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Juvenility and flowering requirements of Chrysanthemum ×superbum Bergmans ex J. Ingram ‘G. Marconi’ and ‘Snow Lady’ were examined by growing plants under short days (SDS) and transferring them to long days (LDs) upon reaching specific true leaf stages. ‘G. Marconi’ plants did not flower in continuous SDS and only sparse flowering occurred in plants transferred to LDs. ‘Snow Lady’ plants transferred from SDS to LDs at the cotyledon stage flowered fastest from seeding (75 days) and had the fewest number of main stem leaves and total leaves (9 and 15, respectively) at the time of first flower. Plants moved from SDS to LDs at the 24 true leaf stage flowered 123 days after seeding and averaged 28 leaves on the main stem and 37 total leaves at the time of first flower. To examine apical floral initiation, plants were distributed between SDS and LDs following transplanting and five plants per treatment were sampled each week thereafter. Microscopic examination revealed floral initiation in plants sampled after just 1 week in LDs and, after 5 weeks, macroscopic terminal flower buds were present. Under SDS, apical floral initiation began after 5 weeks, yet, 9 weeks were required for floral initiation in all five plants sampled. A period of juvenility, described as a physiologically based time when a plant is insensitive to conditions that later promote floral initiation, is common in plants. For many photoperiodic herbaceous species, the passage throughjuvenility can be gradual. characterized by a period of increased sensitivity to daylength rather than a total inability to flower (Bernier et al., 1981). Juvenility has been quantified by determining the minimum leaf number required before the apex was capable of initiating floral organs (Purvis, 1934) and when a critical photosynthetic leaf area was formed to sustain flowering and fruiting (Schwabe, 1976; Wareing and Frydman, 1976). However. it is questionable whether foliage plays the only role in floral induction in all plants. Systematic removal of leaves. even to the extreme of leaving only the rosette stem and shoot apex intact. failed to alter the response of Eschscholzia californica to long-day (LD) induction. an indication that a specific node number may affect floral induction (Lyons and Booze-Daniels, 1986). In addition. Lyons and Neale (1983) identified a negative linear relationship between unfolded leaf stage and rapidity of flowering in E. californica. Flowering occurred most rapidly in E. californica (Lyons and Booze-Daniel\, 1986). Coreopsis lanceolata ‘Early Sunrise’ (Damann and Lyons, 1993). Rudbeckia hirta ‘Marmalade’. and Gaillardia pulchella (Bourke, 1990) when 10, 16, 19. and 20 expanded true leaves. respectively. were present at the start of LDs. Flowering status can be a simple. judgmental determination; a plant is visibly flowering or it is not. However, flowering is not a simple process and additional factors need to be considered. For instance, a plant may be induced to flower by two different treatments. flowering profusely under one. while only sparsely under the other. The rate of reproductive development can also be affected by different photoperiods. Due to the complexity of quantifying flowering, several techniques have been used, includr publication 24 June 1994. Accepted for publication 26 Oct. 1994. The ishing this paper was defrayed in part by the payment of page charges. l regulations, this paper thereforc must be hereby marked advertiseto indicate this fact. ress: The Land. EPCOT Center. P.O. Box 10,000, Lake Buena Vista. rofessor: to whom reprint requests should be addressed. OC. HORT. SCI. 120(2):241-245. 1995. ing calculating the percentage of plants flowering. counting the number of unopened reproductive buds or terminal or axillary flowers per plant. recording the rate of development until flowering occurs. and observing apical development microscopically (Metzger, 1988). Chrysanthemum ×superbum (shasta daisy) cultivars are relatively insignificant lo commercial floriculture, yet they have a perceived market potential as evidenced by rising exposure in garden catalogs and perennial flower guide\. Reports concerning the role of photoperiod in C. ×superbum flowering remain inconsistent. Flowering was hastened by increased day length (Kofranek, 1952: Laurie and Poesch. 