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Richard L. Hassell, Robert J. Dufault and Tyron L. Phillips

Early spring sweet corn (Zea mays var. rugosa) is usually planted in cold soils at sub-optimal temperatures for seed germination. It is important for growers to understand the relationships among temperature, germination, and vigor of sweet corn in order to plan the earliest planting dates that will not significantly reduce plant stand. The objectives of this research were 1) to determine the minimum temperatures to germinate to 75%, (the minimum germination percent for interstate commerce) for 27 new sweet corn su (sugary), se (sugar enhancer), and sh2 (shrunken-2) cultivars; 2) to determine vigor differences among the phenotypes; and 3) to select the most promising se, su, and sh2 cultivars for cold tolerance and vigor for early spring planting. Seeds of each cultivar were placed along a temperature gradient on a thermogradient table, Type 5001 (Seed Processing Holland, Enkhuizen, The Netherlands), and allowed to germinate over a 7-day period. The gradient treatments were [±2 °F (1.1 °C)] 52, 56, 60, 64, 68, 72, 76, 80, 84, and 86 °F (11.1, 13.3, 15.6, 17.8, 20.0, 22.2, 24.4, 26.7, 28.9, and 30.0 °C). Germination data from thermogradient testing were used to determine the minimum temperatures and time required for su, se, and sh2 cultivars to germinate at ≥75%, defined as minimum acceptable germination percent (MAGP); and the minimum temperature to reach the maximum germination rate (MGR) for a cultivar, defined as the ability to germinate to MAGP at the same rate equally at low and high temperatures. Generally, su phenotypes germinated to MAGP within 4 days, with sh2 requiring 6 days, but with se requiring 5 days. We found that within each phenotype, however, cultivars reacted uniquely to temperature. The most vigorous and cold tolerant su cultivars were `NK 199' and `Merit' which germinated to MAGP at 52 °F with `NK 199' more vigorous than `Merit'. The su cultivar `Sweet G-90' was vigorous at warm temperatures, but the least cold tolerant and desirable for planting under cold conditions. Within the se cultivars, `Precious Gem', `July Gold', and `Imaculata' germinated to MAGP at 52 °F with `Precious Gem' requiring 6 days and `July Gold' and `Imaculata' requiring 7 days. `Accord' was the least cold tolerant se cultivar, requiring at least 60 °F for MAGP with a slow MGR, even at warm temperatures. None of the sh2 cultivars reached MAGP within 7 d at 52 °F, as was also observed for certain su and se cultivars.

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Marshall K. Elson, Ronald D. Morse, Dale D. Wolf and David H. Vaughan

High summer temperatures may reduce plant stands of direct-seeded fall broccoli (Brassica oleracea var. italica Plenck). The influence of constant and diurnally alternating temperatures in the range of 5 to 42C on germination and emergence of `Packman' broccoli was evaluated. Germination was defined as protrusion of the radicle from the seedcoat, and emergence as 10 mm elongation of the radicle. The range of constant temperatures from 10 to 30C for 14 days was satisfactory for 90% germination and 75% emergence. However, alternating temperatures extended the acceptable emergence range to 5/17 through 20/32C. Since soil temperatures in warm climates often exceed 20/32C during the summer, high-temperature inhibition of seed germination and seedling emergence is a potentially important factor limiting direct-seeded broccoli stands.

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Richard L. Hassell, Robert J. Dufault and Tyron L. Phillips

Ten triploid and 25 diploid watermelon (Citrullus lanatus) selections were evaluated to determine the temperature range and length of test for which germination index (rate of germination over time) and germination percentages were maximum for expediting vigor and seed testing practices. Temperature interacted with watermelon selection indicating that certain selections germinated faster within specific, but differing temperature ranges. Within 2 days after starting the germination process, 90% of triploid selections and 96% of diploid selections germinated to their greatest level and prolonging germination data collection for one week did not change this relationship. Although optimal temperature ranges may differ among the selections, the one temperature within the range common for all selections evaluated that maximized germination was 85 to 90 °F (29.4 to 32.2 °C) for diploids and 85 °F for triploids.

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Richard L. Hassell, Robert J. Dufault, Tyron Phillips and Teri A. Hale

Seeds of pale coneflower (Echinacea pallida), purple coneflower (Echinacea purpurea), feverfew (Tanacetum parthenium), and valerian (Valeriana officinalis), classified as “old” (1-year-old seed) or “fresh” (seed crop produced in the current year), were germinated at 62, 65, 69, 72, 75, 78, 82, 85, 89, and 92 °F, (16.7, 18.3, 20.6, 22.2, 23.9, 25.6, 27.8, 29.4, 31.6, and 33.3 °C). The optimum germination temperature, defined in this study as the temperature range within which the percent germination is greatest in the shortest period of time, was determined. Old and fresh pale coneflower seed germinated optimally after 5 days at 69 °F. Old purple coneflower seed required 5 d at 78 to 82 °F, but fresh seed germinated optimally after 3 days at 75 °F. Old feverfew germinated optimally after 5 days at 65 °F, but fresh seed germinated to its optimum after 5 days at 69 °F. Old and fresh valerian seed germinated to its optimum after 3 days at 75 °F.

