Grafted plants with NCED OE rootstocks may have improved
assimilate supply to the shoot
Reduced root growth of NCED OE rootstocks may be beneficial (Lambers,
Atkin & Millenaar 2002; Lynch 2018), as excessive lateral root
production increases sink competition for internal and external
resources (primarily carbohydrates, but also water and nutrients) needed
for root growth and exudate production. Decreased assimilate allocation
to the root may also explain the greater vegetative growth and fruit
yield of shoots grafted to NCED OE rootstocks (Table 1). Furthermore,
scions grafted on SP12 rootstocks maintained photosynthetic activity
under low salinity (Figure 2 a, b) without changinggs , thereby increasing intrinsic WUE; this
concurs with the previous observation in reciprocal grafting experiments
under non-stressed conditions where gs was only
reduced when an NCED OE scion was present, and this had modest effects
on AN; NCED OE rootstocks had no effect ongs (Thompson et al. 2007b).
Elevated ABA tissue concentrations, with or without environmental
stresses, can promote developmental changes in stomata and leaf anatomy
that mimic the effects of water deficit (Quarrie & Jones 1977; Franks
& Farquhar 2001; Galmés et al. 2011). Enhanced cuticular wax
deposition and changes in its composition can protect photosynthesis and
water status (Ziv, Zhao, Gao & Xia 2018). In this study, grafting
scions onto NCED OE rootstocks under salinity stress increased the
elongation of leaf epidermal cells and reduced the number of cuticular
wax crystals on leaf adaxial and abaxial surfaces (Figure 2e; Table 2).
In agreement, autotetraploid Rangpur lime rootstocks with high ABA
levels were compared with the diploid equivalent with lower ABA levels:
this showed a reduced expression for the wax synthesis WAX2 gene
in scions grafted to the high ABA autotetraploid rootstocks (Allarioet al. 2013). These results contradict an earlier study in tomato
using ABA-deficient mutants and exogenous application of ABA where there
was a positive relationship between ABA level and wax deposition
(Martin, Romero, Fich, Domozych & Rose 2017). The reduction in wax
deposition due to NCED OE rootstocks could possibly be explained by a
direct down regulation of wax synthesis pathways, or as a secondary
effect where ABA alleviates salinity stress, allows greater leaf
expansion and consequently a dilution of wax deposition or a mitigation
of stress-induced wax synthesis.
Rootstocks can improve photosynthesis by affecting leaf structure to
enhance mesophyll conductance to CO2(gm ) (Fullana-Pericàs, Conesa, Pérez-Alfocea &
Galmés 2020). Indeed, gm was negatively
correlated with sub-stomatal and/or ambient CO2concentration under long-term stress (Flexas et al. 2012, 2013).
Here, grafting onto NCED OE rootstocks disorganized laminar mesophyll
structure (Figure 2c), which could explain the decreased Ci(Figure 2d) through enhanced CO2 diffusion to the cells
(Flexas et al. 2012, 2013).
Overall, NCED OE rootstocks may have improved tomato plant performance
under low salinity via at least two mechanisms that improved assimilate
supply for scion growth: i) altered ABA metabolism and signalling
restricted root growth, consistent with reduced root sink strength,
making more assimilate available for other sinks; ii) increasedAN and decreased sub-stomatal CO2associated with changes in leaf mesophyll structure would have increased
assimilate supply.