Effect of nitrogen form and nutrient solution pH on growth and mineral composition of self-grafted and grafted tomatoes

Three greenhouse experiments were carried out to determine the effect of the nitrogen form and the nutrient solution pH on growth, yield, leaf gas exchange, carbohydrate, N-compound concentrations and mineral composition of tomato cv. Moneymaker (Solanum lycopersicum L.) self-grafted and grafted onto ‘Maxifort’ (S. lycopersicum L. × S. habrochaites S. Knapp and D. M. Spooner) grown in hydroponics. Exp. 1 included five pH levels in the nutrient solution (3.5, 4.5, 5.5, 6.5, and 7.5) while in the Exps. 2 and 3 four different ratios of NO3 − to NH4 + (100:0, 70:30, 30:70, and 0:100) were used. The Exps. 1 and 2 were performed in a short period of time (about 20 days) while Exp. 3 was a long-term experiment. No significant differences among treatments were observed in shoot and root dry biomass of tomato in the pH experiment (Exp. 1), whereas shoot dry biomass, Ca and Mg concentrations decreased sharply when N was exclusively provided as NH4 + (Exp. 2). When averaged over the pH level of the nutrient solution, the highest Ca, Fe, Zn, and Cu concentrations were recorded in grafted than self-grafted plants (Exp. 1), whereas in Exp. 2 shoot and root biomass values recorded in grafted plants were significantly higher than those observed for self-grafted plants, by 20%, and 24%, respectively. In the long-term experiment, the plant growth and yield decreased in response to an increase of NH4 + in the nutrient solution. The decrease in marketable yield with decreasing NO3 −:NH4 + ratio resulted mainly from the increase of blossom-end rot, which reduced the number of marketable fruits per plant. The adverse effects of an increased supply in NH4 + have been associated to a fall in Ca and Mg levels in plant tissues. The carbohydrate concentrations, amino acids and proteins increased under NH4 + in comparison to NO3 − based nutrition. Moreover, NH4 + toxicity was associated with reduced rates of net photosynthesis. Our results also demonstrated that grafting ‘Moneymaker’ into ‘Maxifort’ did not mitigate the negative effects of ammonium nutrition on tomato productivity.