Metabolism of gibberellin A19 is under photoperiodic control in Populus , Salix and Betula , but not in daylength-insensitive Populus overexpressing phytochrome A

Application experiments have suggested that short-day-induced cessation of elongation growth in trees is caused by photoperiodic regulation of the conversion of gibberellin GA 19 to GA 20 . In the present study we examined further the photoperiodic control of GA metabolism in trees with focus on the conversion of GA 19 in Salix pentandra, hybrid aspen (Populus tremula × tremuloides) and silver birch (Betula pendula) using [17,17- 2 H 2 ]-GA 19 or unlabelled GAs in application studies. GA 20 and GA 1 were able to restore growth also in hybrid aspen and silver birch under short days (SD), whereas GA 19 had no or only a very small activity. Contrary to hybrid aspen and S.pentandra, the activity of GAS in silver birch was significantly lower than that of GA 1 . Gas chromatography-mass spectrometry (GC-MS) analysis revealed a smaller turnover of [ 2 H 2 ]-GA 19 in SD than in long days (LD) in hybrid aspen. No such difference in turnover of [ 2 H 2 ]-GA 19 was observed in photoperiod-insensitive hybrid aspen overexpressing PHYA. Application of unlabelled GAs to seedlings of S. pentandra, hybrid aspen and silver birch under SD followed by quantification of metabolites by GC-MS analysis, showed that applied GA 19 was not readily converted to GAS and GA 1 Although the sensitivity to GAs is also known to decrease under SD, the present data are in favour of a photoperiodic regulation of the metabolism of GA 19 in vivo in the woody species S.pentandra, hybrid aspen and silver birch. The data might also suggest that silver birch differs from S.pentandra and hybrid aspen by exhibiting a possible photoperiodic control also of the conversion of GAS to GA 1 .