Abstract
The effects of chilling on growth and development of ginkgo biloba (Ginkgo biloba L.) seedlings were evaluated in a study initiated on February 1, 1999, and terminated on September 10, 2002. Thirteen levels of chilling were applied to trees in increments of 100 hours, with 12 replications per treatment. Increased chilling led to a decreased heat unit requirement, resulting in greater overall growth and development. Within the group of trees receiving the same treatment for three consecutive years, trees receiving 1,100 chill hours had greater caliper, terminal shoot extension, and total plant height. In this same group, greater limb structure was obtained from trees receiving 600 chill hours. Chilling was a determinate factor in growth and development.
Ginkgo biloba L., often referred to as a living fossil, is a valuable and interesting landscape tree. A monotypic genus, G. biloba is native to China. In North America, Ginkgo is grown from southern Canada to central Florida, and from the East Coast to the West Coast (USDA Hardiness Zones 3–9) (Kwant 2002). Ginkgo biloba can reach a mature height of 24.4 m (80 ft) and a width of 12.2 m (40 ft), with a naturally slow to medium growth rate. Trees are often pyramidal when young and become wide spreading when older. Although a common urban tree, resistant to many insects and diseases, G. biloba grows best in full to partial sun and moist, well-drained soils. More than 40 cultivars, with forms ranging from dwarf to majestic or broad to fastigate, are available for landscape plantings (Dirr 1998).
Dormancy in plants has been described as a state in which visible growth is temporarily suspended (Samish 1954), a phase in a plant’s development allowing it to survive winter conditions (Saure 1985), and a state in which deciduous plants are not in leaf or are lacking visible growth (Westwood 1995). Endodormancy release from within plant parts, as controlled by chilling temperatures, is a major factor in determining a plant’s performance in a given climate or hardiness zone (Westwood 1995). Temperate-zone plants must be exposed to a certain period of chilling temperatures above freezing (Westwood 1995) or a minimum number of hours below 7°C (45°F) (Saure 1985) for dormancy release to occur, with this exposure period referred to as the chilling requirement. Dormancy requirements of landscape trees are of particular interest to the arborist and urban forester. Trees noted to perform well in northern climates, such as flowering cherries, spruce, or beech, may perform poorly or not at all in southern climates. In other cases, trees noted to perform well in the south may leaf out too early in the north, resulting in cold and frost damage (Lechowicz 1984).
Much work has been reported on fruit species with respect to dormancy and chilling requirements. In a study with peaches (Prunus persica (L.) Batsch), once rest was satisfied, prolonged chilling was shown to enhance leafing over blooming (Citadin et al. 2001). There are also cultivar differences in heat requirement for bloom. In a study with several fruit tree species, once rest was satisfied, prolonged chilling led to a decreased need for heat to reach budbreak (Couvillon and Erez 1985).
Some work on the dormancy and chilling requirements of ornamental species has also been done. Increased chill hours in dogwood (Cornus florida L.) leads to fewer days to budbreak, indicating that early lifting and transplanting increases survival rates (Ruter et al. 1994). Ashby et al. (1991) report that terminal buds require more chilling than lateral buds in silver maple (Acer saccharinum L.). The optimal chilling requirements of two red maple (Acer rubrum L.) cultivars and one Freeman red maple (Acer × freemanii E. Murray) cultivar have been determined (Sibley et al. 2001; Wilson et al. 2002a). Also, increasing the duration of chilling accelerates the rate of foliar budbreak and reduces the number of heat units required to initiate foliar budbreak in linden (Tilia spp.) cultivars (Sibley et al. 2001; Wilson et al. 2002b).
Most research on G. biloba has been focused on medicinal purposes (Follett et al. 2001; Juretzek 1997), with little attention given to cultural and environmental requirements. No reports have addressed the dormancy requirements of G. biloba. The objective of this work was to evaluate the overall growth and development responses of G. biloba to incremental chilling.
