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Biochemical characterisation of early and late bud flushing in common ash (Fraxinus excelsior L.)

Ash produces fast-growing crops of high timber quality. Nevertheless, young ash trees are well known for their susceptibility to forking, mainly caused by frosts, which occur in late spring resulting in necrosis of the apical bud. The time of flushing is an important selection criterion for tree breeders since it is a heritable trait. To study the physiology of flushing, buds have been harvested in the field (South Belgium), from March to June, and variations of 14 different endogenous metabolites were studied during this time course.

Oxidative products as malondialdehyde or lipid peroxides, polyamines, carbohydrates as hexoses, disaccharides, raffinose family of oligosaccharide and sugar alcohols, presented variations directly related to stages in bud development. An oxidative burst, and a polyamine transitory increase have been measured during this period. The major feature of bud break and further development was a global decrease in the endogenous non-structural carbohydrates. In a second step, an evaluation of the putative markers found in the field population was confirmed by testing provenances with known differences in their flushing time.

Over 14 putative markers were analysed in buds of forced shoots, which were collected from four ash provenances which differed in the timing of their bud flushing. Five markers, (putrescine, mannitol, trehalose, sucrose and raffinose) showed significant differences between late and early flushing provenances. Mannitol, sucrose and trehalose have important functions during bud development and the selection of ash material with an elevated content of these compounds could be an additional selection tool for use by tree breeders.

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