Proposed pre-selection method for identification of dwarfing peach rootstocks based on rapid shoot xylem vessel analysis

Abstract

The association between a genotype's dwarfing ability and a genetic, physiological, or anatomical trait would be of great value for selecting dwarf rootstocks at an early stage of development. Dwarfing peach rootstocks have been associated with decreased hydraulic conductance due to smaller xylem vessel diameters compared to invigorating rootstock genotypes. We evaluated tentative anatomical criteria for selecting dwarfing peach rootstocks based on measurements of the diameter of the largest xylem vessels and the number of xylem vessels surrounding the largest vessels in cross sections of shoots. Epicormic and proleptic shoots were collected from trees of the standard, invigorating ‘Nemaguard’ peach rootstock and a series of dwarfing rootstocks: ‘Controller 9.5’ ™ (HBOK 50), ‘Controller 9’ ™ (P30-135), ‘Controller 8’ ™ (HBOK 10), ‘Controller 7’ ™ (HBOK 32), ‘Controller 6’ ™ (HBOK 27) and ‘Controller 5’ ™ (K146-43). Transverse sections were cut from 2, 3, and 4mm diameter stems of proleptic shoots and from 4, 5, and 6mm diameter stems of epicormic shoots. The samples were fresh sectioned to a thickness of about 150μm, stained with Toluidin-Blue-O and photographed on a light microscope. The largest xylem vessel was visually identified in each microscopic view field, and the surrounding circle of vessels with a diameter of ∼164 micrometers were counted. Rootstock genotype, shoot diameter, and the interactions between rootstock×shoot diameter significantly affected the diameters of the largest vessels of both proleptic and epicormic shoots. Most of the evaluations of the largest vessel diameter were able to distinguish the lowest vigor rootstock from the vigorous, but the intermediate vigor rootstocks were not clearly distinguished by this criterion. The less vigorous rootstocks showed a tendency for having more vessels. The larger vessel diameters in more vigorous rootstocks appeared to be related with fewer vessels. For rootstock screening purposes epicormic shoots appeared to be better for evaluation than proleptic shoots. Young, vigorous seedlings are in the vegetative phase of plant growth, and probably more similar to epicormic shoots than to proleptic onesshoots that are physiologically more related to fruit bearing than to rapid vegetative growth. We propose that the evaluation of the largest vessel of a microscopic view field associated with the number of vessels around the largest vessel, measured on 5mm vigorous shoots would be useful as a criterion for selecting dwarfing peach rootstock genotypes.