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.