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

Abstract

We have examined whether gibberellins (GAs) mediate photoperiodic growth responses in strawberry (Fragaria × ananassa) by measuring GA content and GA metabolism in petioles with accurately defined growth rates. Gibberellin A₁ , GA₈ , GA₁₉ , GA₂₀ , GA₂₉ , GA₃₄ , and tentatively GA₁₇ , were identified by gas chromatography–mass spectrometry, and GA₄ 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 GA₁ , GA₈ , GA₂₉ and GA₃₄ were reduced by about two-fold in short days. GA₂₀ concentration was always low regardless of age or treatment, typically 10-fold less than GA₁ and 40-fold less than GA₁₉ . Application of paclobutrazol (25 g plant^–1 ) reduced growth rate by 43%, somewhat greater than the effect of short days (23%), but resulted in a six-fold decrease in GA₁ 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 GA₁ content. [²H]GA₁₉ and [²H]GA₂₀ were metabolised to GA₁ , GA₈ and GA₂₉ , although conversion of GA₁₉ was slow. GA₄ was converted to GA 34 but not to GA₁ or GA₈ . 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.