Effect of Photoperiod on the Metabolism of Deuterium-Labeled Gibberellin A₅₃ in Spinach

Application of gibberellin A(53) (GA(53)) to short-day (SD)-grown spinach (Spinacia oleracea L.) plants caused an increase in petiole length and leaf angle similar to that found in plants transferred to long days (LD). [(2)H] GA(53) was fed to plants in SD, LD, and in a SD to LD transition experiment, and the metabolites were identified by gas chromatography with selected ion monitoring. After 2, 4, or 6 SD, [(2)H]GA(53) was converted to [(2)H]GA(19) and [(2)H]GA(44). No other metabolites were detected. After 2 LD, only [(2)H] GA(20) was identified. In the transition experiment in which plants were given 4 SD followed by 2 LD, all three metabolites were found. The results demonstrate unequivocally that GA(19), GA(20), and GA(44) are metabolic products of GA(53), and strongly suggest that photoperiod regulates GA metabolism, in part, by controlling the conversion of GA(19) to GA(20).