Twenty petunia (Petunia ×hybrida) cultivars were grown at 14, 17, and 20 °C to quantify the impact of temperature on time to flowering, flowering and development rates, and crop quality parameters. Increasing temperature increased vegetative development rates and reduced time to flower (TTF) for all cultivars. Linear functions generated to describe the effects of temperature on the flowering rate (1/TTF) revealed considerable variability in the temperature sensitivity of flowering rates across cultivars. The minimum temperature for the rate of progress toward flowering (Tbase) ranged from 0.15 °C for ‘Damask Purple’ to 7.1 °C for ‘Wave Purple’. The crop quality parameters plant height and branch and flower bud number were all influenced by interactions between cultivar and temperature. Plant height at flowering was unaffected by temperature for 13 of the 20 cultivars, whereas the height of five cultivars was lower at 20 °C compared with 14 °C, and two cultivars were shortest at 17 °C. The branch number of six cultivars was lower at 14 °C than at 17 or 20 °C, whereas three cultivars produced more branches at 17 °C compared with 20 °C. The branch number of 11 cultivars was not impacted by temperature. For 11 of the 20 cultivars, the flower bud number was greater at 14 °C than at 20 °C, whereas temperature did not influence the flower bud number for the other nine cultivars. The results of this work could help to improve production efficiency by allowing cultivars to be placed in temperature-response groups based on the temperature sensitivity of flowering time and/or crop quality parameters.
Ryan M. Warner
Flowering and morphology of four Petunia Juss. spp. [P. axillaris (Lam.) Britton et al., P. exserta Stehmann, P. integrifolia (Hook.) Schinz & Thell., and P. ×hybrida Vilm.] were evaluated in response to photoperiod and temperature. Photoperiod responses were evaluated under 9-h short days (SD), 9-h photoperiod plus 4-h night-interruption lighting (NI), or a 16-h photoperiod supplemented with high-pressure sodium lamps (16-h HPS). All species flowered earlier under NI than SD and were classified as facultative (quantitative) long-day plants. Increasing the daily light integral within long-day treatments increased flower bud number for P. axillaris only. In a second experiment, crop timing and quality were evaluated in the temperature range of 14 to 26 °C under 16-h HPS. The rate of progress toward flowering for each species increased as temperature increased from 14 to 26 °C, suggesting the optimal temperature for development is at least 26 °C. The calculated base temperature for progress to flowering varied from 0.1 °C for P. exserta to 5.3 °C for P. integrifolia. Flowering of P. axillaris and P. integrifolia was delayed developmentally (i.e., increased node number below the first flower) at 14 °C and 17 °C or less, respectively, compared with higher temperatures. Petunia axillaris and P. integrifolia flower bud numbers decreased as temperature increased, whereas P. ×hybrida flower bud number was similar at all temperatures. The differences in crop timing and quality traits observed for these species suggest that they may be useful sources of variability for petunia breeding programs.
Ryan M. Warner
Celosia argentea L. var. plumosa Voss. (celosia) is a bedding plant crop that often exhibits premature flowering during commercial production, resulting in plants of unacceptable quality. Celosia is a facultative short-day plant. Understanding the photoperiod-sensitive stages of development is critical for management of photoperiodic crops. Limited inductive photoperiod experiments, in which photoperiodic plants are moved from noninductive to inductive conditions for flowering at varying stages of development and for varying durations before returning to noninductive conditions, were conducted to determine when celosia becomes sensitive to floral-inducing short days and how many photoinductive cycles are necessary for floral induction. Plants became receptive to short days ≈9 to 12 days after seedling emergence (DAE). Between six and nine short photoperiods beginning 9 DAE were sufficient to commit plants to flowering, depending on the cultivar evaluated. Early flowering was highly correlated with reductions in plant quality parameters, including the number of inflorescences produced, the number of lateral branches, and shoot dry weight. By the time plants had developed five nodes, photoperiod no longer impacted time to flower, indicating that celosia remains photoperiod-sensitive for floral induction only from ≈9 to 45 DAE at 20 °C.
Joseph Tychonievich and Ryan M. Warner
The wide diversity in the genus Salvia represents an untapped genetic resource to improve and diversify Salvia grown as floriculture crops. Interspecific hybrids have formed naturally or by chance hybridization of cultivated plants, but the degree to which species are cross-compatible is largely unknown. The crossability of nine Salvia species selected to cover a wide range of the diversity in European and American species was evaluated in a full diallel mating scheme. Overall, crossability of the selected species was low with only five of 72 interspecific cross combinations producing viable seed, whereas all nine species were self-fertile. Successful crosses were mostly within close phylogenetic groupings. The majority of successful crosses were between species with different chromosome numbers, suggesting that chromosome number differences alone are not a major barrier to hybridization in this genus. A Salvia nemorosa × Salvia transslyvanica F2 population exhibited transgressive segregation for several horticulturally important traits, including flower size, plant height, and time to flower. Plant height was correlated positively with flower length, inflorescence branch number, and time to flower. Time to flower was correlated positively with flower length. Individuals with desirable trait combinations were identified within the population.
