R. Brian Hardin, D. Joseph Eakes, Jeff L. Sibley, Charles H. Gilliam, and Gary J. Keever

Flowering dogwood is a shade-tolerant native tree ranging from Maine to Florida in the East and extending westward into Michigan and Alabama. It is one of the most popular small landscape trees in the United States. Southern nurseries commonly collect seed in southern areas and sell the seedlings to nurseries in the northern portion of the country. Although considered hardy from Arnold Arboretum Hardiness Zones 5 to 9 (USDA Hardiness Zones 6a to 9b), flowering dogwood seedlings from southern sources are not considered sufficiently hardy in Arnold Arboretum Zone 5 (USDA Zone 6a) and have minimal flower production. Because seedling performance is variable, many cultivars have been selected. There are approximately 60 valid cultivars of flowering dogwood with three natural variations: f. rubra Weston with red bracts, f. xanthocarpa Rehder with yellow fruit, and f. pluribracteata Rehder with more than four bracts. Many cultivars of these have originated in the southeastern United States.

Cultivars of flowering dogwood are often budded or grafted on seedling rootstocks, and the genetic makeup of the rootstock may determine the relative heat tolerance of a given cultivar. The objective of this study was to determine differences in heat tolerance of flowering dogwood seedlings grown from seed collected from native stock in three USDA Hardiness Zones (6a, 7a, and 8a) and two American Horticultural Society (AHS) Heat-Zones (7 and 8).

METHODS
Seed were collected from three sources, representing three USDA Zones and two AHS Zones: Rock Island, Tennessee (35^o41’N x 85^o46’W; 6b; 7), Bankhead National Forest, Lawrence County, Alabama (34^o22’N x 87^o30’W; 7a; 7), and Auburn, Alabama (32^o36’N x 85^o29’W; 8a; 8) in October or November 1996. Following cold stratification at 41.0^oF for 3 months in moist builders sand, seed were planted at a depth of 1 inch in 65 cubic-inch containers in an amended pinebark/sand (6:1 by volume) growth medium on March 6, 1997. Seedlings were grown in a double-wall polyethylene greenhouse in Auburn, Alabama for 6 months with heat (minimum) and venting (maximum temperature) setpoints of 68^oF and 80^oF, respectively. On June 4, 1997, seedlings were topdressed with 0.11 ounces per pot of Osmocote 14-14-14 (0.01 ounce of N, P₂O₅, and K₂O) controlled release fertilizer (O.M. Scotts Co.). Powdery mildew was controlled using 3336-F Turf and Ornamental Fungicide (W.A. Cleary Chemical, Somerset, New Jersey) at the labeled rate every 10 to 14 days. Containers were grouped into trays spaced tray to tray. Trays were lined with landscape fabric (Weed-X; Dalen Products, Knoxville, Tennessee).

Excised portions of young, tender, actively growing roots 1 inch in length (0.018 ounces in weight) were subjected to procedures evaluating leakage of electrolytes from root cells. These procedures involved placing the root tissue in test tubes containing 0.03 fluid ounce of deionized water and exposing it for 30 minutes to 68, 86, 95, 104, 113, 117.5, 122, 131, or 140^oF in a thermostatically controlled circulating water bath. After removal, roots were cut into 0.20 inch segments and returned to test tubes containing 0.6 fluid ounce of deionized water prior to incubation in an ice bath for 24 hours at 39^oF. Measurements of initial conductivity (Accumet 50, Fisher-Scientific, Pittsburgh, Pennsylvania) of these samples were taken, after which the samples were autoclaved for 20 minutes at 250^oF and incubated in an ice bath for 24 hours at 39^oF prior to final solution conductivity measurements.

RESULTS
Electrolyte leakage was an effective method for determining the thermotolerance of flowering dogwood roots, with data appearing sigmoidal in nature for all provenances tested. USDA Zone 6b (AHS Zone 7) seedlings had the highest predicted critical temperature midpoint (Tm) at 126.3 ± 1.08^oF, 2.2^oF higher than that for USDA Zone 7a (AHS Zone 7) seedlings (124.2 ± 0.9^oF), but similar to USDA Zone 8a (AHS Zone 8) seedlings (see table). Likewise, seedlings from USDA Zone 8a (AHS zone 8) at 124.7 ± 0.7^oF were similar to those collected in USDA Zones 7a (AHS 7). Although USDA Zone 6b and 7a seedlings were statistically different, there may be little ecological difference in root thermotolerance among these provenances of flowering dogwood. In practical terms, one factor minimizing the small difference between USDA Zones 6b and 7a provenances is that although in different USDA Hardiness-Zones, Bankhead (USDA Zone 7a) is located in the same AHS Heat-Zone (7) as Rock Island, Tennessee (USDA Zone 6b). Studies have shown a lack of interaction between temperature and provenance for Acer saccharinum (silver maple) from Minnesota and Mississippi, concluding that there was no evidence to support that root-zone heat would affect growth and water relations more in trees native to the northern climate than in those native to southern climate. Furthermore, significant indirect injury may also be realized long before direct injury occurs in natural populations.

No references were found investigating the possibility of genetic differences in heat tolerance of flowering dogwood rootstock. However, sufficient variability must exist in native populations for any improvement to be expected in breeding for a particular trait. While dogwood populations across the native range have been shown to have sufficient variability to warrant extensive selection regarding a number of morphological and physiological traits, the results of this study indicate that there is little genetic variability in root thermotolerance across this part [USDA Zones 6b - 8a (AHS Zones 7 and 8)] of the native range of flowering dogwood. Therefore, there should not be differences in heat tolerance among rootstocks that originate in USDA Zones 6b, 7a, or 8a.

This work provides valuable information regarding root thermotolerance for native populations of flowering dogwood in comparison to other species with a similar native range. However, based on the results of this study, we believe future studies investigating rootstock heat tolerance should be conducted on clonal material rather than open pollinated seed stock, thereby eliminating the possibility of seedling variability. Genetic variability within geographic regions in a provenance test of flowering dogwood seedlings evaluating susceptibility to dogwood anthracnose has been shown by others. Notable differences between two provenances from Oklahoma and between two provenances from Missouri indicated genetics of the parent tree to be more important than location where seed were collected. Therefore, even though minimal differences were found in this study, repeated evaluations from sibling trees in close proximity may reveal greater or lesser variation among seedlings. In addition, future studies evaluating root thermotolerance for seedlings from a wider range of USDA Zones, including zones 5 to 9, would be advantageous.