Chilling and Heat Unit Levels Affect Foliar Budbreak of Selected Red and Freeman Maple Cultivars

MATERIALS AND METHODS

Red maple cultivars October Glory^® and ‘Franksred’ (Red Sunset^®) and Freeman maple cultivar ‘Autumn Fantasy’ from tissue culture origin were obtained as 1.5-m (5-ft) tall bare-root whips from A. McGill & Son Nursery, Canby, Oregon, U.S., in February 1999. Trees were potted into 22-L (7-gal) containers using a 6:1 pinebark:sand substrate amended with 3 kg/m³ (5 lb/yd³) dolomitic limestone, 0.9 kg/m³ (1.5 lb/yd³) Micromax^® (O.M. Scotts Co., Marysville, OH), and 6.3 kg/m³ (11.1 lb/yd³) 18-6-12 Osmocote^® (O.M. Scotts Co.). Trees were grown outdoors with overhead irrigation using standard nursery practices at the Paterson Greenhouse Complex, Auburn, Alabama (32° 36’N x 85° 29’W, USDA Hardiness Zone 8a).

This study included six levels of chilling applied in increments of 200 hours (200 to 1,200), with each treatment consisting of three single-tree replications per cultivar. Chilling hours were calculated using the Old 45 Chilling Model (Powell et al. 1999). In this model, beginning October 1, one hour at temperatures below 7°C (45°F) equals one chilling hour. On accumulation of 200 hours of natural, ambient chilling on December 14,1999, the first treatment was placed in a standard glass greenhouse maintained at a minimum temperature of 22°C (72°F) under natural photoperiods. Trees were positioned in a completely randomized design (CRD). Subsequent treatments were placed in the greenhouse after intervals of 200 chilling hours were accumulated. Treatments 1 through 4 (200 to 800 hours) accumulated chilling under natural conditions. Treatments 5 and 6 (1,000 to 1,200) accumulated 925 hours of natural chilling; the remainder supplied while they were stored at 3°C (38°F) in a thermostatically controlled cooling unit (Funchess Hall, Auburn University, AL). Trees were weeded and watered by hand as needed.

After placement in the greenhouse, trees were monitored twice weekly for foliar budbreak until termination of the experiment on April 7, 2000. The total number of buds were counted for the terminal 30 cm (12 in.) of all branches. Budbreak was considered to be the point where overlapping bud scales began to separate enough to reveal leaves. The highest percentage of budbreak was recorded for each cultivar by April 7, 2000, and was used as the highest percentage attainable. Predicted heat unit values required to reach a percentage of budbreak for different chilling treatments, where one heat unit equals one hour in the greenhouse at a minimum base temperature of 22°C (72°F), were determined using regression analysis, using the SAS stepwise procedure to determine the best model for each cultivar (SAS 1996).

RESULTS AND DISCUSSION

In all cultivars, increasing the level of chilling accelerated the rate of foliar budbreak. Differences in the number of heat units required to reach budbreak at every chilling level were determined for each cultivar (Table 1). The level of chilling exposure required for foliar budbreak was inversely related to heat unit accumulation. By extending the chilling duration, fewer heat units were required to produce budbreak in the terminal 30 cm (12 in.) of each cultivar. The higher chilling treatments also generally exhibited the highest mean percentage budbreak over the course of the experiment. October Glory had a greater minimum chilling hour requirement to initiate budbreak than ‘Autumn Fantasy’ and ‘Franksred’ (Red Sunset). However, once optimal chilling had been satisfied (near 600 hours) October Glory required fewer heat units than the other cultivars to reach 20% and 25% budbreak. For example, estimates for heat units required to reach 25% budbreak for October Glory were less than half that required for ‘Autumn Fantasy’ and ‘Franksred’ following 1,200 hours of chilling. Chilling was a determining factor in foliar budbreak for all cultivars, and with all three cultivars, increasing chilling produced greater budbreak percentages and the rate at which budbreak was initiated.

All cultivars required fewer heat units for budbreak as chilling increased. All cultivars required over 1,700 heat units to initiate budbreak after 200 hours of chilling, but only 780 to 970 heat units after 600 hours and between 200 and 400 following 1,000 hours of chilling. Calculated r² values indicate the models provided a good fit to the data for all cultivars.

These observations were similar to those recorded by Ashby et al. (1991) and Couvillon and Erez (1985). It may be concluded that growers who modify lifting and transplanting schedules based on chilling accumulation could accelerate production of these cultivars. Tissue-cultured plantlets and rooted cuttings could be produced at a faster rate by alternating cold storage with greenhouse growing conditions (Wood and Hanover 1981; Sorenson et al. 1994). Also, the selections evaluated would be suitable landscape choices based on the amount of chilling received for much of the United States.

The interaction of chilling and subsequent heat necessary to release a plant from dormancy is a complex problem to understand. At first glance, an earlier study by Townsend (1977) appears to indicate that some southern red maple progenies have longer chilling requirements than more northern progenies. However, careful study indicates that the chilling levels received by all progenies prior to budbreak would have been the same. What varied was the subsequent heat units necessary to release the different progenies from dormancy. Our study provides good evidence of the importance of heat unit accumulation on budbreak of red maples following satisfactory chilling.

More research will be needed to develop regional planting models. The processes that lead to dormancy and budbreak within a plant consist of many interacting factors (temperature, light, physiological and chronological age of plant, apical dominance, provenance, hormonal balances, environmental conditions, drought, fertility, etc.). These factors as related to chilling and heat must be studied further to present a more accurate picture of specific chilling requirements in individual cultivars. Finally, one of the most critical concerns yet to be addressed is a determination of the optimal temperatures to break dormancy. The study presented here assumed ambient temperatures below 7°C (45°F) and a constant 3°C (38°F) when applied in a cooler as adequate to accomplish chilling, and that maintaining the greenhouse environment above 22°C (72°F) was ideal for flushing. Perhaps lower or higher temperatures could be considered more effective for breaking dormancy. Also, differences between constant versus fluctuating temperatures in a natural or simulated environment merit additional study.