FATE OF L-PHENYLALANINE IN SOIL AND ITS EFFECT ON PLANT-GROWTH

Phenylacetic acid (PAA) is a deamination product of phenylalanine (PHE ) applied to soil, which can have a positive physiological effect on p lant growth and development. The objective of this study was to determ ine the factors limiting deamination of PHE when added to soils, deter mine the major pathway upon detection of metabolic intermediates, and assess the growth-promoting effect on maize (Zea mays L.) of an exogen ous application of PHE to soil. The highest activity of PHE deaminatio n in soils occurred at pH 8.5 and on incubation temperature of 40 degr ees C. The soil assay used to determine these parameters was based on the amount of NH4-N cleaved from L-PHE when incubated for 48 h in the presence of toluene. A sigmoidal relationship was observed between NH4 -N and PHE concentration, indicating cooperative kinetics. Both phenyl pyruvic acid and PAA were detected by high-performance liquid chromato graphy (HPLC) following the addition of PHE to soil. A glasshouse expe riment revealed that an exogenous application of PHE as a soil drench to established uniform seedlings of maize had a significant effect on vegetative growth parameters (shoot height, shoot fresh weight, intern odal distance, uppermost leaf collar base distance, and leaf width). S ince PAA is a well-established plant growth regulator, it was conclude d that this metabolic product derived from PHE in soil was responsible for the stimulatory growth effects in maize.