Abstract: Utilization of organic nitrogen (N) is an important aspect of plant N assimilation and has potential application in sustainable agriculture. A comparative study on plant N uptake using nitrate, ammonium, glycine and CK (N free) was studied in two tomato cultivars (Shenfen 918 and Huying 932). After 8 days of growth with the different N forms, root dry weight followed the series: NO₃^--N > Gly-N > CK > NH₄⁺-N while root volume followed the series Gly-N > NO₃^--N > NH₄⁺-N > CK in both cultivars. Root enzyme activities was the highest in the NO₃^--N or Gly-N treatment followed by NH₄⁺-N treatment with CK showing the lowest activities. The concentration of NO₃^- and phosphate was highest in the NO₃^--N treatment followed by Gly-N and NH₄⁺-N with CK showing the lowest concentrations. Ammonium, free amino acid and soluble sugar content of phloem and xylem sap in the Gly-N and NH₄⁺-N treatments were higher in both cultivars followed by NO₃^--N and CK treatment. Rhizosphere pH was elevated by the NO₃^--N and Gly-N treatment, but decreased in the NH₄⁺-N treatment. Soluble protein content in root exudates followed the series: NO₃^--N> Gly-N > NH₄⁺-N > CK; however, different patterns were observed in xylem and phloem sap (Gly-N >NO₃^--N > NH₄⁺-N > CK). Root enzyme activities were significantly different (P < 0.05) between the two cultivars. Root enzyme activities varied in the two different tomato cultivars with activities being highest in Huying 932 in all four N treatments.

Key words: tomato, mineral nitrogen, organic nitrogen, root exudates, xylem sap, phloem sap