GA4 does not require conversion into GA1 to delay senescence of Alstroemeria hybrida leaves

The biological activity and metabolism of applied GA(1) and GA(4) were studied in leaves of alstroemeria (Alstroemeria hybrida). It appeared that GA(4) was 2 orders of magnitude more active in delaying leaf senescence than GA(1). GA(3)-13-OMe, a GA analog that cannot be hydroxylated on the 13-C position, also retarded chlorophyll loss, although less efficiently. Tritiated and deuterated GA(1), GA(4), and GA(9) were applied to leaves, and their metabolites were analyzed. According to high performance liquid chromatography and gas chromatography-mass spectrometry analyses, GA(9) was converted into GA(4) and GA(3)4, and GA(4) was converted into GA(3)4 and more polar components. No evidence was found for the conversion of both GA(9) and GA(4) into GA(1), even at the relatively high concentrations that were taken up by the leaf. The results strongly suggest that GA(4) is recognized directly by a receptor involved in regulation of leaf senescence in alstroemeria.