Quantification of Free ABA and Free and Conjugated IAA in Strawberry Achene and Receptacle Tissue during Fruit Development

Abstract

Indoleacetic acid (IAA) was identified by gas chromatography-mass spectrometry in extracts of achene and receptacle tissue of ‘Midway’ strawberry ( Fragaria Xananassa Duch.). Free, ester-conjugated, and amide-conjugated IAA present in both tissues from anthesis to maturity were quantified by a double-standard isotope dilution method using 14 C-IAA and 14 C-indolebutyric acid ( 14 C-IBA) as internal standards. Whole fruit at anthesis contained 6.2 μg/g dry weight free IAA, 2.0 μg/g dry weight ester-conjugated IAA, and no detectable amide-linked IAA. Maximum concentrations of free IAA in achenes were 1.2 times higher than those in receptacle tissue; maxima occurred simultaneously in the 2 tissues 14 days after anthesis. The maximum concentration of ester-conjugated IAA in achene tissue 8 days after anthesis was 17.5 times higher than that in receptacle tissue 17 days after anthesis. A high concentration of amide-conjugated IAA was found in achene tissue at 11 days and again at maturity. Maximum concentrations in achenes were 34 times greater than those in receptacle tissue. Correlations between growth rate and levels of free or conjugated auxin were nonsignificant. Abscisic acid (ABA) was detected in whole fruit at anthesis by electron-capture gas chromatography. The concentration in the achenes declined until midway through development, then increased as fruit approached maturity. The total quantities in both achenes and receptacle increased as fruit ripened. The ratio of free IAA to free ABA changed during fruit development but was not well-correlated with fruit growth rate.