Weed species such as kochia, lambsquarters, sunflower, and pigweed also are routinely high in nitrate. Under certain environmental and managerial conditions, wheat, corn, alfalfa, soybeans, oats, Johnsongrass, and other plants can accumulate potentially toxic levels of nitrate. Nitrate content generally is highest in young plant growth and decreases with maturity. Sorghums and sudangrass are exceptions because concentrations usually remain high in mature plants. If plants are stressed at any stage of growth, they can accumulate nitrate.
Nitrates normally accumulate in stems and conductive tissues.
Highest nitrate levels occur in the lower one-third of the plant stalk. Concentrations tend to be low in leaves because nitrate reductase enzyme levels are high there. Grain does not contain appreciable amounts of nitrate. Drought Nitrates accumulate in plants during periods of moderate drought because the roots continually absorb nitrate, but high daytime temperatures inhibit its conversion to amino acids. During a sever drought, lack of moisture prevents nitrate absorption by plant roots.