Vernalization and Devernalization of Campanula ‘Birch Hybrid’ and Leucanthemum ×superbum ‘Snow Cap’

The infl uence of vernalization temperature and duration and devernalization treatments on subsequent growth and fl ower development of Campanula ‘Birch Hybrid’ and Leucanthemum ×superbum Bergman ex J. Ingram ‘Snow Cap’ was determined. In the vernalization experiment, plants of ‘Birch Hybrid’ were vernalized at 0, 2.5, 5, 7.5, or 10 °C for 2, 4, 6, or 8 weeks. ‘Snow Cap’ was vernalized at 0, 2.5, 5, or 10 °C for 2, 4, 6, or 8 weeks. In another devernalization experiment, plants of both species were moved to a high temperature (30/10 °C, day/night) growth chamber for 2 or 4 days at various times during or after the 6-week vernalization period. A 6-week vernalization was necessary to obtain 100% fl owering in ‘Birch Hybrid’, and 8 weeks of vernalization decreased time to fl ower by 7 to 10 days compared with 6-week vernalization. Exposure to high temperature for 2 days during or immediately after vernalization did not devernalize ‘Birch Hybrid’ plants, while a 4-day exposure decreased fl owering percentage in some treatments and delayed fl owering by 7 to 10 days. There were no signifi cant differences in fl owering characteristics of ‘Snow Cap’ plants vernalized at 0 to 5 °C for 4 to 8 weeks. A 2-week vernalization at 0, 2.5, 5, or 10 °C or 4 to 8 week vernalization at 10 °C delayed fl owering by 5 to 10 days compared with those vernalized at 0 to 5 °C for 4 to 8 weeks. Exposure to high temperature for 2 d did not devernalize ‘Snow Cap’ plants regardless of exposure times, but a 4-day exposure delayed fl owering by 4 to 5 days in some treatments. Combined, the data indicate that ‘Birch Hybrid’ has an obligate 6-week vernalization requirement and ‘Snow Cap’ has a facultative 4-week vernalization requirement that can be fulfi lled in the 0 to 10 °C range. Exposure to temperatures of 30 °C (9 h·d–1) for 12 out of 42 days did not devernalize either species but in some cases caused a small delay in time to fl ower. Herbaceous perennial sales and demands for vernalized plants throughout the year are expected to increase. Flowering at the time of sale is an important marketing concern, because many consumers prefer to purchase plants in fl ower. Therefore, producers are interested in understanding fl ower induction requirements and how to bring herbaceous perennials into fl ower at scheduled dates. Many herbaceous perennials require a certain period of cold treatment or vernalization for fl owering. Vernalization is defi ned as a cold treatment that promotes subsequent fl owering when given to imbibed seeds, bulbs, or whole plants (Vince-Prue, 1975). The effectiveness of vernalization is governed by both genetic and environmental factors (Napp-Zinn, 1987). Devernalization is a complete or partial reversal of vernalization (Lang, 1965). It results from an interaction between the degree of vernalization and environmental conditions during or immediately after the vernalization. High temperatures (>25 °C) during or immediately after vernalization may promote devernalization. Some Materials and Methods Plant materials and culture. Vegetative cuttings were taken from stock plants grown under short days (9 h·d) of C. ‘Birch Hybrid’ and L. ×superbum ‘Snow Cap’ and were rooted in 72-cell trays (50-mL cell volume) for 3 weeks in a propagation house at air and medium temperatures of ≈23 and 26 °C. Rooted cuttings were then grown in a glass greenhouse under a 12-h photoperiod at 20 ± 2 °C and a daily light integral of about 5 mol·m·d (natural sunlight) for 3 weeks from January to early February 2001 before the treatment. Photoperiod was maintained by supplemental lighting from 5:00 to 8:00 PM using incandescent light and by pulling black cloth at 5:00 PM and opening at 8:00 AM. A peat-perlite mix (Sure-Mix, Michigan Grower Products, Galesburg, Mich.) was used for rooting and potting media. Plants were fertilized at every irrigation with a nutrient solution of well water [electrical conductivity (EC) of 0.70 d·Sm and 105, 35, and 85 mg·L Ca, Mg, and S, respectively] acidifi ed with H 2 SO 4 to a titratable CaCO 3 alkalinity of 130 mg·L and water soluble fertilizer composed of 125N−12P−125K−13Ca (mg·L) plus 1.0Fe–0.5Mn–0.5Zn–0.5Cu–0.1B–0.1Mo (mg·L; MSU Special, Greencare Fertilizers, Chicago, Ill.). Vernalization. Plants