True recovery coefficient of 15Nurea sprayed on wheat canopies at flowering

The efficiency of recovery of N-urea by plants or true recovery coefficient of N-urea [TRC(N-urea) %] was determined after fertilization of wheat via the leaves. A chemical preparation containing N-15(urea) (0.55 %) was sprayed on crops of two varieties at flowering. Two concentrations were given: 20 and 40 kg N-urea. ha(-1). The crops were previously fertilized (NH4NO3) at optimum or at sub optimum doses at 1-cm ear stage. Leaf damage due to urea spray remained in a low range. At flowering, the tips of the leaves already exhibited necroses (10 % of the total leaf area). These necrotic area proportions doubled at the largest urea dose 72 h after the spray. A hypothesis involving leaf urea metabolism is proposed. The true recovery coefficients of N-urea [TRC(N-urea) %] were determined 6 h after spraying whole plants. They were found to be between 50 and 72 %, N-urea being localized mainly in the ears and the leaves, and only slightly in the roots. The coefficients increased to 80-90 % at maturity. Thus, N-urea assimilation was rapid and efficient. Eighty-six percent of the absorbed N-urea was recovered in the seeds. The seed TRC(N-urea) values were between 65 and 81 %. The high and constant values of the TRC(N-urea) values described above contrasted with the low and variable data found in the literature. A metabolic compartmentation between N-urea and the remobilized N from the other plant organs is proposed. ((C) Academie des sciences / Elsevier, Paris.).