Stock Quality of Black Walnut (Juglans Nigra) Seedlings as Affected by Half-Sib Seed Source and Nursery Sowing Density

Morphological quality of black walnut (Juglans nigra L.) seedlings largely dictates their capacity to establish well in plantations and their performance potential for use as rootstocks in grafting. Larger seedlings and rootstocks, attained from propagation at relatively low nursery sowing densities, have comparatively more expansive root systems for exploitation of soil resources and greater nutrient/carbohydrate reserves to support aboveground growth. The relative role of genetic selection in dictating morphological quality of nursery-grown black walnut seedlings, however, has not been thoroughly documented. We collected seed from three half-sib families (A, B, C) of black walnut in Indiana, USA which were sown as sprouted seeds into bareroot nursery beds at three sowing densities (L, 11.2 m; M, 24.2 m; H, 29.4 m). Sowing density significantly affected mean shoot height (H > M > L) and root volume (L > M > H), but not stem diameter. Shoot height was increased by 14.4 cm (43%) in H vs. L and root volume was increased by 32 cm (39%) in L vs. H. Seedling response to family selection was significant for height, root volume, and stem diameter (C > B > A). The magnitude of the difference in response was greater for family selection compared to nursery sowing density, as height and root volume were increased by 15.2 cm (46%) and 44 cm (60%) in C vs. A, respectively. Further, family C had greater mean stem diameter and root volume in H than did family A in L; mean height for these two families were about equal between H and L. Our results suggest that identification of superior genetic sources may be relatively more important than sowing density in nursery propagation of high quality black walnut seedlings. INTRODUCTION Black walnut (Juglans nigra L.) is an important component of forests in the Central Region of the USA, producing valuable timber and nut resources (Williams, 1990). Black walnut is among the most common species grown in forest tree nurseries in this region (Jacobs et al., 2004), with seedlings being used for afforestation plantings and to a lesser extent as rootstock for grafting of genetically improved germ plasm. Planted hardwood seedlings in this region often grow poorly or fail to survive (Jacobs et al., 2004), which may be partially attributed to variable nursery stock quality. Morphological attributes such as shoot height and stem caliper are easily measured and often used to evaluate nursery stock quality (Thompson, 1985). In addition, large root system morphology appears to provide a useful assessment of potential hardwood seedling performance (Schultz and Thompson, 1996; Jacobs and Seifert, 2004) presumably due to increased access of a more expansive root system to soil resources, as well as supplying greater carbohydrate and nutrient reserves to promote aboveground growth. Benefits to using high quality nursery stock are likely to be realized following seedling planting, as well as when using rootstock for grafting. Control of black walnut nursery stock quality is largely achieved through cultural practices conducted in the nursery (Jacobs, 2004). Among the most important nursery cultural variables is bed sowing density, with larger seedlings consistently being produced Proc. V th Int. Walnut Symp. Eds. M.E. Malvolti and D. Avanzato Acta Hort. 705, ISHS 2006 376 at relatively low sowing densities (Schultz and Thompson, 1996). Low seedbed densities, however, limit overall nursery stock production as only a fixed amount of nursery growing space is generally available for production. Thus, examination of additional factors which might enhance nursery stock quality at a given density could help improve the quality of black walnut nursery stock for seedling planting and grafting. Though substantial investigation into genetic improvement of black walnut has been conducted on plantation trees (Beineke, 1989), relatively little attention has been given to the role of genetic identity in dictating stock quality of nursery seedlings. Operationally, the majority of black walnut seed attained for nursery stock production is collected from sources of unknown genetic origin (Jacobs, 2003), limiting the positive contribution that genetic identity might have on improving black walnut nursery stock quality. Thus, the objective of this study was to examine the relative role of half-sib seed source and nursery sowing density in the propagation of high quality black walnut nursery stock. MATERIALS AND METHODS In October 2002, seeds were collected from three half-sib families (A, B, C) of black walnut in Indiana, USA. The seeds were temporarily stored in mesh bags in an area with adequate air circulation to allow excess moisture in the husks (pericarps) to dry. After drying in November 2002, husks were removed from the seeds, which were then placed in perforated plastic bins filled with lightly moistened and finely milled sphagnum peat moss. The bins were placed into cold storage at 4 °C for 90 days of stratification. Seeds were pre-sprouted (Davis et al. 2004) to ensure high probability of successful seedling development under nursery density treatments and that all seeds began from a similar physiological state of development following sowing into nursery beds. In February 2003, the seeds were sown in plastic greenhouse flats with Metro-Mix 360 with ScottsCoir growing medium (O.M Scotts Co., Marysville, OH, USA) and placed into a greenhouse. Greenhouse environmental conditions were maintained at 24/17 °C (day/night) with natural photoperiod and ambient light intensity. The flats were watered as necessary to maintain moist medium. Twice weekly, the seeds from each flat were removed and carefully rinsed free of planting medium to check for sprouting. Optimum sprouting occurred when the shell had cracked open at the cleavage line but the radicle, while visible, had not yet emerged. This protected the radicle from physical damage. If root formation had begun, or if the epicotyl had emerged above the media, those seeds were discarded. Sprouted seeds were placed in flats and refrigerated at 4 °C until a sufficient number had been collected, at which time the seeds for each family were packed with moistened peat moss into separate plastic bins for transportation to Vallonia Nursery, Indiana, USA (38°48’N, 86°06’W) in April 2003. Sprouted seeds from each family were sown by hand into bareroot nursery bed plots at three sowing densities (L, 11.2 m; M, 24.2 m; H, 29.4 m). Densities in each plot were kept consistent using planting jigs. The jigs were made of plywood with wooden dowels spaced at regular intervals to mark the desired density. L plots were 2.85 m (32 seeds), M plots were 1.24 m (30 seeds), and H plots were 1.02 m (30 seeds). Plots were arranged in a randomized complete block design consisting of 4 blocks, where every block contained plots for each family x density combination. Seedlings were then grown under operational nursery conditions (Jacobs, 2003) until lifting in March 2004. Seedlings were then stored at 2 °C until measured for morphological characteristics including stem caliper, shoot height, and root volume using the water displacement method of Burdett (1979). All analyses were performed using SAS 9.1 (SAS Institute Inc., Cary NC, USA). Multivariate analysis of variance was performed using the general linear model where stem diameter, shoot height, and root volume were dependent variables and family, sowing density, family x density interactions, and block were independent variables. For each dependent variable the least-squares means and standard deviations were generated for each combination of family and density by invoking LSMEANS. Means separation