Role of nitrate in regulating germination of Sinapis arvensis L. (wild mustard)

Depending on the applied concentration, nitrate can either stimulate or inhibit germination of dormant seeds of Sinapis arvensis L. (wild mustard). Seed NO⁻ ₃ levels that corresponded to the maximal germination frequencies recorded, ranged from 0.3 to 4.4 nmol seed⁻¹ for applied NO⁻ ₃ concentrations between 2.5 and 20 mol m⁻³. Germination was significantly lower in seeds containing more than 5 nmol NO⁻ ₃. Although the presence of NO⁻ ₃ within the seed was required to promote germination, seed NO⁻ ₃ levels were 5 to 15 times less than levels calculated from the volume of solution taken up by the seeds. Seed NO⁻ ₃ levels also responded in a passive fashion to the external NO₃ ⁻ concentration when the seeds were incubated on filter paper or in soil. In a representative soil containing 26 mg NO⁻ ₃‐N per kg dry weight and 8 to 16% water by weight, the amount of NO⁻ ₃ taken up by the seeds reached inhibitory levels. Our results suggest that levels of inorganic nitrogen (NO⁻ ₃, NH⁺ ₄) in managed soils may play an important role in regulating germination of dormant S. arvensis seeds.