Redox rather than carbohydrate metabolism differentiates endodormant lateral buds in walnut cultivars with contrasting chilling requirements

- Sugar and redox metabolisms were compared during bud break in two walnut cultivars.
- Bud sugar metabolism is alike in both low and high chilling requirement cultivars.
- ROS scavenging activities persist longer in high chilling requirements cultivars.
- Oxidative signals develop earlier in low chilling requirement cultivars.
- Cultivars with contrasting chilling requirements are different in redox reactions.

Release of bud dormancy in temperate woody plants requires a period of exposure to low temperatures known as chilling requirements. The biochemical bases of differences in chilling requirements of plants are unknown. Carbohydrate and redox metabolisms are considered important for bud dormancy release but their significance in relation to differences in chilling requirements has not yet been investigated. Accordingly, these metabolisms were studied in the lateral buds of two walnut (Juglans regia L.) cultivars with contrasting high (Hartley) and low (Serr) chilling requirements. Lateral buds were collected monthly from leaf drop stage in November till initiation of bud break (March to April) and used for biochemical analyses. Of the investigated parameters related to carbohydrate metabolism, buds from walnut cultivars differed only in monthly patterns of starch content and invertase activity however, they were different in nearly all parameters related to redox metabolism. Thus, from November till February the activities of NADPH-oxidase and superoxide dismutase declined in 'Hartley', but increased in 'Serr'. Meanwhile the activity of soluble peroxidase declined in 'Serr' from December but persisted till February in 'Hartley'. Rise in ascorbate peroxidase activity of 'Serr' buds in January however, was earlier than that of 'Hartley'. 'Serr' buds generally had greater hydrogen peroxide than 'Hartley' but lipid peroxidation in the former declined from January while in the latter it happened from February. Exogenous hydrogen peroxide promoted burst of ecodormant 'Hartley' and 'Serr' buds whereas hydrogen peroxide scavengers like dimethylthiourea (DMTU) postponed the process only in the latter cultivar. It was proposed that redox interactions rather than sugar metabolism govern bud dormancy release. Furthermore, bud dormancy release requires accumulation of oxidative signals and cultivars with lower chilling requirements are accordingly able to develop and respond to these signals earlier.