Effects of poly-γ-glutamic acid (γ-PGA) on plant growth and its distribution in a controlled plant-soil system

To demonstrate the responses of plant (Pakchoi) and soil to poly-gamma-glutamic acid (gamma-PGA) is essential to better understand the pathways of the promotional effect of gamma-PGA on plant growth. In this study, the effects of.-PGA on soil nutrient availability, plant nutrient uptake ability, plant metabolism and its distribution in a plant-soil system were tested using labeled gamma-PGA synthesized from C-13(1)-N-15-L-glutamic acid (L-Glu). gamma-PGA significantly improved plant uptake of nitrogen (N), phosphorus (P), and potassium (K) and hence increased plant biomass. gamma-PGA greatly strengthened the plant nutrient uptake capacity through enhancing both root biomass and activity. gamma-PGA affected carbon (C) and N metabolism in plant which was evidenced with increased soluble sugar contents and decreased nitrate and free amino acids contents. About 26.5% of the gamma-PGA-N uptake during the first 24 h, after gamma-PGA application, was in the form of intact organic molecular. At plant harvest, 29.7% and 59.4% of gamma-PGA-N-15 was recovered in plant and soil, respectively, with a 5.64% of plant N nutrition being derived from gamma-PGA-N. The improved plant nutrient uptake capacity and soil nutrient availability by gamma-PGA may partly explain the promotional effect of gamma-PGA, however, the underlying reason may be closely related to L-Glu.