MATERIALS AND METHODS
Two hundred, 46 cm (18 in.) tall liners of Ginkgo biloba L. seedlings were purchased from Musser Forests, Inc. (Indiana, PA) from a Mississippi seed source, in winter 1999. Trees were initially potted into 3.8 L (#1) containers using a 6:1 pinebark:sand substrate amended with 2 kg m−3 (3.3 lb/yd3) dolomitic limestone, 0.6 kg m−3 (1 lb/yd3) Micromax (The Scotts Co., Marysville, OH), and 4.2 kg m−3 (7.3 lb/yd3) 18N-6P-12K Osmocote (The Scotts Co.). Trees were transplanted to 7.9 L (#2), 13.5 L (#3), and 22 L (#5) containers in 2000, 2001, and 2002, respectively, of the study. Trees were grown outdoors with overhead irrigation at the Paterson Greenhouse Complex, Auburn, Alabama (32° 36′ N × 85° 20′ W, USDA Hardiness Zone 8a).
The 4-year study included 13 treatment levels of chilling applied in increments of 100 hours (0–1,200). Each treatment was applied to four single tree replications at the end of year 1 (1999–2000) for a total of 52 trees. Trees not selected for treatment at the end of year 1 received ambient chilling (998 hours for the season), while remaining on the container pad area. In year 2 (2000–2001), four single-tree replications were added to each treatment for a total of 104 trees. Trees not selected for treatment at the end of year 2 received ambient chilling (1,487 hours for the season), while remaining on the container pad area. In year 3 (2001–2002), four single-tree replications were added to each treatment for a total of 156 trees. Treatment levels were applied to the same replicates each year. Therefore, four replications received the same treatment for 3 years (group one); four replications received the same treatment for 2 years (group two); and four replications received the same treatment for 1 year (group three). Chilling hours were calculated using the Old 45 Chilling Model (Powell et al. 1999), which counts hours below 7°C (45°F) as chilling hours beginning October 1 for each year.
Each September before receiving any chill hours, four trees were randomly selected to receive 0 chill hours and placed in a heated greenhouse maintained at a minimum of 22°C (72°F). In late fall 1999, with an accumulation of 100 and 200 natural, ambient chill hours, four trees were randomly selected to receive 100 and 200 chill hours, respectively, and placed into the greenhouse. After receiving 200 ambient chill hours, the 40 trees to receive 300 to 1,200 chill hours were placed into a thermostatically controlled cooling unit (Funchess Hall, Auburn University, AL) maintained at 3°C (38°F) until the desired chill-hour increments were accumulated. Trees were removed in 100-hour increments and placed into the greenhouse. Trees were arranged in a completely randomized design (CRD), hand-weeded, and watered as needed. The same pattern of treatment management was used in fall 2000 and fall 2001 on the first four replicates (year 1 trees) and followed by the same pattern for year 2 trees and year 3 trees.
After placement in greenhouse each year, trees were evaluated at the end of the annual growth cycle for overall growth and development. Terminal new shoot extension, number of limbs, and average length of each limb were measured. At the end of the summer growing period, trees were measured for total plant height and caliper. Data were analyzed with regression analysis, performed using orthogonal contrast statements in order to prevent masking of significant effects due to multicollinearity (Neter et al. 1996). All analyses were performed with SAS Software (SAS 1999).
RESULTS AND DISCUSSION
This report presents data following four growing seasons. Chilling is a determinate factor in growth and development of ginkgo biloba (Ginkgo biloba L.). As chilling levels were increased, different aspects of growth increased, such as terminal shoot extension, caliper, total plant height, and total limb length.
Caliper
A linear treatment effect in caliper was observed in trees that received 3 consecutive years of forced chilling (group one) (y = 18.0 + 0.0053x, r = 0.39, p = 0.007). Mean caliper ranged from 17 mm (0.7 in.) (300 hours) to 26.7 mm (1.1 in.) (1,100 hours). Trees receiving 1,100 hours had greater caliper than trees receiving 200 to 600 and 1,000 hours. Trees receiving 700, 900, and 1,200 hours had greater caliper than trees receiving 300 and 500 hours (Table 1).
Final caliper of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
A linear treatment effect in caliper was observed in trees that received 1 year of ambient chilling and 2 years of forced chilling (group two) (y = 18.0 + 0.0049x, r = 0.35, p = 0.016). Mean caliper ranged from 16.3 mm (0.65 in.) (100 hours ) to 26.1 mm (0.1 in.) (700 hours). Trees receiving 700 hours had greater caliper than trees receiving 100, 300, and 500 hours. Trees receiving 1,100 hours had greater caliper than trees receiving 100 and 300 hours. Trees receiving 1,200 hours had greater caliper than trees receiving 100 hours (Table 1).
