Composition of the xylem sap of tomato seedlings cultivated on media with HCO-3 and nitrogen source as NO-3 or NH+4

The results of the experiments discussed here present changes in the chemical composition of xylem sap of tomato seedlings cultivated in hydroponics on media containing 5 mmol HCO3- and an N-source given as NO3- NH4+ or these two forms in different proportions. The occurrence of free NH4+ in the xylem sap of NH4+-seedlings and in NO3--seedlings indicates that the process of N-assimilation was not only confined to roots. The application of HCO3- to the medium effected a decrease in the concentration of NH4+ in the xylem sap of NH4+-seedlings, having no effect on changes in the concentration of NO3- or NH4+ in NO3--seedlings. Malate, citrate, fumarate, and succinate were identified in the xylem sap. The concentration of carboxylates in NO3--seedlings exceeded by about 50% that recorded in NH4+-seedlings. The highest concentration of malate constituting from 80% to 93.5% of this fraction, was determined in this group of compounds. The enrichment of the medium with HCO3- ions induced an increase in the content of carboxylates, chiefly of malate. In these experimental conditions an increase in the malate concentration in the xylem sap of NO3- and NH4+-seedlings reached relative values of 100% and 36%, respectively. The total concentration of amides and amino acids was about 2.6 times higher in the xylem sap of NH4+-seedlings than in NO3--seedlings. Amide glutamine was the main component of this fraction in xylem sap and its total concentration was about 3.3 times higher in NH4+-seedlings than that determined in NO3--seedlings. Glutamine, glutamate, aspargine, and aspartate constituted from 69% to 77% of this fraction. The concentration of the remaining amino acids varied from 0.6% to 7%. The enrichment of the medium with HCO3- ions also effected an increase in the concentration of amides and amino acids in the xylem sap by about 17% and 56% in the case of NO3- and NH4+-seedlings, respectively, in comparison with the respective controls (without HCO3-).