Agronomy Journal Journal of Natural Resources and Life Sciences Education
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Published online 27 April 2005
Published in Agron J 97:746-754 (2005)
DOI: 10.2134/agronj2004.0181
© 2005 American Society of Agronomy
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Applying Thermal Time Scales to Sunflower Development

R. M. Aiken*

Kansas State Univ., NWREC, Colby, KS 67701



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Fig. 1. Computation of 3-h thermal time interval (TTi), used on the optimized developmental response to temperature (OR) algorithm (Eq. [2]), is depicted for conditions where daily temperature minimum (Tmn) is less than base temperature (Tb = 4°C) or daily maximum temperature (Tmx) exceeds the optimum temperature (Topt = 28°C). Eight provisional values (TTi') are computed from daily thermal extremes and a temperature factor (TFi). The effective value for each of the eight TTi is computed according to its relation to Tb, Topt, and the upper temperature limit (Tul = 40°C).

 


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Fig. 2. Daylength, computed from latitude and day of year, is depicted for Colby, KS (39°24'), the northern border of Colorado (41°), Carrington, ND (47° 30'), and Watrous, SK, Canada (51°30'). Photoperiod effects have been demonstrated for daylengths less than 15 h (also depicted).

 


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Fig. 3. Monthly means of daily temperature [average (•), maximum, and minimum] are depicted for April–September from 1999 through 2003 at Colby, KS. Base temperature (Tb = 4°C), optimum temperature (Topt = 28°C), and upper temperature limits [Tul, linear developmental response to temperature (LR) = 34°C; Tul, optimized developmental response to temperature (OR) = 40°C] also are indicated.

 


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Fig. 4. (a) Cumulative precipitation and (b) cumulative pan evaporation are shown for April–September from 1999 through 2003 at Colby, KS. Normal values (1971–2000) are provided for reference.

 


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Fig. 5. Cumulative growing degree days required for hybrid SF 187 to develop from emergence to visual appearance of the floral bud (R1) and to physiological maturity (R9) are shown in relation to daylength. Daylength corresponds to the date of floral bud appearance (R1) for cumulative growing degree days from emergence to R1 and for the presumed date of floral initiation (assumed 295 oCd following emergence) for cumulative growing degree days from emergence to R9. Degree days were calculated by assuming an optimized developmental response to temperature (Eq. [2], Fig. 1). Coefficients for linear regression models depicted here are presented in Table 4.

 


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Fig. 6. Sunflower development [vegetative (PSV) and reproductive (PSR) phenostages] from three to five planting periods are shown, with respect to cumulative growing degree days after emergence (PSV) or to scaled thermal time (PSR), which accounts for photoperiod effects on the duration of reproductive phenostages. Linear and logistic relationships fitted to PSV and PSR, respectively, for the hybrid SF 187 are reproduced for all three cultivars observed. Growing degree days and scaled thermal time (Eq. [5]) were calculated by assuming an optimized developmental response to temperature (Eq. [2], Fig. 1).

 





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