Plant Physiol, March 2002, Vol. 128, pp. 1129-1136

Leaf Urea Metabolism in Potato. Urease Activity Profile and Patterns of Recovery and Distribution of ¹⁵N after Foliar Urea Application in Wild-Type and Urease-Antisense Transgenics¹

Unit of Plant Biochemistry, Scottish Crop Research Institute, Invergowrie, Dundee DD2 5DA, United Kingdom

The influence of urease activity on N distribution and losses after foliar urea application was investigated using wild-type and transgenic potato (Solanum tuberosum cv Désirée) plants in which urease activity was down-regulated. A good correlation between urease activity and ¹⁵N urea metabolism (NH₃ accumulation) was found. The general accumulation of ammonium in leaves treated with urea indicated that urease activity is not rate limiting, at least initially, for the assimilation of urea N by the plant. It is surprising that there was no effect of urease activity on either N losses or ¹⁵N distribution in the plants after foliar urea application. Experiments with wild-type plants in the field using foliar-applied ¹⁵N urea demonstrated an initial rapid export of N from urea-treated leaves to the tubers within 48 h, followed by a more gradual redistribution during the subsequent days. Only 10% to 18% of urea N applied was lost (presumably because of NH₃ volatilization) in contrast to far greater losses reported in several other studies. The pattern of urease activity in the canopy was investigated during plant development. The activity per unit protein increased up to 10-fold with leaf and plant age, suggesting a correlation with increased N recycling in senescing tissues. Whereas several reports have claimed that plant urease is inducible by urea, no evidence for urease induction could be found in potato.