Aaron E. Walworth and Ryan M. Warner
Freezing tolerance of many plant species increases after exposure to low, nonfreezing temperatures, a process termed cold acclimation. In some species, shortened photoperiods also bring about an increase in freezing tolerance. Within the plant family Solanaceae, species vary widely in cold acclimation ability. The objectives of this work were to examine the effects of low temperature and photoperiod on cold acclimation of Petunia hybrida (Hook.) Vilm. ‘Mitchell’ and to evaluate cold acclimation of several Petunia species by measuring freezing tolerance using an electrolyte leakage assay on leaf tissue discs. Temperature, but not photoperiod, influenced cold acclimation of P. hybrida. Whether grown under long days or short days, nonacclimated plants had an EL50 value (temperature at which 50% of cellular electrolytes are lost) of ≈–2 °C. Plants acclimated by gradual cooling at temperatures of 15 °C, 10 °C, and 3 °C for 7 days each reached an EL50 of ≈–5 °C regardless of photoperiod. Exposure to 3 °C under short days for 1 or 3 weeks resulted in EL50 temperatures of –3.9 and –4.9 °C, respectively. Freezing tolerance of petunia species P. exserta Stehmann, P. integrifolia (Hook.) Schinz & Thell., P. axillaris (Lam.) Britton et al. (USDA accessions 28546 and 28548), and P. hybrida ‘Mitchell’ was similar before cold acclimation, but varied from –5 °C for P. exserta to –8 °C for P. axillaris (accession 28548) after cold acclimation. Our results demonstrate the cold acclimation ability of Petunia spp. and identify wild germplasm sources with potential usefulness for improving freezing tolerance of cultivated petunia.
Ryan M. Warner and John E. Erwin
One-time spray applications [about 6 mL (0.2 fl oz)] of chlormequat chloride [1000 or 2000 mg·L-1 (ppm)], daminozide (2500 or 5000 mg·L-1), paclobutrazol (20 or 40 mg·L-1) and uniconazole (5 or 10 mg·L-1) varied in efficacy in reducing Hibiscus coccineus (Medic.) Walt., H. radiatus Cav., and H. trionum L. (flower-of-an-hour) stem elongation. Chlormequat chloride inhibited stem elongation of all species, with a 2000 mg·L-1 application reducing stem length of H. coccineus, H. radiatus, and H. trionum by 87%, 42%, and 52%, respectively, compared to untreated plants, 28 d after application. Paclobutrazol also inhibited stem elongation of all species. Uniconazole reduced stem elongation of H. coccineus and H. radiatus, but not H. trionum. Daminozide applied at 5000 mg·L-1 reduced H. radiatus stem elongation only. Growth retardants examined in this study did not delay flowering of H. trionum, the only species that flowered during the experiment. (Chemical names used: ancymidol (α-cyclopropyl-α-(4-methoxyphenol)-5-pyrimidinemethonol), chlormequat chloride(2-chloroethyltrimethylammonium chloride), paclobutrazol ((+)-(R*,R*)-beta((4-chlorophenyl)methyl)-alpha-(1,1-dimethyl)-1H-1,2,4-triazol-1-ethanol), daminozide ([butanedioic acid mono(2,2-dimethylhydrazide)], uniconazol-P ((E)-(+)-(s)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)pent-1-ene-3-ol)).
Ryan M. Warner and John E. Erwin
Thirty-six Hibiscus L. species were grown for 20 weeks under three lighting treatments at 15, 20, or 25 ± 1.5 °C air temperature to identify flowering requirements for each species. In addition, species were subjectively evaluated to identify those species with potential ornamental significance based on flower characteristics and plant form. Lighting treatments were 9 hour ambient light (St. Paul, Minn., November to May, 45 °N), ambient light plus a night interruption using incandescent lamps (2 μmol·m-2·s-1; 2200 to 0200 hr), or ambient light plus 24-hour supplemental lighting from high-pressure sodium lamps (100 μmol·m-2·s-1). Five day-neutral, six obligate short-day, six facultative short-day, three obligate long-day, and one facultative long-day species were identified. Fifteen species did not flower. Temperature and lighting treatments interacted to affect leaf number below the first flower and/or flower diameter on some species. Hibiscus acetosella Welw. ex Hiern, H. cisplatinus St.-Hil., H. radiatus Cav., and H. trionum L. were selected as potential new commercially significant ornamental species.