A linear treatment effect in caliper was observed in trees that received 2 years of ambient chilling and 1 year of forced chilling (group three) (y = 20.1 + 0.0039x, r = 0.30, p = 0.036). Mean caliper ranged from 17.5 mm (0.7 in.) (700 hours) to 29.1 mm (1.2 in.) (600 hours). Trees receiving 600 hours had greater caliper than trees receiving 100 to 500 and 700 to 900 hours. Trees receiving 1,100 hours had greater caliper than trees receiving 100, 200, 400, 700, and 800 hours. Trees receiving 1,000 and 1,200 hours had greater caliper than trees receiving 700 hours (Table 1).
A linear treatment effect in caliper was observed when all three groups were combined. While year started was not significant, chill hours were. There was also an interaction between the two. Mean caliper results were similar to those from group one.
Total Plant Height
A linear treatment effect in height was observed in trees that received 3 consecutive years of forced chilling (group one) (y = 166.0 + 0.053x, r = 0.40, p = 0.005). Mean total plant height ranged from 160 cm (63 in.) (300 hours) to 243 cm (95.7 in.) (1,100 hours). Trees receiving 1,100 hours had greater height than trees receiving 300 to 600 hours. Trees receiving 900 and 1,200 hours had greater height than trees receiving 300 hours (Table 2).
Final total plant height of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
A linear treatment effect in height was observed in trees that received 1 year of ambient chilling and 2 years of forced chilling (group two) (y = 166.3 + 0.052x, r = 0.41, p = 0.004). Mean total plant height ranged from 149 cm (58.7 in.) (300 hours) to 248 cm (97.6 in.) (1,100 hours). Trees receiving 700, 1,100 and 1,200 hours had greater height than trees receiving 100 and 300 hours (Table 2).
A linear treatment effect in height was observed in trees that received 2 years of ambient chilling and 1 year of forced chilling (group three) (y = 192.5 + 0.029x, r = 0.34, p = 0.017). Mean total plant height ranged from 186 cm (73.2 in.) (200 hours.) to 253 cm (99.6 in.) (1,100 hours). Trees receiving 1,100 hours had greater height than trees receiving 100, 200, 400, 700, and 800 hours (Table 2).
With all three groups combined, a linear treatment effect in height was observed. While year started was not significant, chill hours were. Mean caliper results were similar to those from groups one through three.
Terminal Shoot Extension (TSE)
There was no statistical significance with terminal shoot extension between the treatments that received 3 consecutive years of forced chilling (group one). Mean terminal shoot extension ranged from 39.3 cm (15.5 in.) (400 hours) to 64.8 cm (25.5 in.) (1,100 hours) (Table 3). A linear treatment effect in TSE was observed in trees that received 1 year of ambient chilling and 2 years of forced chilling (group two) (y = 42.0 + 0.017x, r = 0.30, p = 0.040). Mean terminal shoot extension ranged from 31 cm (12.2 in.) (500 hours) to 71.8 cm (28.3 in.) (1,100 hours). Trees receiving 700 and 1,100 hours had greater TSE than trees receiving 100 and 500 hours. Trees receiving 200 hours had greater TSE than trees receiving 500 hours (Table 3).
Terminal shoot extension of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
There was little difference between the treatments that received 2 years of ambient chilling and 1 year of forced chilling (group three) (y = 55.5 + 0.012x, r = 0.24, p = 0.096). Mean terminal shoot extension ranged from 52.3 cm (20.6 in.) (1,000 hours) to 80.5 cm (31.7 in.) (800 hours). Trees receiving 800 hours had greater TSE than trees receiving 1,000 hours (Table 3). With all three groups combined, a linear treatment effect in TSE was observed. Chill hours and year started were significant, but there was no interaction. Trees started in year 3 had greater TSE than trees started in years 1 and 2. This result could be due to receiving more ambient chilling and or other factors.