1932). and individual cultivars were later found to be either obligate LD plants (Griffin and Carpenter, 1964) or in need of a chilling period to induce flowering (Shedron and Weiler, 1982). There is also some confusion about the flowering requirements of individual cultivars, indicating a need for research so that flowering may be better manipulated and scheduled. The objectives of the present study were to quantify the end of juvenility in C. ×superbum G. Marconi and ‘Snow Lady’, describe vegetative and reproductive responses to photoperiod treatments. and document the apical events during floral initiation in C. ×superbum ‘Snow Lady’ grown in continuous short day (SD) and LD conditions. Materials and Methods Seeds were sown in June in a medium of 2 vermiculite: 1 peatmoss (v/v) and placed under intermittent mist and a natural photoperiod al aconstant 19 to 21C day and night. Upon germination. misting ended and all seedlings were moved to SDs held predominantly at 16 to 18C nights with the assistance of fan and pad cooling. SDs were created by covering the plants from 1700 to 0800 HR with 100% lightexclusion, black sateen cloth. LDs werc created by night interruption from 2200 to 0200 HR with 60-W incandescent bulbs strung overhead to provide 3 to 4 μmol·m ·s photosynthetic photon flux density (PPFD) at plant height. Plants were fertilized weekly) during irrigation with 400 mg N/liter from 20N-6.6P-17.6K. Juvenility and flowering requirements of C. ×superbum ‘G. Marconi’ and ‘Snow Lady’. Following germination, plants were transplanted into 500-ml (10-cm diameter) plastic pots containing a mixture of 3 peatmoss : 1 perlite : 1 vermiculite (by volume). Ten plants (replications) were transferred from SDS to LDs upon reaching the following true leaf stages: 0 (cotyledons only), 3, 6, 9, 12, 15, 18, 21, and 24, where a leaf was defined as possessing a fully expanded lamina. Ten plants of each cultivar remained in SDS for the duration of the experiment as controls. Leaf number transfer treatments were assigned randomly at the time of sowing. Data taken at first flower included the number of days clapsed from seeding and start of LDs, number of expanded leaves along the main stem, stem length (from cotyledonary node to the most distal node possessing a visible, reproductive, axillary branch), nonreproductive scape length (from the node where stem length ended to the most distal node possessing a leaf), and leafless scape length (from the node where nonreproductive scape length ended to the point of calyx attachment). First flower was defined as the date when ray florets had expanded perpendicular to the stem. Regression and ANOVA methods were used to analyze the data. Apical examination of floral initiation in C. ×superbum ‘Snow Lady’. Seeds were sown in January. At about four leaves per plant, all plants were transplanted into 500-ml (10-cm diameter) plastic pots containing a mixture of 1 peatmoss : 1 perlite : 1 vermiculite (by volume). Plants were randomly selected and evenly distributed between SDs and LDs at the time of transplanting. Beginning I week after transplanting, five plants (replications) were sampled weekly from LDs and SDs. After all expanded leaves and roots were removed, the shoot apices were fixed in 10% formalin-% 15 acetic acid-%15 alcohol (FAA). The number of expanded leaves removed per plant was recorded. Sampling continued for 12 weeks, at which time macroscopic terminal flower buds were present. Samples were dehydrated using a histomatic tissue processor (model 166; Fisher Scientific, Lexington, Mass.), imbedded in Leaf Number At Transfer to Long Days Fig. 1. The effect of leaf number at transfer to long days on number of days from seeding to flower for Chrysanthemum ×suprbum ‘Snow Lady’, y = 75.4 + 1.97x, r2 = 0.70**. 242 paraffin blocks and sequentially and longitudinally sectioned at 8 μm on a rotary microtome (Spencer Lens Co., Buffalo, N.Y.) until reaching the approximate center of the apex. Tissue was mounted on glass slides, cleared, stained with Mayers Hemalum, and counter-stained with Erythrosin (Sass, 1958). Slides were examined via light microscopy for indications of floral initiation. Results and Discussion Juvenility and flowering requirements of C. ×superbum ‘G. Marconi and ‘Snow Lady’. ‘G. Marconi’ did not flower in SDs and only 14 plants did so in LDs, indicating that the scope of this experiment could not define the juvenile period for this cultivar. The limited flowering was random and could not be attributed to a specific leafnumber treatment. These plants exhibited wide variability in total height, ranging from 40 to 74 cm (data not shown). All ‘Snow Lady’ plants flowered except for those in the 24-leaf stage and the SD-control treatments, which exhibited 70% and 90% flowering, respectively. With SD-control plants deleted from the data set, regression analysis indicated linear relationships between leaf number at transfer to LDs (x) and the number of days to flower (y) from onset of LDs (y = 61.9 – 1.07x, r2 = 0.54**) and number of days from seeding to flower (Fig. 1). Plants transferred to LDs at the cotyledon stage flowered fastest from seeding, 75 days, whereas those transferred to LDs at 24 leaves flowered 123 days after seeding. The end ofjuvenility has been quantified in other species by identifying the leaf number associated with the fastest flowering after the onset of inductive conditions (Bourke, 1990; Damann and Lyons, 1993; Lyons and Booze-Daniels. 1986). For C. ×superbum, however, this technique is inappropriate. In this experiment, plants grown in SDs to 21 leaves required 43 LDs to flower once transferred co

Juvenility and Photoperiodic Flowering Requirements of Chrysanthemum ×superbum `G. Marconi' and `Snow Lady' Grown under Short- and Long-day Conditions