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

thermal time ( Slafer and Savin, 1991 ). Thermal time, defined as the accumulation of daily mean temperature above a base temperature, is widely used in scheduling ornamental crop production. The approach can be more useful for predicting plant development

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D. Scott NeSmith

Different planting dates were used to study the influence of thermal time on leaf appearance rate of four summer squash (Cucurbita pepo L.) cultivars. During the first year (1991), thermal time or growing degree days (GDD) were calculated using a base temperature of 8C and a ceiling temperature of 32C for several planting dates. Leaf numbers per plant were determined every 2 to 3 days. Leaves that were beginning to unfold with a width of 2 cm or greater were included in the counts. The relationship between leaf number and GDD was established from the initial data set, and data from subsequent years were used for model validation. Results indicated that single equation could be used to predict leaf appearance of all four cultivars in response to thermal time. The response of leaf appearance to GDD was curvilinear, with a lag over the first five leaves. After five leaves, the increase in leaf number per plant was linear with increased GDD. Segmented regression with two linear functions also fit the data well. With this approach, leaf 5 was the node, and a separate linear function was used to predict the leaf number below five leaves and above five leaves. The results of this model should prove to be useful in developing a model of leaf area development, and eventually a crop growth model, for summer squash.

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Sonali R. Padhye and Arthur C. Cameron

., 2001 ; Rawson et al., 1998 ; Streck et al., 2003 ; Suzuki and Metzger, 2001 ; Yan and Hunt, 1999 ), thermal time to flower ( Rawson et al., 1998 ), number of reproductive buds at flowering ( Clough et al., 2001 ; Suzuki and Metzger, 2001 ), and

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Andrés Javier Peña Quiñones, Melba Ruth Salazar Gutierrez and Gerrit Hoogenboom

fixedness associated with flower bud development stages by using the thermal time concept to relate the air temperature to the LT. Materials and Methods Field samples Three sweet cherry cultivars ( Prunus avium L.), Bing, Chelan, and Sweet Heart, and two

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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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P. Inglese, G. Barbera and T. La Mantia

Flowers and stems (cladodes) of cactus pear [Opuntia ficus-indica (L.) Mill.] appear simultaneously in spring, and a second vegetative and reproductive flush can be obtained in early summer by completely removing flowers and cladodes of the spring flush at bloom time. The seasonal growth patterns of cactus pear fruits and cladodes were examined in terms of dry-weight accumulation and cladode extension (surface area) to determine if cladodes are competitive sinks during fruit development. Thermal time was calculated in terms of growing degree hours (GDH) accumulated from bud burst until fruit harvest. Fruits of the spring flush had a 25% lower dry weight and a shorter development period than the summer flush fruits, and, particularly, a shorter duration and a lower growth rate at the stage when most of the core development occurred. The duration of the fruit development period was better explained in terms of thermal rather than chronological time. The number of days required to reach commercial harvest maturity changed with the time of bud burst, but the thermal time (40 × 103 GDH) did not. Newly developing cladodes may become competitive sinks for resource allocation during most of fruit growth, as indicated by the cladode's higher absolute growth rate, and the fruit had the highest growth rate during the final swell of the core, corresponding to a consistent reduction in cladode growth rate. Cladode surface area extension in the first flush ceased at the time of summer fruit harvest (20 Aug.), while cladodes continued to increase in dry weight and thickness until the end of the growing season (November), and, eventually, during winter. The growth of fruit and cladodes of the summer flush occurred simultaneously over the course of the season; the cladodes had a similar surface area and a lower (25%) dry-weight accumulation and thickness than did first flush cladodes. The proportion of annual aboveground dry matter allocated to the fruits was 35% for the spring flush and 46% for the summer flush, being similar to harvest increment values reported for other fruit crops, such as peach [Prunus persica (L.) Batsch.]. Summer cladode pruning and fruit thinning should be accomplished early in the season to avoid resource-limited growth conditions that could reduce fruit and cladode growth potential.

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Julie M. Tarara, Paul E. Blom, Bahman Shafii, William J. Price and Mercy A. Olmstead

(i.e., shoot length, number of leaves per shoot, number of clusters per shoot). Linear regressions were performed in SAS (Release 9.1; SAS Institute Inc., Cary, NC) using the REG procedure. Thermal time, expressed as degree-days (DD, °C), was computed

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Keith A. Funnell

Containerized plants of Scadoxus multiflorus subsp. katharinae (Baker) Friis & Nordal were forced to anthesis under three environments of contrasting temperature. Flowering performance, growing degree-days (GDD) requirements for timing of anthesis, and the influence of cold storage (12 °C for 4 weeks) before forcing were evaluated. Total forcing time from the beginning of the experiment until anthesis decreased with warmer forcing environment, ranging between 129 and 86 days. Across all forcing environments, use of GDD was readily able to explain differences in time to anthesis resulting from both cold storage and forcing. Using a base temperature of 5 °C, GDD requirements between beginning of the experiment and anthesis was 1166 ± 124 GDD, emergence of the vegetative shoot and anthesis 1075 ± 118 GDD, and appearance of the tips of the leaf lamina and anthesis 883 ± 91 GDD. Using a base temperature of 11.5 °C, GDD requirements between appearance of tip of the involucre and anthesis was 180 ± 44 GDD, and the whole involucre being visible and anthesis 144 ± 42 GDD. In the next year, validation of the GDD requirements was achieved by subsequently forcing a second population of plants to anthesis for a specific date.

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Douglas G. Bielenberg and Ksenija Gasic

. We used the observed developmental rates at 12, 14, 16, 18, and 20 °C to estimate a thermal time parameter (base temperature) for the development of two different developmental events: floral budbreak in peach [ Prunus persica (L.) Batsch] and seed

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Matthew G. Blanchard and Erik S. Runkle

/b 1 ) and the amount of thermal time (units of degree-days) that were required from VI to flower (°C·d −1 = 1/b 1 ) in each Odontioda clone ( Roberts and Summerfield, 1987 ). Results During Year 1, plants of both clones displayed