Xylem manipulation techniques affecting tree vigour in peach and olive trees

Water relations are among the major factors affecting shoot growth and plant vegetative vigor. In many tree crops, dwarfing rootstocks regulate scion vegetative vigor by reducing plant hydraulic conductance. In Prunus spp. reduced xylem vessel size leads to reduced hydraulic conductance, but reduction of annual xylem tissue growth could have similar results. The aim of the present work was to investigate the effects of techniques that allow to manipulate xylem tissue growth on tree vegetative vigor. Two different techniques, early trunk girdling and trunk constriction were applied on peach and on olive trees, respectively. In 2010, at the KAC, Parlier, CA, USA, very early girdling (March 31) and late girdling (April 15) were carried out on 8-yearold ‘Springcrest’/‘Nemaguard’ trees. On peach trees, a deciduous species, early girdling (few weeks after sprouting) caused a significant decrease of early season stem water potential and vegetative growth in comparison with trees in which girdling was carried out later in the season and control trees, respectively. On olive trees a plastic strap was applied in December 2008 to cause a trunk constriction limiting trunk growth during the vegetative season. Three-year-old trees of five cultivars grown in super high-density orchard (1600 tree ha-1) near Perugia, Italy, were used for the experiment. Midday stem water potential was lower in constricted trees than in control trees. At the end of the experiment constricted trees had smaller vegetative growth than control trees. Constriction increased yield efficiency, but severe reduction of vegetative growth triggered alternate bearing in the following year. The results of these experiments underline the role of xylem conductance in regulating tree vegetative growth. Techniques aimed at manipulating trunk/branch xylem growth can be effective in reducing tree vegetative growth in tree horticultural species.