**2. Metabolic origins of urea in plants**

Mammals synthesize urea via the Krebs-Henseleit cycle (also called the arginine, ornithine, or urea cycle) as nontoxic form of jettisoned ammonia [15]. Plants usually have the opposite problem, i.e., how to conserve nitrogen, which after carbon, is the most limiting element in plant nutrition [16]. This contention is consistent with the presence of urease in plants and in most bacteria and fungi [17] and its absence in mammals. Whereas in the latter, urea is a nontoxic "waste" form of ammonia in excess, in the former, ureolytic activity is necessary to recycle urea nitrogen (urea is 47% nitrogen). We discuss in this section the metabolic and tissue origins of plant urea.
