Possible membrane-associated effects in gibberellic acid and phenylalanine-induced rose coloration enhancement

When combined with GA, phenylalanine produced marked enhancement of red colouration of rose (cv Baccara) petals, this effect significantly exceeding that of GA applied alone, while phenylalanine when administered alone was virtually ineffective in this respect. Parallel assays of phenyl-ammonialyase (PAL) activity indicated essentially similar endogenous levels of enzyme irrespective of treatment. These results are interpreted as indicating that mode of GA action in this system is probably not by direct PAL activation but rather via GA-induced enhancement of membrane permeability thus providing more phenylalanine precursor for anthocyanidin pigment synthesis. This hypothesis is substantiated by the results of plasmolysis studies executed on rose petal epidermal strips in hypertonic solutions where GA significantly cut down time required to achieve a given degree of plasymolysis.