THE LEVEL OF ABSCISIC-ACID IN THE ROOT-TIP OF RUNNER BEAN SEEDLINGS - IMPLICATIONS FOR THE ABA-UPTAKE CARRIER

High performance liquid chromatography and gas chromatography using an electron-capture detector have been combined in a method for the determination of the content of free abscisic acid in small amounts (< 100 mg FW) of Phaseolus coccineus L. cv. <Prizewinner, root-tip tissue. The identification of abscisic acid was verified by mass spectrography and the method gave estimated overall recoveries of abscisic acid from the tissue consistently greater than 60 %. A stereological analysis of transmission electron micrographs of thin sections was used to determine the relative volumes of cellular compartments at either end of that region of the root tip in which carrier-mediated abscisic acid transport has been shown (Astle, M. C. and P. H. RUBERY (1980) Planta, 150, 312-320). Using the two sets of data and published estimates of the pH of cytoplasmic compartments, the concentration of abscisic acid in the major compartments was calculated. The results define (for unstressed seedlings as used by Astle and Rubery) the range of concentration of abscisic acid occurring in the cytoplasm and cell wall compartments in the region of the root in which the carrier was detected. The values for abscisic acid concentration in the cell wall compartment are so low relative to the K(m) of the transport carrier that it is not reasonable to suppose that the presence of the carrier will affect the distribution of abscisic acid between compartments to any appreciable extent under normal conditions.