The endogenous gibberellin (GA) content of spinach (Spinacia oleracea) was reinvestigated by combined gas chromatographymass spectrometry analysis. The 13-hydroxy GAs: GAw3, GA44, GA19, GA17, GA2o, GA5, GA1, GA29, and GA$; the non-3,13-hydroxy GAs: GA12, GA15, GA,, and GA51; and the 3#-hydroxy GAs: GA4, GA7, and GA34, were identified in spinach extracts by comparing full-scan mass spectra and Kovats retention indices with those of reference GAs. In addition, spinach plants contained GA7-isolactone, 16,17-dihydro-17-hydroxy-GAw, GA29-catabolite, 3-epi-GA1, and 10 uncharacterized GAs with mass spectra indicative of mono-and dihydroxy-GA12, monohydroxy-GA25, dihydroxy-GA24, and dihydroxy-GA9. The effect of light-dark conditions on the GA levels of the 13-hydroxylation pathway was studied by using labeled intemal standards in selected ion monitoring mode. In short day, the GA levels were higher at the end of the light period than at the end of the dark period. Levels of GAs at the end of each short day were relatively constant. During the first supplementary light period of long day treatment, GA,1 and GA19 declined dramatically, GA." and GA1 decreased slightly, and GA2o increased. During the subsequent high-intensity light period, the GA2o level decreased and the levels of GA53, GA", GA19, and GA1 increased slightly. Within 7 days after the beginning of long day treatment, similar patterns for GAs3 and GA19 occurred. Furthermore, when these plants were transferred to darkness, an increase in the levels of GA,3 and GA1,9 was observed. These results are compatible with the idea that in spinach, the flow through the GA biosynthetic pathway is much enhanced during the highintensity light period, although GA tumover occurs also during the supplementary period of long day, both effects being responsible for the increase of GA29 and GA1 in long day.under noninductive photoperiods stimulated stem elongation, whereas inhibitors of GA biosynthesis arrested growth induced by LD conditions (24). Later, six members of the early-13-hydroxylation pathway, GA53, GA44, GA19, GA17, GA20, and GA29, were identified as endogenous GAs in spinach shoots (16). It was also shown that, under LD conditions, levels of GA19 decreased, whereas levels of both GA20 and GA29 increased. This observation led to the suggestion that the photoperiod regulates the conversion of GA19 to GA20 (17). Furthermore (25) indicated that these GAs were only metabolized to GA20 when plants were kept in continuous light. Results obtained with cell-free extracts from spinach leaves confirmed that the oxidations of both GA53 to GA44, and of GA19 to GA20 are under photoperiodic control (9). Thus, LD regulates GA metabolism by modulating the activity of two enzymes of the GA biosynthetic pathway, viz. GA53-and GA19-oxidases. These enzyme activities are only present in plants growing in light, which enhances the levels of the active GA(s) stimulating stem growth. Although there is increasing evidence that, for promoting stem elongation, the critical feature in ...