To elucidate the role of abscisic acid (ABA) in tomato (Solanum lycopersicum L.) responses to suboptimal root temperature (T), a near-isogenic line carrying the notabilis null mutation in the ABA biosynthesis gene LeNCED1 was reciprocally grafted with its parental cultivar Ailsa Craig. Exposure of tomato to sub-optimal root T (15◦C) decreased leaf area expansion, shoot elongation and plant biomass in comparison with optimal root T (25◦C). Both suboptimal root T and null mutation of the LeNCED1 gene in root and shoot reduced leaf area and total plant biomass, but these two factors did not interact. Transpiration rates and stomatal conductances decreased, while net CO2assimilation was not influenced by root exposure to suboptimal T. However, notabilis scions exhibited higher net assimilation rates, stomatal conductances and transpiration rates than Ailsa Craig scions. Moreover, notabilis plants invested much more biomass into the root than Ailsa Craig self-grafts, thereby improving their water uptake capacity. Lipid peroxida-tion as well as polyamines and guaiacol-peroxidase (G-POD), which are considered to possess antioxidant properties, increased in the leaves of all grafting combinations of tomato when exposed to suboptimal root T. All grafting combinations showed an increase in shoot ABA levels when exposed to suboptimal root T. However, the levels of ABA in the shoot of notabilis did not differ significantly from those found in Ailsa Craig, indicating that inactivation of LeNCED1 did not impair ABA accumulation in the leaves at low root-zone T. The genes LeNCED2 and LeNCED6 were not up-regulated at low root-zone T and therefore not responsible for ABA biosynthesis in notabilis. Increased ABA levels in tomato exposed to suboptimal root T were associated with elevated levels of the polyamine putrescine which contributes to ABA biosynthesis. In conclusion, the introgression in Ailsa Craig that contains the mutation resulting from inactivation of the LeNCED1 gene includes further genetic differences that influence responses to suboptimal root-zone T, such as plant growth restriction and ROS scavenging modification.