Boron and Salinity Stress Interaction and Effects on Grafted and Ungrafted Tomato Plants

Boron toxicity represents one of the most feared abiotic stresses limiting vegetable production in certain coastal areas of the Mediterranean Basin and it is often associated to salinity stress. A greenhouse experiment was carried to evaluate the effects of boron and salinity excess on growth, yield and fruit quality performance of tomato (Solanum lycopersicum L.) plants cv ‘Ikram’, ungrafted or grafted onto interspecific tomato hybrid rootstocks (S. lycopersicum × S. habrochaites) ‘Arnold’ and ‘Big Power’. Grafted and ungrafted plants were grown in pots containing perlite and fertigated by drip irrigation with a complete nutrient solutions containing: a standard boron level (25 μM B), an excess of boron (925 μM B) or a combined excess of boron and NaCl (925 μM B and 30 mM NaCl). Growth analysis, 30 days after treatments differentiation, revealed that in presence of 925 μM of B grafted plants produced, on average, a leaf area and a shoot fresh and dry biomass 62.5, 66.0 and 40.6% higher than ungrafted plants, respectively. Under combined boron and salinity excess, grafted plants leaf area, fresh and dry shoot weight resulted to be on average 29.5, 40.7 and 29.8% higher than that of ungrafted plants, respectively. On average, plant analyses showed that boron content in bottom and apical leaves resulted to be 40 and 71% lower, respectively, in plants grown with the combined excess of boron and salinity as compared to those grown under boron excess. Grafted plants showed a lower leaf boron content as compared to ungrafted ones, when subject to a combined excess of boron and salinity, either in bottom (about 30%, on average) and apical (about 22%, on average) leaves. While in presence of boron excess, only plants grafted onto ‘Arnold’ showed a lower leaf boron content with respect to ungrafted plants either in bottom (39%) and apical (47%) leaves. In terms of yield, the excess of boron and the combined excess of boron and salinity caused on average a fruit yield reduction of 50 and 70%, respectively, as compared to plants nourished with the control nutrient solution. Grafted plants produced on average 30% more than ungrafted ones. Fruit quality was positively influenced by the salinity stress, while was not affected by boron excess or by grafting. In conclusion, grafting can improve tomato tolerance to boron excess and to the combined excess of boron and salinity by reducing leaf boron content and enhancing plant growth and productivity.