Absorption, translocation, and assimilation of foliar-applied urea compared with nitrate and ammonium in tomato plants

Abstract

To evaluate the use of foliar application of N fertilizer and the occurrence of leaf injury in tomato plants (Lycopersicon esculentum Mill., cv. Momotaro), the effects of the form and concentration of N and solution pH on the leaf injury were studied in the first experiment (Expt. 1). The effects of solution pH and leaf surface on the absorption, translocation, and assimilation of urea were compared with those of nitrate and ammonium in the second experiment (Expt. 2). In Expt. 1, no leaf injury was observed regardless of N sources applied at the N concentration of 1.0 g L^-1. Compared with nitrate or ammonium, the index of leaf injury was the lowest in the leaf to which urea had been applied (hereafter referred to as “urea-applied leaf”), when the N level increased from 2.0 to 10.0 g L^-1. Leaf injury was not affected by the solution pH in the case of urea, but it increased in the case of ammonium and decreased when nitrate was applied with increasing solution pH. In Expt. 2, the absorption of nitrate and ammonium by a leaf within 4 d was 34% and 74% of that of urea, respectively. N absorption at the lower leaf surface was much greater than that at the upper leaf surface for each N source. No apparent effect of solution pH on the absorption of urea was detected. With increasing solution pH, however, the absorption of nitrate decreased. The absorption of ammonium was the greatest at solution pH 7.5. Total-¹⁵N translocation from applied leaf to other plant parts within 4 d was the largest in the urea-applied plants. Effects of solution pH and leaf surface on ¹⁵N distribution were not appreciable. ¹⁵N assimilation was the quickest in the urea-applied plants. Two days after application, ¹⁵N assimilation in the whole plant was up to 76.9% in the urea-applied plants, but only 33.7% and 43.0% in the nitrate- and ammonium-applied plants, respectively. Urea was an appropriate foliar N source due to the low ability to injure foliage because of the rapid absorption and translocation, fast assimilation, and the wide and suitable range of solution pH. Foliar application of N to the lower leaf surface was recommended.

Key Words:

• assimilation
• leaf injury
• Lycopersicon esculentum
• nitrogen
• urea