**3.2 Effects of urease on germination**

What is the consequence of blocking urease action during germination when large of amounts of arginine and ureides are mobilized and generate urea? In soybean we observed a 7–8 h delay in germination of its protein-rich seeds imbibed in the potent urease inhibitor [52–54] phenylphosphorodiamidate (PPD) [55]. Proteinpoor *Arabidopsis thaliana* seeds imbibed in water with 50 PPD did not germinate at all. That PPD acts specifically on urease and brings about inhibition of germination by nitrogen limitation is indicated by parallel dose-response curves for germination and urease inhibition, and by reversal of inhibition of germination with added nitrogen sources, NH, NO, (5 mM), or casein hydrolysate (1 mg/ml) [55]. Similar respose was observed for a cyclotriphosphazatriene urease inhibitor [56]. Within *A. thaliana*, large seeds germinated in distilled water tend to produce seedlings that survive longer than those from small seeds [57]. Phenyl phosphorodiamidate may effectively reduce seed size by depriving the seedling of that portion of its nitrogenous reserves catabolized to urea. Hydrolyzed *A. thaliana* seed meal contains 5.5 g arginine per 16 g total nitrogen [26]. Thus, 1.76 g or 11% of seed nitrogen (free plus that bound in protein and nucleic acids) is in arginine and, potentially, 5.5% (or more, if arginine is proportionally higher in "reserves") seed nitrogen can be converted to urea by arginase. Seed nitrogen in nucleic acids is another generator of urea; greater than 40% of urea generated in 6-day *Arabidopsis* seedlings is eliminated by allopurinol [55]. In other species, e.g., *Lupinus texensis* [58] and wild radish [59], larger seeds tend to germinate at a higher frequency than small seeds. We have observed a crude correlation, across species, between seed size and resistance to PPD inhibition of germination and will extend studies of relative PPD sensitivity to large seeded soybeans vs. selected small-seeded sister lines [60], when these lines are made available to the public.
