Submitted to: Journal of Plant Growth Regulation
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 22, 2005
Publication Date: December 27, 2005
Citation: Tworkoski, T., S. Miller, and R. Scorza. 2006. Relationship of pruning and growth morphology with hormone ratios in shoots of pillar and standard peach trees. J. Plant Growth Regul. 25:1-11. Interpretive Summary: Narrow-top tree growth forms of peach (Pillar peach) enable growers to plant more trees in an acre of orchard space and this increased tree density may increase yield efficiency (i.e. the number of bushels of peaches per acre). Our understanding of the biology responsible for growth forms such as Pillar peach is incomplete and we may be able to further improve tree forms by targeting and modifying the underlying biochemical mechanisms. In this experiment we discovered that auxin levels were consistently higher and cytokinin levels were consistently lower in Pillar than Standard trees and these biochemical signals, that are plant hormones, were related to branch angle and branching density. The results indicated that improved tree growth forms, such as those needed for productive, high-density plantings, can be achieved by targeting auxin and cytokinin signals as breeding selection criteria and for cultural management
Technical Abstract: Peach trees [Prunus persica (L.) Batch] with reduced crown width and upright branches are being developed to increase yields for high-density plantings. While breeding and cultural management are being used to shape tree crowns for high density plantings, the relationships between hormone signals and branching, which are fundamental to these approaches, are poorly understood. In this experiment, two peach growth habits (Pillar, KV930479 and Standard, 'Harrow Beauty') were budded on 'Lovell' rootstock, planted in the field in 1998, and characterized for shoot morphology and hormone concentrations in 2002 and 2003 (the fourth and fifth leaf, respectively). Auxin (indole-3-acetic acid) and cytokinins (largely trans-zeatin riboside, dihydrozeatin riboside, and isopentenyladenine) were measured by mass spectrometry in shoot tips and current-year shoots during 2002 and 2003, respectively. Also in 2003, the effects of pruning on both growth habits and the interaction of pruning and crown position in Pillar (upper and lower position within the crown) were measured. In 2002, bud break on one-year-old branches did not differ between growth habits, but there were fewer sylleptic branches and greater distances from terminal buds to the first sylleptic branches in Pillar. In addition, branch angles were narrower in Pillar than in Standard trees. Less sylleptic branching and more upright growth was accompanied by higher auxin and auxin-to-cytokinin ratios in Pillar, suggesting stronger apical dominance. In 2003, auxin concentrations appeared to be related to shoot growth vigor as well as growth habit. Auxin concentrations were highest in current year shoots of pruned trees. The auxin to cytokinin concentration was greater in current year shoots of Pillar than of Standard trees. In Pillar trees, significant branch growth occurred in the upper crown between 58 and 87 days after bud break. This branch growth, despite the high auxin to cytokinin ratio, suggested that resource availability (e.g. light) had a coordinate effect with hormones on branch growth. Results from this experiment suggest that existing peach tree growth habits may have resulted from selection of genotypes with altered hormone metabolism or sensitivity and that superior growth forms may be achieved by selection and management for target hormone concentrations and ratios.