Bud break accompanies with the enhanced activities of hemicellulase and pectinase and the mobilization of cell wall thickenings in Persian walnut bud scales

Abstract

The endo-dormant bud scales of walnut have storage parenchyma cells with thickened walls. During bud dormancy release, cell wall hydrolytic enzymes mobilize pectin and hemicellulose reserves of these cells to fuel bud break. Cell wall hydrolytic enzymes might define specific stages of bud dormancy release. Bud dormancy release is important in woody plants as it is linked to final biomass and yield. This process requires enzyme activities which affect the bud cell wall physico-chemical properties, however, little information is available. Accordingly, the activities of some cell wall hydrolytic enzymes were studied during endo-dormancy release in lateral buds of two Persian walnut (Juglans regia L.) cultivars having low (‘Serr’) and high (‘Hartley’) chilling requirements (CRs). Buds were collected monthly from the leaf shedding stage (November) till bud break and used for enzymatic analysis. In both cultivars, the activities of 1,4-β-D-glucanase, xylanase, mannanase and pectinase increased during dormancy release. In ‘Serr’, first rises in mannanase and pectinase activities occurred before transition to eco-dormancy (January) and the second rises occurred before bud break (March). Similarly, in ‘Hartley’, first rise in mannanase activity observed before transition to eco-dormancy (February) and the second rise before bud break (April). ‘Serr’ buds also displayed significant increase in the activities of hemicellulases and pectinase earlier than ‘Hartley’, however, the latter showed quantitatively higher means for the enzymatic activities. Endo-dormant bud scales had very thick-walled storage parenchyma cells which turned into thinner-walled ones during bud break, suggesting cell wall polysaccharide mobilization by hemicellulases and pectinases. Thus, the activation of cell wall hydrolytic enzymes during endo-dormancy release may contribute to the bud scale cell wall carbohydrate reserve mobilization and/or growth-associated wall relaxation in preparation for bud break. Furthermore, buds from low CR plants may show earlier activation of cell wall-loosening mechanisms during dormancy release.