The absorption, translocation, and assimilation of urea, nitrate or ammonium in tomato plants at different plant growth stages in hydroponic culture

Abstract

The absorption, translocation, and assimilation of urea, nitrate, and ammonium in tomato plants within 24 h after 15N labeled compounds were applied at four different growth stages: seedling, flowering, fruiting, and harvesting. The absorption of urea-N was only 25% of NO 3-N at seedling stage, but it was up to about 80% of NO 3-N at the subsequent growth stages. The translocation of urea-N was limited at seedling stage, but it was as fast as that of NO 3-N at the subsequent growth stages. 15N was found higher in the lamina of urea- or nitrate-fed plant, but higher in the stems and fruits of ammonium-fed plant. The assimilation of urea-N at seedling stage was less than half of that at the subsequent growth stages. The poor absorption, limited translocation, and slow assimilation of hydroponically applied urea may be the cause of growth reduction at seedling stage. Because as much as 94% of total- 15N in the leaves of urea-fed plant at seedling stage and about 84% of that at the subsequent growth stages were found in the form of urea- 15N, urea is not only absorbed but also translocated by the plant in the form of urea itself. At reproductive growth stage, the absorption, translocation, and assimilation of hydroponically applied urea were greatly improved, and urea should be a suitable hydroponic N source for tomato plants.