Variation in the nitrotoxin concentration of 13 species of Astragalus (Fabaceae) over a 6-year period.

The purpose of this study was to examine the variation in nitrite production in the leaves of various species of Astragalus over a 6-y period. It was discovered that in drought years the nitrate concentration in leaves was consistently higher than in wet years or those of adequate moisture. Among the plant examined, A. Hallii contained the greatest quantity in drought years (30-35 mg NO2/g leaf, dry wt basis) and A. aesclepiadoides had no detectable amounts in wet years. A two-way analysis of variance showed NO2 concentrations to be significantly different (p < 0.0001) in the two climatic years.