Salinity stress in tomatoes can be alleviated by grafting and potassium depending on the rootstock and K-concentration employed

Plant production under salinity requires increased capacity for K+ homeostasis. For this purpose, supplementary K2SO4 in the nutrient solution and grafting on a tolerant rootstock were employed in two experiments to test whether grafting, potassium and their interactions can alleviate salinity stress in tomato (Solanum lycopersicum L.). In Exp-ion, plants were cultivated for 122 days to compare different ionic compositions: EC 9 dS m−1 in ECall (by macro-nutrients) and in ECNaCl (by 64.2 mM NaCl), EC 12 dS m−1 in ECK (ECNaCl + 25.8 mM K+). Exp-K+ was established to compare K+ concentrations of 6, 16 and 36 mM at 150 mM NaCl. In both Experiments, ‘ZS-5’, selected as a salt sensitive cultivar, was either self-grafted or grafted onto the cultivar ‘Edkawi’, reported as salt tolerant. Yield and growth, minerals, gas exchange, soluble sugars, and proline were analyzed. Different ionic treatments affected almost all characteristics considered while differences between rootstocks were rarely observed. No pronounced differences were found in shoot growth, yield and gas exchange between ECall and ECNaCl. ECK did not show any salinity alleviative effects but inhibited even growth compared with the other treatments. In Exp-K+, 16 mM K+ increased plant growth, leaf soluble sugars and proline concentrations. 36 mM K+ did not further reduce upper leaf Na+ although leaf K+ concentration increased significantly. The results indicated that the response of tomato plant to NaCl stress was principally attributed to the osmotic component in Exp-ion, excessive K+ showed no mitigating effect on fruit yield and shoot growth. However, 16 mM K+ in the root environment enhanced the salt adaptive capacity of plants stressed at 150 mM NaCl. The use of the tolerant rootstock resulted in no ameliorative effects, owing to its susceptibility to blossom-end rot, failure in enhancing photosynthesis, and ineffectiveness of restraining the long-distance transport of Na+.

► Grafting and K nutrition were applied as treatments to test possible alleviation in salt stress.
► Rootstocks did not differ significantly in response to salinity.
► The K treatment of 16 mM K+ enhanced the salt adaptive ability of plants stressed at 150 mM NaCl.
► The response of tomato plant to NaCl stress was mainly due to the osmotic component in Exp-ion.
► Excessive K+ showed no mitigating effect on fruit yield and shoot growth.