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In this research three greenhouse tomato cultivars (C1:NIKITA, C2:HAMARA, C3:ELENA) were cultured in a hydroponic system using five nutrient solutions. The main difference between nutrient solutions (S1...S5)
was the form of of Nitrogen used in their formula. In S1 solution all Nitrogen was in the form of Nitrate and the amount of N-NH4 increased from S2 to S5 and S5 solution had 12 meqlL nitrate and 2 meqlL ammonium. In This
research two experiment in two different seasons (winter and spring) were conducted. Results of two experiments showed that form of nitrogen can
affect quantitative and qualitative charactristics of greenhouse tomato. Increasing the proportion of N- NH4 in nutrient solutions reduced fruit yield,fruit vitamin-C, leaf and fruit dry weight, fruit titrable acidity fruit T.S.S.
and fruit nitrate content. Form of nitrogen affeced the elemental composition of leaves. Increasing the proportion of N-NH4 reduced leaf Ca and K and increased leaf Na and N. There were some significant differences between
cultivars used in this research.