Research Approaches for Determining Cold Requirements for Forcing and Flowering of Geophytes John M. Dole1 Department of Horticultural Science, Campus Box 7609, North Carolina State University, Raleigh, NC 27695-7609

Many of the numerous geophytic plant spe- Terminology a covering, e.g., FritillariaL. and Lilium. Other cies are commercially important floriculture plant materials are occasionally lumped into crops, including Gladiolus L., Hyacinthus L., Geophytes are plants in which the perennial the term geophyte, such as woody crowns and Iris L., Lilium L., Narcissus L., and Tulipa L. buds are situated below ground on a storage pseudobulbs. Woody crowns, in particular, can Lilium and Tulipa are two of the world·s major organ such as a rhizome, tuber, corm, or bulb. be difficult to differentiate from geophytes and floriculture crops with hundreds of cultivars Rhizomes, such as with Alstroemeria L. are are often cold stored and forced. However, being grown as potted flower plants, fresh cut modified, elongated, underground stems geophytes will be defined in the strictest sense flowers, and garden ornamentals. Geophytes which grow horizontally with well-defined for this discussion. are especially suitable for commercial floricul- nodes. Tubers have nodes marked only by ture production because the storage organs can small buds and are separable into three types: Role of storage organs be harvested, stored, and forced into flower- root and stem tubers and enlarged hypoctyls. ing (programmed). Production time required Root tubers, e.g., Dahlia Cav., have vegetative Storage organs permit plants to survive peri- for forcing is often short because the storage buds only at the apex of the storage organ and ods of unfavorable weather conditions, such as organ provides stored photosynthates for rapid the primary storage tissue is the root. Stem high or low temperatures, drought, or improper growth. Unfortunately, only a few genera have tubers, such as with Solanum tuberosum L., light levels. Consequently, the success of a been extensively studied, including Gladiolus, have buds distributed over the entire surface geophytic species depends on growing rapidly Hyacinthus,Iris,Lilium,Narcissus, and Tulipa. and the primary storage tissue is the stem. En- when environmental conditions are favorable. Hundreds of other species may also have high larged hypocotyls, such as with Cyclamen L., The growth period is often brief and plants commercial potential but remain unstudied. are similar to stem tubers but the primary stor- become dormant when the conditions are not One key factor in the cultivation and possible age tissue has been derived from the hypocotyl. favorable. Geophytic species respond to many commercialization of new geophytes is that Corms, e.g., Gladiolus, are modified stems with environmental signals that determine when to many species have cold requirements that must well-defined nodes and can be differentiated enter or exit dormancy, including temperature, be characterized (Hartsema, 1961). Procedures from rhizomes in that they are typically round, moisture, and photoperiod. For example, with for breaking dormancy are often complex and have a vertical axis of growth and form on top Dahlia hybrids (D. coccinea Cav. x D. pinnata cannot be transferred from one species to the of the previously planted and senescing corm. Cav.) tuberous root formation is induced by next. However, a number of basic patterns have In bulbs the primary storage organ is the swol- photoperiods of 11–12 h or less. The dormant emerged (Table 1). len leaf bases and/or scales (modified leaves), tuberous roots do not immediately resume which are positioned atop a compressed short growth unless they have been exposed to 0 to stem (basal plate). Hippeastrum Herb. is an 10 °C for 6 weeks (Konishi and Inaba, 1967; Received for publication 4 Apr. 2002. Accepted for example of a bulb composed of compressed Moser and Hess, 1968). Geophytes are found publication 4 Sept. 2002. Approved for publication leaf bases and Tulipa and Lilium exemplify in a range of climates from tropical to arctic by the Director, North Carolina Agricultural Experi- bulbs with scales. In addition, bulbs can be and, therefore, differ greatly in response to ment Station. either tunicate, enclosed in dry leaf bases, e.g., temperature. Species such as Tulipa require 1E-mail: [email protected]. Tulipa andHyacinthus, or nontunicate, without exposure to temperatures averaging 5 °C for

