PHOTOPERIODIC CONTROL OF GIBBERELLIN METABOLISM IN SPINACH

[3H]GA20 applied to spinach plants (S. oleracea L.) was metabolized to several products. Two of these were identified by combined GLC-radio counting as [3H]GA29 and [3H]3-epi-GA1. since GA20 and GA29 are endogenous GA in spinach (Metzger et Zeevaart 1980), the conversion of GA20 to GA29 is a natural process. 3-epi-GA1 was not detected in extracts of spinach shoots analyzed by combined gas chromatography-mass spectrometry. This indicates that the conversion of exogenous [3H]GA20 to [3H]3-epi-GA1 may be an artifact. Long-day pretreatment of spinach shoots caused a 2 fold increase in the rate of [3H]GA20 metabolism over the rate of metabolism in plants maintained under short-day conditions. [3H]GA29 accumulated more rapidly under long than under short days, whereas photoperiodic treatment had no effect on the accumulation of [3H]3-epi-GA1. The long-day-induced increase in the level of endogenous GA29 in spinach shoots appears to be the result of an increased capability to convert GA20 to GA29.