of ‘Birch Hybrid’ in 72-cell trays were placed in coolers set at 0, 2.5, 5, 7.5, or 10 °C for 2, 4, 6, or 8 weeks under 5 to 10 μmol·m·s photosynthetic photon fl ux provided by fl uorescent lamps for 9 h·d. For ‘Snow Cap’, vernalization temperature was 0, 2.5, 5, or 10 °C, since a 7.5 °C cooler was not available when this species was vernalized. The experimental design was a two-way factorial arrangement of treatments in a randomized complete block design. Devernalization. Plants of both species in 72-cell trays were placed in a cooler set at 5 °C under 5 to 10 μmol·m·s provided by fl uorescent lamps for 9 h·d for 6 weeks. During the 6-week vernalization, plants were moved to a high-temperature growth chamber at the end of the second, fourth, or sixth week for 2 or 4 d. Therefore, plants were moved to a high temperature growth chamber for 2 or 4 d for once (week 2; week 4; week 6), twice (week 2, 4; week 2, 6; week 4, 6), or three times (week 2, 4, 6). The growth chamber was set at temperature of 30/10 °C (day/night), a photoperiod of 9 h d, and a PPF of 120 μmol·m·s provided from cool-white fl uorescent lamps during the day. The control plants were vernalized at 5 °C continuously for 6 weeks without being exposed to high temperatures. While in the coolers or growth chamber in all experiments, plants were irrigated as necessary with well water (CaCO 3 at 340 mg·L) acidifi ed (93% H 2 SO 4 ) to a titratable alkalinity of CaCO 3 at 100 mg·L. There were 10 plants in each treatment in both experiments. Plants were potted to 13-cm square plastic (1.1-L volume) containers before being placed in greenhouses after vernalization. The temperature in the greenhouse after vernalization was maintained at 20 ± 2 °C. Photoperiod (16 h·d) and supplemental lighting were provided daily herbaceous perennial producers vernalize their plants in simple growing structures, relying on the naturally cold conditions of winter. In this case, plants may experience warm temperatures (>25 °C) on sunny days, particularly in late winter or early spring. Devernalization by high temperature has been reported in some cold-requiring plants, such as Daucus carota L. (Hiller and Kelly, 1979) and Apium graveolens L. (Booij and Meurs, 1993). Exposure to high temperature (30 °C) before or after vernalization delayed fl owering in a range of genotypes of spring rape (Brassica napus var. nanura L.) and 4 d of exposure had more effect than that of 2 d (Dahanayake and Galwey, 1998). However, information on devernalization based on experimental data is limited in herbaceous perennials. The objective of this study was to better understand the processes of vernalization and devernalization in order to ensure uniform fl owering of herbaceous perennials. Specifi cally, we aimed to characterize the effect of a range of vernalization temperatures and durations on subsequent growth and fl owering of Campanula ‘Birch hybrid’ and Leucanthmum ×superbum ‘Snow Cap’. We also aimed to examine if these species would be devernalized by exposing the plants to high temperatures for certain periods of time during or immediately after vernalization. HORTSCIENCE 39(7):1647–1649. 2004. Received for publication 12 Nov. 2003. Accepted for publication 12 Apr. 2004. Current address: Agriculture Research and Extension Center at El Paso, Texas A&M University, 1380 A&M Circle, El Paso, TX 79927. Author to whom correspondence should be addressed; e-mail [email protected]. 136-Prod.indd 1647 10/14/04 11:05:23 AM HORTSCIENCE VOL. 39(7) DECEMBER 2004 1648 by HPS lamps between 0600 to 2200 HR at a PPF of about 65 μmol·m·s at plant canopy level when PPF in the greenhouse was lower than 400 μmol·m·s. Data collection and analysis. Dates of visible bud and fi rst fl ower were recorded for both species in both experiments. For ‘Birch Hybrid’, number of fl ower buds per shoot and number of reproductive or fl owering shoots per container were recorded when plants fi rst fl owered. For ‘Snow Cap’, number of fl ower buds per container and height at fl ower were recorded. Statistical Analysis System’s (SAS Institute, Cary, N.C.) PROC MEANS was used to calculate the means and their 95% confi dence intervals. PROC general linear model was used to test the signifi cance or contrast among the treatments (control vs. 2or 4-d exposure and exposure once versus twice).