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at least 10 to 12 weeks to break dormancy ‘Champion· at 8–9 leaves (Cavins and Dole, (De Hertogh and Gallitano, 1997; Jansen van (Le Nard and De Hertogh, 1993a). In contrast, 2001), the juvenility period for geophytes is Vuuren, 1997; Roh et al., 1995). The plants are Leucocoryne coquimbensis F. Phil. requires typically determined by minimum storage or- subsequently forced in a greenhouse or planted 20 °C for at least 16 weeks (Ohkawa et al., gan circumference (Fortanier, 1973). Juvenility in fields. These studies provided both practical 1998). Some species, e.g., Iris ×hollandica periods range from §1 year and 3 to 4 cm in information for growers and data on which to hort., require a combination of temperatures, circumference for Triteleia Lindl. corms to 4 base further studies. The next step is to refine e.g., 30 °C for at least 15 weeks followed by 6 to 7 years and 6 to 10 cm in circumference the cold treatment process and answer one or to 13 weeks of 9 to 15 °C for dormancy release for Tulipa bulbs. more of the following questions: 1) what is (Hartsema, 1961). Storage organ size. Organ size is important (are) the optimum temperature(s); 2) what is the in that a plant may not be able to produce quality optimum duration; 3) should storage organs be Effects of low treatment on plant flowers, if the storage organ is too small prior unplanted (dry or moist) or planted and moist development to cold treatment. For example, large Liatris during the cold period; and 4) is light required corms produce more flowering stems than during the cold treatment if shoots are pres- The role of low temperatures in the life cycle small corms (Waithaka and Wanjao, 1983). ent? Research can precisely defined the cold of the geophyte varies and plants can be catego- Plants must attain proper photosynthate storage treatment, which may be expressed as degree- rized into three groups: 1) Cold is required for capacity before being subjected to reproduc- hours—the number of hours below a specific growth and development (endodormancy) and tive conditions. In addition, early exposure to temperature. For example, the optimum cold the plant cannot complete its life cycle without reproductive conditions before storage organs period for Lilium longifl orum Thunb. is 1000 a cold period. For example, Tulipa requires reach sufficient size (end of juvenility) may h at temperatures of 2 to 7 °C (Stuart, 1954). a cold treatment for shoot elongation and decrease uniformity of flowering (Cameron The bulbs must be moist to perceive the cold flowering (Le Nard and De Hertogh, 1993a). et al., 1996). temperature and the highest quality plants and 2) Cold is required for continued growth and Pretreatments. For some species, cold flowers are obtained when the bulbs are potted development if dormancy has been induced treatments are required after specific envi- and allowed to form roots prior to the treatment. (endodormancy), however, the plant can flower ronmental stimuli, e.g., high temperatures or Light is not required but is beneficial if shoots and complete its life cycle without a dormancy drought stress, have been completed (Boyle and emerge during the cold treatment. period. For example, exposure to daylengths Stimart, 1987; Hartsema, 1961; Lewis, 1951). Morphological markers. Morphological of 12 h or less induces hypocotyl enlargement Such pretreatments are imposed to either delay markers, such as root and/or shoot growth, and dormancy in tuberous begonias (Lewis, development and allow extended storage of are often used to determine when chilling is 1951) and several weeks of 1 to 5 °C are re- the organs or to accelerate development and sufficient or when a specific cold treatment quired to break dormancy (Haegeman, 1993). allow forcing as quickly as possible after bulb stage has been completed. For example, the However, when tuberous begonias are grown harvest. For example, Iris×hollandicabulbs are temperature is usually dropped from 9 to 5 °C under 14-h daylengths without exposure to a exposed to warm (30°C) temperatures to retard when sufficientTuliparoot growth has occurred cold period, the plants do not develop dormancy further development prior to cool (9–15 °C) and then to 0–2°C when sufficient shoot growth and flower (Lewis, 1951). 