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Isabel Pimentel and Samuel Contreras

light or darkness at a range of temperatures from 12 to 36 °C, provided by a thermogradient table, or growing chambers at constant temperatures of 20 and 25 °C. Germination in dark was performed using aluminum-foil-covered petri dishes. When evaluated in

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Robyn L. Cave, Colin J. Birch, Graeme L. Hammer, John E. Erwin and Margaret E. Johnston

one-way thermogradient plate with 10 insulated chambers (Lindner and May Pty. Ltd., Windsor, Brisbane, Australia). The table surface temperature for each chamber was logged every 10 min using a standard thermistor probe (PB-5001) coupled to a data

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Anne K. Carter and Charles S. Vavrina

The germination of five commercial cultivars of jalapeño and cayenne pepper were tested to determine cultivar response of Capsicum annuum L. to supra-optimal temperatures. Two seedlots of `Cayenne, Large Red Thick', `Ole', `Jalapeño M', `Mitla', and `Tam Veracruz' were evaluated on a thermogradient table at temperatures of 20, 25, 30, 35, and 40 °C. Percent germination and time to 50% of final germination (T50) were calculated. All cultivars exhibited thermodormancy, but the degree of inhibition varied within temperature and cultivar. No cultivar had >1.0% germination at 40 °C. Generally, the T50 varied among cultivars, but not among temperatures within a cultivar (T50 at 40 °C was not measured). Cultivar selection should be considered when growing fall transplants in Florida.

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Yu Sung, D.J. Cantliffe and R.T. Nagata

Lettuce seeds differentially fail to germinate at temperatures above 21C according to genotype. Twenty-one lettuce lines were screened for their ability to germinate at temperatures from 24C to 36C. Four cultivars, `Dark Green Boston', `Valmaine', `Floricos 83', and `PI251245', were selected for this study because of their range of ability to germinate at temperatures above 24C. Seeds of the four cultivars were collected from mother plants grown in growth chambers at 20/10C(day/night temperature), 25/15C, 30/20C and 35/25C. Seeds were germinated on a thermogradient table from 24C to 36C under light (12 h). Seeds from `Floricos 83' produced above 30C had higher germination percentage at 33C and 36C than those produced below 30C temperatures. At 30C germination temperature seeds of `Valmaine' produced above 30C had 98% germination compared to 45% of those produced below 30C. `Dark Green Boston' seeds produced at 35C had higher germination percentage(70%) at 30C than those produced at other temperatures. Seeds collected from the mother plant grown above 30C day temperatures had greater germination than those grown below 30C.

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Erin G. Wilkerson, Richard S. Gates, Sérgio Zolnier, Sharon T. Kester and Robert L. Geneve

Root zone temperature optima for root initiation and root elongation stages for rooting in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch `Freedom Dark Red') cuttings was determined to be 28 and 26 °C, respectively. Threshold temperatures where rooting development was slow (>24 days) or did not occur were ≤20 and ≥32 °C. Time to visible rooting and postemergent root elongation was modeled based on cumulative daily mean root zone temperatures in growth chamber studies using a thermogradient table to provide simultaneous temperatures between 19 to 34 °C. Time to root emergence at different root zone temperatures was best described using a nonlinear growth rate derived mathematical model, while postemergent root elongation up to 100 cm could be described using either a linear thermal time model or a nonlinear equation based on elongation rate. These temperature-based mathematical models were used to predict rooting in six greenhouse experiments. Using a root zone base temperature of 21 °C, observed vs. predicted time to visible root emergence was highly correlated (r 2 = 0.98) with a mean prediction error (MPE) of 1.6 d. Observed vs. predicted root length using the linear thermal time model had a r 2 = 0.69 and an MPE of 14.6 cm, which was comparable to the nonlinear model with an r 2 = 0.82 and an MPE of 14.8 cm.

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A.K. Carter and C.S. Vavrina

Chile (Capsicum annuum L.) seeds sown in July for fall transplants in the greenhouse often yield poor stands due to thermoinhibition. To determine cultivar response to high temperature, five jalapeno cultivars and one cayenne cultivar used commercially in Florida and New Mexico were tested. Two seedlots of `Cayenne, Large Red Thick', `Ole', `Jalapeno M', `Mitla', and `Tam Veracruz', and one seedlot of `Ebano' were evaluated on a thermogradient table at a temperature range of 20, 25, 30, 35, and 40°C. Percent germination, mean daily germination, and germination performance index were calculated. Additional seeds were soaked for 24 hours at 25, 30, 35, and 40°C. The exudate was then measured for electrical conductivity and glucose equivalents as an indication of seed leakage. All cultivars exhibited thermoinhibition, but the critical temperature of onset varied among cultivars. `Ebano' had the highest germination performance index across all temperatures among the six cultivars. At 40°C, however, no cultivar exceeded 4.0% germination. There was significant variation in germination performance between seedlots of some cultivars. The electrical conductivity and the number of glucose equivalents leached from the seed varied among cultivars, but did not correlate with decreased germination. Thermoinhibition in chile is probably due to factors other than those associated with membrane leakage.