Limb Length and Average
A linear treatment effect was observed for total limb length and number of limbs, and a quadratic effect for average limb length, for trees receiving forced chilling for 3 consecutive years (group one). Mean total length ranged from 168 cm (66.1 in.) (200 hours) to 446 cm (175.6 in.) (600 hours). Trees receiving 600, 900, and 1,100 hours had greater total limb length than trees receiving 200 hours (Table 4) (y = 224.3 + 0.122x, r = 0.27, p = 0.062). Mean limb number ranged from 4.5 (200 hours) to 13.3 (600 hours). Trees receiving 600 hours had greater limb numbers than trees receiving 200 and 300 hours. Trees receiving 900 hours had greater limb numbers than trees receiving 200 hours (Table 5) (y = 5.5 + 0.0044x, r = 0.33, p = 0.022). Mean limb length ranged from 29.4 cm (11.6 in.) (600 hours) to 45.3 cm (17.8 in.) (100 hours). Trees receiving 100 hours had greater mean limb length than trees receiving 500 and 600 hours (Table 6).
Total limb length of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
Total limb number of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
Final limb length average of Ginkgo biloba L. following 3 years of forced chilling (group one); 1 year of ambient chilling (998 hours) and 2 years of forced chilling (group two); and 2 years of ambient chilling (998 and 1,487 hours) and 1 year of forced chilling (group three).z
While there was no statistical significance, a trend indicated that trees receiving 1 year of ambient chilling and 2 years of forced chilling had noticeable effects (group two). Mean total limb length ranged from 132 cm (52 in.) (300 hours) to 338 cm (133.1 in.) (800 hours) (Table 4). Mean limb number ranged from 3.5 (300 hours) to 9 (400 hours) (Table 5). Mean limb length ranged from 35.1 cm (13.8 in.) (1,000 hours) to 45 cm (17.7 in.) (600 hours) (Table 6).
A linear treatment effect was observed for total limb length and number of limbs for trees receiving 2 years of ambient chilling and 1 year of forced chilling (group three). Mean total limb length ranged from 147 cm (57.9 in.) (400 hours) to 483 cm (190.2 in.) (1,100 hours). Trees receiving 1,100 and 1,200 hours had greater total limb length than trees receiving 100, 300, 400, 700, and 800 hours (Table 4) (y = 102.0 + 0.279x, r = 0.51, p = 0.001). Mean limb number ranged from 4.3 (400 hours) to 12.3 (1,200 hours). Trees receiving 1,200 hours had greater total limb length than trees receiving 100 to 400 and 700 hours. Trees receiving 1,100 hours had greater total limb length than trees receiving 100, 300, 400, and 700 hours. Trees receiving 900 hours had greater total limb length than trees receiving 100, 400, and 700 hours (Table 5) (y = 2.9 + 0.007x, r = 0.53, p = 0.001). Mean limb length ranged from 22.3 cm (8.8 in.) (800 hours) to 41.8 cm (16.4 in.) (1,100 hours). Trees receiving 200, 1,000, and 1,100 hours had greater mean limb length than trees receiving 800 hours (Table 6).
CONCLUSIONS
Tree growers often must make decisions about which species best fit into their production cycle. Many aspects influence the product-mix decision, most notably whether a species can be produced in a given region in a timely manner. For G. biloba, satisfying dormancy requirements is of great importance for the production process. This study reveals chilling to be a determinate factor in growth and development of G. biloba. Optimal chilling for overall growth and development appears to begin in the 600 to 700 chill hour range. Increasing chill hours results in greater overall growth and development with a decreased need for heat. For growers using greenhouse production methods, adjusting environmental conditions to allow chilling can accelerate and lead to more efficient production. For growers using field production methods, modifying lifting and transplanting schedules based on chilling accumulation can lead to improved growth in the first year for the customer, providing more rapid foliar emergence. It is reasonable to predict that tissue-cultured plantlets and rooted cuttings may be produced at a faster rate by alternating cold storage with greenhouse growing conditions (Sorenson et al. 1984; Wood and Hanover 1981). Although G. biloba is a suitable landscape choice based on the amount of chilling received for much of the United States, this study shows that field production may be better suited to areas receiving 700 or more hours of chilling. This study may also provide a basis for future evaluation of chilling response for G. biloba cultivars.
- © 2004, International Society of Arboriculture. All rights reserved.
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