Can grafting onto suitable rootstocks contribute to less discharge of drainage water in semi-closed soilless cultivations of tomato?

The commercial tomato (Solanum lycoperiscum) hybrids ‘Merillia’ and ‘Belladonna’, either self-grafted or grafted onto the rootstock ‘Armstrong’ (S. lycopersium L. × S. habrochaites), were cultivated in a closed soilless system in which the nutrient solution (NS) was continuously recirculating. The NaCl concentration in the fresh NS supplied to the system to compensate for plant uptake was either low (0.6 mM) or high (5 mM). The hetero-grafted plants contained much more Na+ and C(l-) in the roots and much less Na+ and C(l-) in the leaves compared to the self-grafted plants. The reduced transport of Na+ and C(l-) to the leaves of ‘Merilia’ and ‘Belladona’ plants grafted onto ‘Armstrong’ did not result in significantly higher rates of Na+ and C(l-) accumulation in the recirculating NS during cultivation, compared to those recorded in self-rooted plants. This is ascribed to the increased deposition of Na+ and C(l-) to the roots of hetero-grafted plants, which compensated for the appreciable reduction of Na+ and C(l-) translocation to the leaves. Nevertheless, the appreciable reduction of Na+ and C(l-) transport to the leaves of plants grafted onto ‘Armstrong’ enables growers to recycle the drainage water (DW) for longer time before discharging it, when irrigation water salinity is a constraint in semi-closed hydroponic tomato crops. Further research is needed to select new tomato rootstocks with an even more extensive root system and an even higher ability to retain Na+ and C(l-) in the roots, while maintaining low rates of their translocation to the leaves. The availability of such rootstocks can increase the acceptable concentrations of Na+ and C(l-) in irrigation water used in closed or semi-closed hydroponic tomato crops, and restrict or even eliminate the need to discharge DS.