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.