Endogenous gibberellin content does not correlate with photoperiod-induced growth changes in strawberry petioles

We have examined whether gibberellins (GAs) mediate photoperiodic growth responses in strawberry (Fragaria x ananassa) by measuring GA content and GA metabolism in petioles with accurately defined growth rates. Gibberellin A, GA, GA, GA, GA, GA, arid tentatively GA, were identified by gas chromatography-mass spectrometry, and GA was detected by selected ion monitoring. Although petiole growth rates were reduced within 2 d of a long-day to short-day transfer, we found no consistent changes in GA content until 8 d, when GA, GA, GA and GA were reduced by about twofold in short days. GA concentration was always low regardless of age or treatment, typically 10-fold less than GA and 40-fold less than GA. Application of paclobutrazol (25 μg plant) reduced growth rate by 43%, somewhat greater than the effect of short days (23%), but resulted in a six-fold decrease in GA content, much greater than the maximum two-fold effect of short days. However, paclobutrazol-treated petioles in long and short days differed in growth rate by 30%, yet had no difference in GA content. [H]GA and [H]GA were metabolised to GA, GA and GA, although conversion of GA was slow. GA was converted to GA but not to GA or GA. Photoperiod had little effect on any of these metabolic steps. For the following reasons, we suggest that the photoperiod growth response is not mediated primarily through altered GA concentrations: (1) a lack of a rapid photoperiod effect on GA concentrations or metabolism, (2) changes in growth before or independent of changes in GA concentrations and (3) a normal photoperiod effect on growth in petioles with artificially lowered GA content