Discussion
Rootstocks that strongly differed in their dwarfing capacity of the
scion also affected stem water potential, leaf gas exchange, and carbon
isotope composition in both leaves and stems. Rootstocks with lower stem
water potential also showed more strongly restricted growth and trunk
diameter. The influence of rootstocks on scion growth and canopy
development is an important component in the understanding of
rootstock-scion water use (Higgs and Jones 1990). In this experiment,
rootstocks with a high dwarfing capacity such as B.9. had lower stem
water potential, net CO2 assimilation, stomatal
conductance, and transpiration. These rootstock-induced changes in leaf
function translated to differences in carbon isotope composition in both
leaves and stems.
Changes to stem water potential are normally induced via changes in soil
water availability (Suter et al., 2019). Stem water potential
contributes to the control of stomatal conductance (Buckley, 2019).
Stomatal behavior depends on the physiological traits of the plant and
the host environment. At night, water is taken up and stored within the
plant (Knipfer et al., 2019). In the morning, stomata open and
transpirations rates increase, reducing leaf water potential and turgor
pressure (Rodriguez-Dominguez et al., 2019). Drought stress forces
earlier stomatal closure due to loss of leaf turgor and, consequently, a
decline in leaf xylem hydraulic conductance to avoid excessive negative
pressure in xylem (Knipfer et al., 2020). Stomatal closure is an effect
of a decline in root hydraulic conductance and changes within the
root-to-soil hydraulic continuum (Knipfer et al., 2020).
In apple, stem water potential has been reported to vary between – 0.8
and – 1.0 MPa in fully irrigated trees (Naor and Cohen. 2003). Here,
stem water potential measurements corresponded with those previously
reported. Valverdi et al. (2021) reported that rootstocks with a higher
dwarfing capacity like B.9 did not vary as much in shoot growth between
well-watered conditions compared to semi-dwarfing rootstocks like G.890.
Similar to Valverdi et al. (2019), more dwarfing rootstocks increased
inherent water limitations indicated by lower stem water potential,
stomatal conductance, and transpiration. This was especially true in
this study as trees matured from 2018 to 2019. Rooting volume may, in
part, contribute to some of these responses (Costes and
Garcia-Villanueva, 2007; Ma et al., 2013; Harrison et al., 2016; Foster
et al., 2017). However, in the semi-arid environment where this study
occurred, more than 95% of the water was supplied through drip
irrigation and with a young orchard, irrigated volume was similar for
all rootstocks. Therefore, differences observed among rootstocks were
more likely a result of elevated limitations in water uptake and
transport to the scion rather than simply access to water within the
soil profile.
In this study, rootstocks modulated scion leaf water relations by
affecting stomatal closure and leaf gas exchange which had downstream
effects on δ13C composition. There have been previous
studies which have reported differences in water relations when
different rootstocks were used in woody plants (Bongi et al., 1994;
Padgett-Johnson et al., 2000; Gibberd et al., 2001; Cohen and Naor,
2002; Ma et al., 2010; Liu et al., 2012a; Galbignani et al., 2016;
Peccoux et al., 2018; Villalobos-Gonzales et al., 2019; Frioni et al.,
2020). However, these studies did not show the linear relationship
between these traits among many phenotypically diverse rootstocks like
this study. More often, differences have been observed among scion
cultivars or among treatments that affect water supply to the roots.
However, Liu et al (2012b) described how rootstocks manipulated stomatal
conductance and photosynthetic capacity in ‘Gale Gala’. Here, carbon
isotope composition was different for Honeycrisp when grafted to apple
rootstocks with differences in dwarfing capacities. A reduction in
discrimination against 13CO2 may
indicate an increase in water use efficiency (Jones; 1984; Farquhar et
al. 1989). In apples, carbon isotope composition was reported to be
lower for Fuji compared to Braeburn apples because of limitations in
stomatal and leaf respiration rates (Massonnet et al., 2007). In this
study, elevated carbon isotope composition in more dwarfing rootstocks
like B.9 indicated increased conservation of water through reduced
transpiration rates and lower stomatal conductance in ‘Honeycrisp’
leaves. These effects on water relations have downward effects on scion
vigor and were consistent with our knowledge of dwarfing capacity of the
rootstocks tested in this study (Seleznyova et al.,2008; Kosina, 2010;
Tworkoski and Fazio, 2015; Foster et al., 2017). The most obvious
influence of dwarfing rootstock is the ability to produce lower
vegetative biomass relative to more vigorous rootstocks. Trunk diameter
and shoot extension measurements are two of the main traits used to
characterize tree growth (Lauri et al; 2006). When we measured trunk
diameter at rootstock above and below the graft union, we notice
differences in growth rates. In both years, B.9 rootstock was smaller
than all other rootstocks and our observations were similar to Gjamovski
and Kiprijanovski (2011). Corresponding reductions in stem water
potential, an accurate measure of plant water status (Shackel et al.,
1997) were observed among rootstock cultivars.