Ryan M. Warner and John. E. Erwin
Flowering of many herbaceous ornamentals is reduced or eliminated under high temperatures. On warm, sunny days, greenhouse growers often cover crops with light-reducing screening materials to reduce air and plant temperature. However, low irradiance can also reduce flowering on many species. To examine the impacts of temperature and irradiance on herbaceous ornamental flowering and to select a model to study high temperature-reduced flowering, Antirrhinum majus L. (snapdragon) `Rocket Rose', Calendula officinalis L. (calendula) `Calypso Orange', Impatiens wallerana Hook.f. (impatiens) `Super Elfin White', Mimulus ×hybridus Hort. ex Siebert & Voss (mimulus) `Mystic Yellow', and Torenia fournieri Linden ex E. Fourn (torenia) `Clown Burgundy' were grown at constant 32 ± 1 °C or 20 ± 1.5 °C under a 16-hour photoperiod with daily light integrals (DLI) of 10.5, 17.5, or 21.8 mol·m-2·d-1. Flower bud number per plant (all flower buds ≥1 mm in length when the first flower opened) of all species was lower at 32 than 20 °C. Reduction in flower bud number per plant at 32 compared to 20 °C varied from 30% (impatiens) to 95% (torenia) under a DLI of 10.5 mol·m-2·d-1. Flower diameter of all species except snapdragon was less at 32 than 20 °C. Decreasing DLI from 21.8 to 10.5 mol·m-2·d-1 decreased flower diameter of all species except snapdragon. Calendula, impatiens, and torenia leaf number below the first flower was greater at 32 than 20 °C, regardless of DLI. Increasing DLI from 10.5 to 17.5 mol·m-2·d-1 increased shoot dry mass gain rate of all species, regardless of temperature. Further increasing DLI from 17.5 to 21.8 mol·m-2·d-1 at 20 °C increased shoot dry mass gain rate of all species except snapdragon and mimulus, indicating that these species may be light saturated below 21.8 mol·m-2·d-1. Under DLIs of 17.5 and 21.8 mol·m-2·d-1 shoot dry mass gain rate was lower at 32 than 20 °C for all species except torenia. Torenia shoot dry mass gain rate was 129 mg·d-1 at 20 °C compared to 252 mg·d-1 at 32 °C under a DLI of 17.5 mol·m-2·d-1. We suggest torenia may be a good model to study the basis for inhibition of flowering under high temperatures as flowering, but not dry mass gain, was reduced at 32 °C.
Emma Bradford, James F. Hancock, and Ryan M. Warner
Strawberry (Fragaria ×ananassa) cultivars vary greatly in the expression of remontancy, or repeat flowering. To more clearly define the roles of temperature and daylength in flowering control of strawberry, the non-remontant cultivar Honeoye, and two remontant genotypes classified as day-neutral, ‘Tribute’ and an elite clone of Fragaria virginiana ssp. virginiana, RH 30, were grown at 14, 17, 20, 23, 26, or 29 °C, under a short (9 h) or long (16 h) photoperiod. Differential flowering responses of genotypes across temperature and photoperiod treatments resulted from variation in 1) the photoperiod-insensitive permissive temperature range for flowering, 2) photoperiodic requirement if temperature exceeded the photoperiod-insensitive range, and 3) the rate of development of axillary meristems. The photoperiod-insensitive temperature range varied from 14 up to 20 °C for ‘Honeoye’, 23 °C for RH 30, and 26 °C for ‘Tribute’. When temperature exceeded the photoperiod-insensitive range, ‘Honeoye’ and RH 30 required short days for flowering, while ‘Tribute’ required long days. Flowering of ‘Honeoye’ lagged behind ‘Tribute’ and RH 30 by about 84 days in treatments where flowering occurred. These results indicate that temperature and floral initiation/development rate strongly influence the expression of remontancy, and that screening genotypes for day neutrality alone is unlikely to result in the development of robustly remontant cultivars.
Wook Oh, Erik S. Runkle, and Ryan M. Warner
Increasing the photosynthetic daily light integral (DLI) during the seedling stage promotes seedling growth and flowering in many bedding plants. Our objective was to determine the impact of increased DLI for different periods during the seedling stage on young plant quality and subsequent growth and development. Seeds of petunia (Petunia ×hybrida Vilm.-Andr. ‘Madness Red’) and pansy (Viola ×wittrockiana Gams. ‘Delta Premium Yellow’) were sown into 288-cell plug trays and placed under a 16-h photoperiod provided by sunlight plus 90 μmol·m−2·s−1 [supplemental lighting (SL)] or 3 μmol·m−2·s−1 [photoperiodic lighting (PL)] from high-pressure sodium lamps when the ambient greenhouse photosynthetic photon flux was less than 400 μmol·m−2·s−1 from 0600 to 2200 hr. Plants were grown at 20 °C under PL or SL for the entire seedling stage or were exposed to SL for one-third or two-thirds of the seedling stage. Seedlings were then transplanted into 10-cm pots and grown until flowering with SL at 20 °C. Shoot dry mass of transplants increased linearly with increasing DLI provided to seedlings in petunia (y = −4.75 + 1.86x, R 2 = 0.76) and pansy (y = −3.94 + 3.47x, R 2 = 0.78) in which y = dry mass (g) and x = DLI (mol·m−2·d−1). SL during the last two-thirds or the entire plug stage increased shoot dry mass and the number of leaves in both species compared with SL during the earlier stage or PL. SL during the last two-thirds or the entire plug stage accelerated flowering, but plants had a lower shoot dry mass and flower bud number at first flowering compared with that in SL during the first third or two-thirds or that in PL. Therefore, SL generally had greater effects on transplant quality and subsequent flowering when provided later in the plug stage than if provided earlier in production.