Effect of Photoperiod on the Levels of Endogenous Gibberellins in Spinach as Measured by Combined Gas Chromatography-selected Ion Current Monitoring

Abstract

The changes in the levels of five endogenous gibberellins (GAs) in spinach in relation to photoperiodic treatment have been examined by combined gas chromatography-selected ion current monitoring. Long-day treatment caused a 5-fold decline in the level of GA19 while the levels of GA20 and GA29 increased dramatically during the same period. In absolute terms, the level of GA20 increased from 0.8 microgram per 100 grams dry weight in short days to 5.5 micrograms per 100 grams dry weight after 14 long days. The levels of GA17 and GA44 did not change significantly with long-day treatment. These results are consistent with the hypothesis that GA19 is converted to GA20 and that this conversion is under photoperiodic control. Since stem growth in spinach is correlated with an increase in the level of GA20, one major aspect of photoperiodic control of stem growth might be the availability of GA20 through regulation of the conversion of GA19 to GA20.