The rootstock-modulated effect on scion water relations represents an
important component in the understanding of rootstock-scion water use
(Higgs and Jones., 1990). When net carbon assimilation was measured
among rootstocks, we observed that less conservative rootstocks like
semi-dwarfing G.890 had higher photosynthetic rates. These results
follow similar patterns with those reported by Fallahi et al (2001), who
reported lower net photosynthetic rates for B.9 compared to more
vigorous rootstocks like Ottawa 3 and M.7. Clearly, the capacity of the
tree to improve net carbon assimilation and thus, increase carbon
fixation is related to the ability of the hydraulic system to supply
water to the leaves (Koepke and Dhingra. 2013). Therefore, the
hypothesis that dwarfing in rootstocks is associated with
rootstock-mediated water restrictions at a particular point in the tree
(Cohen and Naor, 2002; Atkinson et al., 2003; Cohen et al; 2007; Cohen
et al, 2003; Atkinson et al, 2001; Tworkoski and Fazio, 2015)..These
restrictions could either occur at the root, the graft union, or in the
impact of rootstocks on stem and leaf hydraulic traits. We acknowledge
that this isn’t fully answered by this study and more work is needed to
better discern how rootstocks might affect each restriction point.
The positive relationship between carbon isotope composition between
leaf and stem demonstrates short-term and long-term impacts of
rootstocks on scion water relations and carbon assimilation. We observed
enrichment of stems compared to leaves and this is well aligned with
previous studies (Arndt and Wanek, 2002; Cernusak et al., 2005).
Specifically, B.9 showed a clear divergence in carbon isotope
composition between stem and leaves where leaves were depleted relative
to stems that could be explained by several phenomenon. Cernusak et al.,
2009 presented six hypotheses for this effect. One of the outlined
hypotheses to explain enrichment of non-photosynthetic tissues compared
with their leaves rely on post-photosynthetic carbon fractionation due
to seasonal separation of growth (Cernusak et al., 2009). Development
and growth of leaves occur in spring with plentiful soil water content.
Since variability in carbon discrimination is also affected by
intercellular to ambient CO2 partial pressure, the
photosynthetic carbon assimilation is heavily discriminating against13C at this time (Werner and Gessler, 2011). Wood
growth occurs later in the season, when temperatures and VPD are
elevated which can reduce stomatal conductance producing less
discrimination against 13C at the time carbon is used
for synthesis of non-photosynthetic tissues (Cernusak et al., 2009).
Differences in δ13C between leaves and stems were less
pronounced when discrimination and vegetative vigor were also greater.
Since more vigorous rootstocks extend growth into later parts of the
season, we would expect a greater agreement in δ13C
between leaves and stems than for less vigorous rootstocks like B.9.
In conclusion, we report the effect of rootstocks on carbon isotope
composition in leaves and stems and show the close association between
leaf gas exchange and stem water potential. Rootstocks were able to
strongly affect water-use efficiency and carbon assimilation for the
same apple scion. Furthermore, there was a close association between
rootstock-influenced gas exchange and overall shoot vigor when soil
water content was uniform. Rootstocks that are more vigorous have more
negative carbon isotope composition indicating lower water restrictions
in transport to above ground parts compared to rootstocks with lower
vigor. These results increase our understanding of the physiological
mechanisms underlying dwarfing in composite woody plants like apple and
show how below-ground traits imparted from rootstocks can affect
water-use efficiency and carbon isotope composition in leaves and stems.