Longleaf Pine Root System Development and Seedling Quality in Response to Copper Root Pruning and Cavity Size

rootcollar diameter as the standard, nondestructive measure of seedling quality. Other nondestructive measures of seedling quality would be valuable. One option, root-bound index (RBI), reflects the ratio of rootcollar and cavity diameters (South et al. 2005, South and Mitchell 2006). This variable is an indirect, nondestructive measure of how well the cavity accommodates both the taproot and fibrous roots. With the determination of acceptable ranges of values, rootcollar diameter and RBI could be used together to indicate seedling size, the progression of fibrous root growth, and the potential for root binding. It is hypothesized that cavity size and copper root pruning affect root system development and, subsequently, standard measures of seedling quality for longleaf pine. We further hypothesize that copper root pruning affects the vertical distribution of RGP along the length of the root plug. Our objectives were to assess how cavity size and copper root pruning affect longleaf pine root system development during production, seedling quality at the end of the cultural period, and RGP before planting. Root system development was assessed by the fraction of root dry weight as taproot and primary and secondary lateral roots. Seedling quality was evaluated by the standard, nondestructive measure of rootcollar diameter, an alternative nondestructive measure, RBI, and two other destructive variables: number of primary lateral roots and shoot-root ratio. Materials and Methods Greenhouse Experiment Longleaf pine seeds of mixed seedlots from Florida were sown in six container types in April 2004. Treatments were Superblock and Copperblock containers (Beaver Plastics Ltd., Acheson, Alberta, Canada) that differed by three cavity sizes (Table 1). Superblock and Copperblock containers were distinguished by the application of Root Trim (US Environmental Protection Agency Reg. No. 81293-2, containing copper oxychloride) to the cavity wall of Copperblock cavities. Using standard cultural practices (Barnett and Brissette 1986), longleaf pine seedlings were grown in four, seven, and nine Superblock and Copperblock containers having small, medium, and large cavities, respectively. In April, seeds were sown in container cavities containing a commercial peat moss and vermiculite mixture and Osmocote 19-6-12 slow-release fertilizer (Scotts Miracle Grow Co., Marysville, OH) at a rate of 3.6 kg/m. Containers were placed in random locations on greenhouse benches, and seedlings were fertilized weekly by broadcast application of a 0.05–0.06% solution of water-soluble fertilizer (i.e., 20-20-20) to saturation. Seedlings were fertilized between early June and the end of September 2004. Seedlings were grown for 27 weeks under ambient light in a greenhouse where air temperature was maintained at 20–25°C. At 10, 17, and 27 weeks after sowing, 25 seedlings were randomly selected from each container type. The growing medium was washed from the root system, rootcollar diameter (RCD) was measured, and the stem was severed from the root system. Primary lateral roots 0.5 cm in length were counted. Primary lateral roots were excised from the taproot, and all secondary lateral roots were stripped or cut from the primary lateral roots. Dry weights of the shoot, taproot, primary lateral roots, and secondary lateral roots were determined after drying to a constant weight at 70°C. Shootroot ratios and fractions of root system dry weight as taproot and as primary and secondary lateral roots were calculated. RBI was calculated by dividing RCD by cavity diameter and expressing this value as a percentage (i.e., 0–100%) (South et al. 2005, South and Mitchell 2006). RGP Experiment At the end of the 27-week cultural period, 25 seedlings were randomly selected from each container type. Root plugs remained intact, and seedlings were planted in pots (1.5 l volume) of masonry sand. Potted seedlings were placed on benches in a greenhouse using a randomized complete block layout with five blocks representing location in the greenhouse. Each block contained five potted seedlings from each of the six container types. Seedlings were well watered and maintained for 28 days under ambient light and air temperature ranging between 20 and 25°C. A root plug template of each cavity size was drawn and delineated into three root plug depths (top, middle, bottom). Two root plug depths were similar among all container types: 0–5 and 5–10 cm. For the small container types, the third root plug depth was 10–13.3 cm, and the depths for the medium and large container types were 10–14.9 and 10–15.2 cm, respectively. After 28 days, the masonry sand was washed from seedling root systems. Root plugs were laid over the appropriate template and visually partitioned into the three root plug depths. Then, egressed roots, 0.5 cm in length, were excised from the outer face of each root plug depth and counted. Statistical Analyses All data were transformed to square root or natural logarithm values as needed to establish normally distributed experimental errors. Ten, 17, and 27 weeks after seedlings were sown in the greenhouse study, seedling variables (RCD, number of primary lateral roots, shoot-root ratio, RBI) were evaluated by analysis of variance (ANOVA) using a completely random 2 3 factorial design with two main effect treatments: copper root pruning (no copper root pruning: Superblock; copper root pruning: Copperblock), and cavity size (small, medium, large). Fractions of total root system dry weight as taproot and as primary and secondary lateral root dry Table 1. Container types varying in copper root pruning treatment and cavity size used to produce longleaf pine seedlings. Container type Cavity size treatment Copper root pruning treatment Cavities per unit/cavities per m Cavity volume (ml) Cavity depth Cavity top diameter . . . . . . . . .(cm) . . . . . . . . . Superblock 198/60 Small None 198/936 60 13.3 2.8 Superblock 112/105 Medium None 112/530 108 15.0 3.6 Superblock 77/170 Large None 77/364 170 15.2 4.2 Copperblock 198/60 Small Root Trim 198/936 60 13.3 2.8 Copperblock 112/105 Medium Root Trim 112/530 108 15.0 3.6 Copperblock 77/170 Large Root Trim 77/364 170 15.2 4.2 a Roots pruned with copper oxychloride applied to cavity wall. 6 SOUTH. J. APPL. FOR. 35(1) 2011 weights were evaluated by ANOVA using a completely random 2 3 factorial split plot in time design. The whole-plot effects were copper root pruning and cavity size, and the subplot effect was cultural period (10, 17, and 27 weeks after sowing). Cultural period and its interactions with copper root pruning and cavity size were tested to evaluate developmental responses in root system morphology over the 27-week cultural period. At the end of the 28-day RGP test, numbers of egressed roots from the root plug depths were evaluated by ANOVA using a randomized complete block 2 3 factorial split plot in space design with five blocks. The whole-plot effects were copper root pruning and cavity size, and the subplot effect was root plug depth (top, middle, bottom). Normality tests and main and interaction effects were considered significant at an level of 0.05. As warranted, means were compared by the Tukey-Kramer procedure and considered significantly different at an level of 0.05.