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over time (°C·d −1 ), a simple regression analysis was conducted in which LT 10 was the dependent variable and the number of days after the branch segments were placed in the chamber (DAC) was the independent variable. Degree-day calculation. A similar

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crops, the most common approach used for harvest scheduling is based on the relation of harvest date with accumulated degree days often in combination with other factors ( Everaarts, 1999 ; Perry et al., 1997 ). Well-characterized degree day

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The objective of this study was to determine the most advantageous time to collect cuttings of Chinese pistache, a commonly recommended ornamental shade tree that is difficult to propagate by cuttings. In 1993, calendar date and degree days (daily mean temperature -7.2C) were used to estimate an appropriate cutting time. The greatest percentage of rooted cuttings occurred in male cuttings harvested on 13 May 1993 (397 degree days) and treated with 17,500 mg·liter-1 IBA or in male cuttings harvested on 20 May 1993 (482 degree days) and treated with either 8750 or 17,500 mg·liter-1 IBA. In 1994, cutting time was associated with calendar days, degree days, and morphology. The most rooted cuttings (44%) were from green softwood cuttings taken on 9 May 1994, which was 380 degree days from orange budbreak using a threshold temperature of 7.2C. Orange budbreak was characterized by separation of the outer bud scales such that the orange, pubescent inner bud scales were visible. Cuttings taken on 9 May 1994 and treated with 8750 mg·liter-1 IBA produced the most primary and secondary roots and the longest primary roots per cutting. Male Chinese pistache cuttings should be collected from green softwood or red semi-softwood stems when about 380 to 573 degree days have accumulated after orange budbreak. Chemical names used: indolebutyric acid (IBA).

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seedling growth rates vary with temperature. It would, therefore, be interesting to assess the accuracy of degree-days, a common indicator of plant phenology, to predict seedling development ( Bonhomme, 2000 ; Brisson et al., 2003 ; Jones et al., 2003

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Various instruments and contract services can be used to calculate degree-days. This study compared instruments and services to the Wescor Biophenometer, an instrument used by cooperators of the Southeast Pennsylvania IPM Research Group (SE PA IPM RG) throughout Delaware and southeastern Pennsylvania for 10 years. Instruments evaluated in the study were the Wescor Biophenometer Datalogger, Avatel HarvestGuard, Avatel Datascribe Junior, Davis Weather Monitor II, Accu-Trax, and the HOBO H8 Pro Temperature Data Logger. The services were SkyBit and national weather data. Different combinations of instruments and services were used at three locations in Pennsylvania and four locations in Delaware over a 2-year period. We checked the degree-day accumulation of each instrument and service weekly and made statistical comparisons among the instruments and services at each site. To further construct a comparison of the instruments, we noted distinctive qualities of each instrument, interviewed the manufacturers, and received feedback from SE PA IPM RG members who used the instruments. We evaluated the instruments' algorithms, durability, cost, temperature sampling interval, ease of use, time input required by the user, and other distinctive factors. Statistically, there were no significant differences in degree-day accumulations between the Biophenometer, Harvest-Guard, Datascribe, Weather Monitor II, Skybit, or weather service data. However, cost and time required to access/interpret data and personal preference should be major considerations in choosing an instrument or service to measure degree-days.

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Carrot (Daucus carota) L.) seed quality is affected by the environment in which it matures. Substantial differences in germination from year to year and from field to field have been recognized for many years for umbelliferae seed. Part of the explanation for low germination appears to be the harvest of immature seed. Data was collected for two years, from fields of the cultivars Chantenay and Nantes. Approximately 550 growing degree days were accumulated from anthesis until maturity for seed from the primary umbel. Growing degree days were calculated using a 10°C base temperature and without truncating for temperatures in excess of 35°C. Secondary, tertiary, and quaternary umbel seed maturity sequentially followed primary umbel seed. Secondary and tertiary umbels produced approximately 80 percent of the total seed yield while the primary and quaternary umbels produced approximately 20 percent. Seed maturity was determined by measuring the germination rate. Immature seed germinate at a slower rate than mature seed. The implications of these results for obtaining high quality carrot seed will be discussed.

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The relationship of leaf area and stem weight, with stem length at different times during the year, was studied on 5 apple (Malus domestica Borkh.) cultivars. Leaf area was related linearly to stem length, but the slope differed by up to 2-times between cultivars. Stem dry weight increased curvilinearly with respect to stem length and showed less variation among cultivars than did leaf area. These 2 relationships combined indicated that the stem constitutes an increasing proportion of the total shoot weight with increasing stem length. Dormant stems had a greater weight per unit length than stems of growing shoots. Both stem length and leaf area showed strong fitting linear relationships with accumulated growing degree-days, but the stem length relationship showed less variability among cultivars than did leaf area.

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The phellogen of cultivated apple, sweet cherry, and peach trees was wounded at regular intervals beginning in early May and ending in late Aug. 1983. Bark tissue supporting the wounds was excised, sectioned, treated with phloroglucinol + HCl, and examined under a bright field to determine the extent of lignification. The same sections were examined under ultraviolet epi-illumination to determine the extent of suberin deposition in the boundary zone tissue formed from cells present at the time of wounding. Mean daily temperature, time post-wounding, and accumulated degree days (base = −5°, 0°, 5°, and 10°C) were used to predict the percentage of wounds lignified and suberized. A segmented quadratic equation incorporating accumulated degree days (base = 0°) was the best model for predicting lignification for the 3 species and for suberin deposition in peach and sweet cherry.

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Abstract

Base temperature (temperature at which growth begins) and thermal time required for fruit development (degree-days needed from full bloom to commercial harvest) were calculated for 5 low-chilling peach and nectarine [Prunus persica (L.) Batsch] cultivars grown in different climatic conditions. Base temperatures ranged from 2.5° to 4.4°C, and thermal time from 1028 to 1432 degree-days when the calculated base temperatures were used. Use of the calculated base temperatures rather than 10° base temperature reduced variability in predicting thermal time for fruit development.

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A modification of the chilling and heating model for pecan budbreak was used to describe the interactive effects of chilling and heating on the date of first entry of the pecan nut casebearer (PNC; Acrobasis nuxvorella Neunzig) into the pecan [Carya illinoinensis (Wangenh.) K. Koch] fruit. Selected data from unpublished and published sources were used to construct the model. Base temperatures of 9.4 and 13.9C for chilling and heating, respectively, provided the best fit (r 2 = 0.981) for the model used to predict PNC activity. An inverse relationship [1/Y = 0.0037259(1 – 0.1e–0.0028069x – 574.9638969)] was found between chilling (1 Dec. through February) and heating (beginning 1 Feb.) degree-days accumulated until entry of first-generation PNC into the pecan fruit. This model can be used to predict entry of first-generation PNC larvae into fruit over a range of geographic and climatic conditions and pecan genotypes. Model validation using 1994 data from two sites in Texas suggests precision is sufficient to use the model as a guide in managing nut casebearer control.

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