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

We have examined whether gibberellins (GAs) mediate photoperiodic growth re sponses in strawberry (Fragaria x ananassa) by measuring GA content and GA metabolism in petioles with accurately defined growth rates. Gibberellin A( 1), GA(8), GA(19), GA(20), GA(29), GA(34), and tentatively GA(17), were ide ntified by gas chromatography-mass spectrometry, and GA(4) 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(1), GA(8), GA(29) and GA(34) were reduced by about two-fold in short days. GA(20) concentration was always low regardles s of age or treatment, typically 10-fold less than GA(1) and 40-fold less t han GA(19). Application of paclobutrazol (25 mu g plant(-1)) reduced growth rate by 43%, somewhat greater than the effect of short days (23%), but res ulted in a six-fold decrease in GA(1) content, much greater than the maximu m 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(1) content. [H-2]GA(19) and [H-2]GA(20) were metabolised to GA(1), GA (8) and GA(29), although conversion of GA(19) was slow. GA(4) was converted to GA(34) but not to GA(1) or GA(8). Photoperiod had little effect on any of these metabolic steps. For the following reasons, we suggest that the ph otoperiod growth response is not mediated primarily through altered GA conc entrations: (1) a lack of a rapid photoperiod effect on GA concentrations o r metabolism, (2) changes in growth before or independent of changes in GA concentrations and (3) a normal photoperiod effect on growth in petioles wi th artificially lowered GA content.