Four Hypotheses to Explain Axillary Budbreak after Removal of Flower Shoots in a Cut-rose Crop

When flower-bearing shoots in cut-rose (Rosa xhybrida) are harvested (removed), a varying number of repressed axillary buds on the shoot remainder start to grow into new shoots (budbreak). Besides removing within-shoot correlative inhibition, it is hypothesized that shoot removal leads to 1) increased light intensity lower in the crop canopy; 2) changes in the light spectrum (particularly red:far-red ratio); and 3) changed source:sink ratio (i.e., the ratio between supply and demand of assimilates). As a fourth hypothesis it is proposed that the degree of budbreak on a shoot remainder is also influenced by the correlative inhibition exerted by other shoots on the plant. It is the goal of this work to determine which of these four hypotheses is most important for budbreak in a cut-rose crop. Four experiments were conducted, in which these factors were varied by leaf removal, removal of mature shoots, varying the number of young shoots, shading of the crop, and application of direct light on the buds. Increase in source:sink ratio was not consistently associated with higher budbreak. If source:sink ratio was decreased by removal of leaves or a mature shoot, budbreak showed even a tendency to increase. Budbreak was subject to correlative inhibition exerted by other shoots on the plant. Treatments where more light reached the bud (as a result of less shoots, no shading of the crop, application of local light) increased budbreak. Increased red:far-red ratio had the same result as more light reaching the bud but was often interrelated with light intensity. It was concluded that after removal of the flower-bearing shoot, among the factors tested, light intensity on the buds was an important and consistent factor explaining budbreak on the shoot remainder, whereas the effect of light spectrum should be further investigated.