3) Cold is not re- temperatures for dormancy release (Hartsema, has occurred (De Hertogh, 1996; Le Nard and quired but prevents growth and development 1961). Heat treatment for retardation allows De Hertogh, 1993a). In this case, root growth and reduces desiccation (ecodormancy). For the iris to be forced year-round for cut flower and shoot growth are used as physiological example, Hippeastrum bulbs do not require a production. markers indicating the completion of specific cold treatment, but bulbs are stored at 5–9 °C Anatomical markers. Many studies have stages of growth during the cold treatment. to delay flower and leaf emergence and allow been based on time after harvest or anatomical Biochemical markers. Studies are being storage and shipping (Boyle and Stimart, 1987; markers, such as size of the apical meristem, conducted to provide accurate indications of Rees, 1985). and anatomical development of floral or vegeta- the end of the cold treatment by biochemical Low temperatures also effect specific tive organs (Le Nard and De Hertogh, 1993a). means (Boonekamp et al., 1990). The levels of aspects of growth, and the optimum tempera- For example, Tulipa bulbs are considered to be various carbohydrates and endogenous plant ture varies with stage of growth. For example, physiologically responsive to a cold treatment growth regulators have been studied in several flower scape development in Allium afl atu- when the apical meristem has reached “Stage species, including Freesia ecklonex Klatt. (Gil- nense B. Fedtsch. bulbs is greatest at 11 °C, G”—the stage at which all floral organs have bertson-Ferriss et al., 1981a, 1981b; Masuda but the greatest root dry weight occurs at 8 to been differentiated and are visible upon dis- and Asahira, 1978), Hyacinthus(Rudnicki and 11 °C (Zimmer and Renken, 1984; Zimmer section of the bulb (Le Nard and De Hertogh, Nowack, 1976), Iris (De Munk and Schipper, et al., 1985). Sixteen weeks of 5 °C is opti- 1993a). 1993), Liatris (Keren-paz et al., 1989), Lilium mum for leaf growth and 20 weeks of 5 °C is Biochemical markers. Several researchers (Lin et al., 1975; Miller and Langhans, 1990; optimum for flower scape elongation. Tulipa have attempted to provide more accurate indi- Ohkawa, 1977; Takayama et al., 1993; Wang bulbs initiate foliage, flowers, and roots at 18 cations of when to commence the cold treatment and Roberts, 1970), Narcissus (Aung et al., °C (Le Nard and De Hertogh, 1993a; Shoub by biochemical means, e.g., specific levels of 1969; Edelbluth and Kaldewey, 1976; Hanks and De Hertogh, 1975). However, flower scape various carbohydrates and endogenous plant et al., 1986; van Staden, 1978), and Tulipa elongation commences after at least 10 to 12 growth regulators (Farooq and Koul, 1983; (Hanks and Rees, 1980; Hobson and Davies, weeks at 0–9 °C. Gilbertson-Ferriss et al., 1981c; Rudnicki and 1977; Rebers et al., 1996; Rietveld et al., 2000; Nowak, 1976). Theoretically, after specific and Terry et al., 1982). A decline in the levels of Commencing cold treatment reliable markers have been discovered, the endogenous gibberellin-like substances in challenge would be to find reliable methods Lilium speciosum rubrum Mast. ex Bak. bulbs Juvenility.Aprimary consideration for flo- for rapidly and accurately testing sufficiently has been associated with dormancy breaking riculturists is to determine when to begin the large numbers of bulbs to allow commercial and shoot elongation (Ohkawa, 1977). When cold treatment. Geophytes must be physiologi- use of the methodology. accurate markers have been established, the cally capable of perceiving the cold treatment. challenge will be to find methods for rapidly Juvenility is the early stage of plant growth Cold treatment and accurately testing sufficiently large num- in which the plant is incapable of flowering bers of bulbs to allow commercial use of the despite being exposed to reproductive condi- Duration. Another key consideration for methodology (Boonekamp et al., 1990). tions. While the minimum number of leaves floriculturists is to determine the length of the (nodes) is commonly used to indicate the end cold treatment. Research on cold treatment of Other considerations for cold treatments of juvenility for many nongeophytic plant spe- geophytes can involve whole plants; the storage cies such as Antirrhinum majus L. at 18–22 organs, either potted or unpotted, are subjected Optimum vs. acceptable conditions. Other leaves (Cockshull, 1985) and Campanula L. to cold temperatures for a range of durations factors must be considered with regard to cold

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