Grafting improves tomato salinity tolerance through sodium partitioning within the shoot

Two greenhouse experiments were carried out to analyze the shoot sodium (Na⁺) partitioning, yield, and fruit quality of 'Cuore di Bue', a salt-sensitive heirloom tomato (Solanum lycopersicum L.), ungrafted or grafted onto interspecific tomato hybrid rootstocks (S. lycopersicum × S. habrochaites) 'Maxifort' and 'Arnold' in 2009, 'Arnold' and 'Armstrong' in 2010, grown at different salinity stress (SS) levels (0, 20, and 40 mM of NaCl in 2009; 0 and 20 mM of NaCl in 2010). In both experiments, an interaction was observed between grafting combinations and SS levels in terms of fruit yield, and fruit juice Na⁺ content. Under no SS conditions, plant grafted onto 'Maxifort' and 'Armstrong' provided the highest yield in 2009 and 2010 experiments, respectively. In the presence of 20 mM of NaCl, plants grafted onto 'Arnold' provided a marketable yield 23.5% (on average) higher than plants grafted onto 'Maxifort' or ungrafted in 2009 and 33% (on average) higher than plants grafted onto 'Armstrong' or ungrafted in 2010. The further increase of SS to 40 mM of NaCl considerably reduced the productivity of all grafting combinations. At 20 mM of NaCl, plants grafted onto 'Arnold' showed also a higher capacity to modulate shoot Na⁺ partitioning with respect to ungrafted plants by increasing Na⁺ accumulation in older leaves (52%) and reducing Na⁺ content in younger and most active leaves (24%), thus enabling the maintenance of higher K⁺/Na⁺, Ca²⁺/Na⁺, and Mg²⁺/Na⁺ ratios compared with ungrafted plants. Fruit total soluble solids content, titratable acidity, and dry matter were unaffected by grafting at any SS level, whereas under SS, the fruit juice Na⁺ content of grafted plants was consistently lower (from 19% up to 68%) than that of ungrafted plants. Under moderate SS conditions (20 mM of NaCl), the use of rootstock genotypes such as 'Arnold' having a particular ability to reduce Na⁺ accumulation in younger and most active leaves may increase tomato yield and enhance tomato nutritional value by reducing the fruit juice Na⁺ content.