Involvement of heme synthesis in the growth stimulation of maize seedlings by 5-aminolevulinic acid

The growth of maize seedlings was stimulated by shoot-applied 5-aminolevulinic acid 2days after treatment at 90 and 120molL(-1). The effects of 5-aminolevulinic acid on the activity of nitrate reductase (EC 1.6.6.2) and nitrite reductase (EC 1.7.7.1) and the chlorophyll, ammonium, heme and total free amino acid content were investigated by using maize seedlings to clarify the involvement of nitrogen metabolism and heme synthesis in the growth stimulation. 5-Aminolevulinic acid increased the level of nitrate reductase activity at 90molL(-1) and the ammonium and heme content at 90 and 120molL(-1) 2days after treatment. The total amino acid content increased by 90 and 150molL(-1) 5-aminolevulinic acid 2 and 3days after treatment, respectively. However, no significant change was observed in the activity of nitrite reductase or the chlorophyll content after the 5-aminolevulinic acid treatment. These results suggest that the enhancement of nitrogen metabolism by nitrate reductase activation is involved in the 5-aminolevulinic acid-induced stimulation of maize seedling growth. The activation of nitrate reductase might be related to an increase in the heme content following the 5-aminolevulinic acid treatment.