Abstract
Role of endogenous gibberellins (GAs) in the thermoperiodic regulation of stem elongation in Dendranthema grandiflorum was investigated by gas chromatography-mass spectrometry with deuterated GAs as internal standards. GA1, GA20, GA19%gt;, GA<44, and GA53, which all belong to the early 13-hydroxylation pathway of GA-biosynthesis, and GA9, which belongs to the early nonhydroxylation pathway, were identified from the stem. The thermoperiodic treatments employed were a higher day temperature than night temperature (DT > NT) and several hours of temperature drop from beginning of the day (TD). TD markedly decreased the stem elongation rate when compared to the effect of DT > NT. Further, TD decreased the concentrations of stem GA1, GA20, and GA19 more than DT > NT, while TD increased the concentration of stem GA44 more than DT > NT. Thus, TD probably retarded the conversion of GA44 to GA?<19>, resulting in a low concentration of GA1, the probable biologically-active GA. These changes in endogenous GA concentration are probably an important physiological factor for the thermoperiodic regulation of stem elongation in D. grandflorum.
