to their capability in reducing scion vigor and increasing precocity (Czynczyk, 1981; Webster, 2001; Marini and Fazio 2018). Currently, there are four size categories for apple rootstocks; dwarfing, semi-dwarfing, semi-vigorous, vigorous (Roper, 2001). Despite extensive characterization of rootstock traits in the last century, the mechanisms by which rootstocks control scion growth are still not well defined (Westwood., 1970; Jones., 1971; Jones., 1984).
When plants are grafted, fusing of vascular tissue occurs and can be affected by both rootstock and scion genotype. Dwarfing is the key rootstock-induced trait in apple. Differences in growth among rootstocks can often be seen immediately upon grafting (Roberts, 1927) and rootstocks can modify the graft union and stem xylem morphology, both of which can affect scion water relations (Olien and Lasko, 1984, 1986; Albacete et al., 2015). Several potential mechanisms of dwarfing have been proposed involving hormonal regulation, mechanical limitations, or genomic control (Beakbane and Thompson., 1940, 1947; Tubbs., 1980; Spangelo., 1974; Higgs and Jones., 1990; Soumelidou et al., 1994; Webster., 1995; Seleznyova et al., 2003; Tworkoski and Miller., 2007; Dolgun et al., 2009; Gregory et al., 2013; Harrison et al; 2016). Phenotypic variation in these traits are difficult to characterize but they affect plant hydraulics in composite plants affecting overall tree vigor. Higher throughput approaches are needed that capture the effect of rootstocks on scion water relations and downstream effects on plant growth and development.
Measurement of stable carbon isotope ratios in plant material are used to study plant water relations (Saugier et al, 2012). Carbon isotope ratios are used to calculate intrinsic water use efficiency (iWUE) and it can reflect the balance between carbon acquisition and water consumed by transpiration (Cernusak et al., 2003, Seibt., 2008; Ma et al, 2010; Bchir et al; 2016). Vigorous plants, which can be less water limited, will have less enriched isotopic values compared to water limited species or cultivars (Lehmann et al., 2018). Carbon isotope composition is discriminated during the fixation by Rubisco and during the diffusion across the mesophyll membrane (Guy et al., 1993). The extent of discrimination relates to stomatal conductance which affects CO2 fluxes between the atmosphere and the site of assimilation by Rubisco and subsequent water fluxes from the xylem through the stomata into the atmosphere. Carbon isotope discrimination is an effective seasonally integrated proxy for gas exchange and water relations throughout the season (Mills et al., 1998). Since these effects on stomatal function and gas exchange are downstream from changes in hydraulics, the effect that rootstocks have on root water uptake, limitations at the graft union, or induced hydraulic restrictions in the scion stem or leaves should be clearly reflected in the isotope signature.
The objective for this study was to assess rootstock-mediated variability in water relations for a common apple scion. We hypothesized that phenotypic differences in vigour among rootstocks will be reflected in their effect on gas exchange and water relations of the scion. Consequently, these effects should have downstream effects on carbon isotope composition of leaves and stems as a carbon source and sink, respectively. These findings will help understand the relationship between water relations and rootstock dwarfing capacity in the scion. Furthermore, clear rootstock-mediated effects on carbon isotope ratios will improve selection efficiency for dwarfing capacity. The knowledge acquired with this study will hopefully be applied to better pair rootstocks with specific cultivars, environment conditions, and improve efficiencies for rootstock breeding programs.