Rooting ability of Persian walnut as affected by seedling vigour in response to stool layering By K. VAHDATI 1 * , R. REZAEE 2 , V. GRIGOORIAN 3 , M. VALIZADEH 4 and A. MOTALLEBI AZAR 3 1 Department of Horticultural Science, College of Abooraihan, University of Tehran, P.C. 3391653755, Tehran, Iran 2 Department of Seed and Plant Improvement, Agricultural and Natural Resources Research Center, West Azerbaijan, P.O. Box 365, Uremia, Iran 3 Department of Horticulture, Faculty of Agriculture, University of Tabriz, P.C. 5166614766,Tabriz, Iran 4 Department of Crop Production and Breeding, Faculty of Agriculture, University of Tabriz, P.C. 5166614766, Tabriz, Iran (e-mail: kvahdati@ut.ac.ir) (Accepted 8 January 2008) SUMMARY Walnut (Juglans regia L.) is an important nut crop with a “difficult-to-root” characteristic. In this study, the rooting ability of low-vigour and precocious seedlings of 3-year-old Persian walnut was compared with semi-vigorous and high-vigour seedlings using the stool layering method. The results indicated that low-vigour seedlings rooted more (40%) than semi-vigorous (31.42%) and high-vigour (17.14%) seedlings. The average number of roots per shoot (layer) and rooting score (on a scale of 1 – 5) in the low-vigour group were 7.83 and 4.19, respectively, which differed significantly from the high-vigour group. Moreover, most of the high quality adventitious roots formed on low-vigour seedlings appeared to originate from internal tissues compared to the low quality and brittle roots that originated from callus in high-vigour seedlings. Improved rooting of low-vigour seedlings, together with a significant negative correlation between layer size and root number (r = –0.29), reflects substantial structural or hormonal differences among seedlings of different vigour. Approx. 70% of rooted layers survived after transferring to field condition. Our results provide more support for the possibility of vegetative propagation of walnut by conventional stool layering, as well as the selection of easy-to-root, dwarf walnut cultivars or rootstocks on their own roots. P roblems associated with present methods of walnut production on seedling rootstocks of walnut or hybrids include: lack of genetic uniformity, inconvenient orchard management due to large tree size, the threat of lethal black–line disease, as well as a desire for hedgerow growing systems. These have caused renewed interest in the use of Persian walnut (Juglans regia L.)-based rootstocks or own-rooted cultivars (Kuniyuki and Forde, 1985; McGranahan and Leslie, 1988; Forde and McGranahan, 1996; Vahdati, 2003; Vahdati et al., 2004). Precocious (early maturing) and low-vigour walnut genotypes, which are frequently found in some seed sources in Iran (Rezaee et al., 2006) or in central Asia (Germain et al., 1997), could provide the genetic material for tree-size control. To date, tree-size reduction using genetically dwarf rootstocks is a key component of high- density orchard systems (Cousins, 2005) and many walnut growers are now interested in shifting to high- density planting systems (McGranahan and Leslie, 1988; Olson et al., 2001; Ramos et al., 2001). The main limiting factor in exploiting this valuable germplasm is the lack of efficient vegetative propagation due to the difficult-to-root nature of Persian walnut species (Kuniyuki and Forde, 1985; Gunes, 1999;Vahdati et al., 2004). Micropropagation could provide an effective technique for large-scale multiplication; however, only specialised laboratories could apply this, as there are problems in the rooting and acclimatisation phases (Kuniyuki and Forde, 1985; McGranahan and Leslie, 1988; Vahdati et al., 2004). In contrast, propagation through conventional methods such as layering is not only cost-effective, but also much easier to carry out and most nurseries have the facilities required. A number of attempts have been made to propagate walnut by cutting and/or various types of layering. These showed that the continuity of the sclerenchymatous layer encircling the phloem, as well as the oxidation of juglone during wounding, inhibited rooting and the emergence of walnut roots (Vahdati, 1996; Gunes, 1999). Recently, Vahdati and Khalighi (2001) reported a higher rooting efficiency using a modified “stool layering” method. Stool layering, also known as “tie-off” or “mound-layering”, is presently used by an increasing number of nurseries to propagate rootstocks of apple, pear, quince and hazelnut (Hartmann et al., 1990; Erdugan and Smith, 2005). Meanwhile, in Persian walnut, a high variability in rooting ability was reported among different seed sources (genotypes) both by layering (Vahdati and Khalighi, 2001) and by micropropagation methods *Author for correspondence. Journal of Horticultural Science & Biotechnology (2008) 83 (3) 334–338