Phyllochron Change in Winter Wheat with Planting Date and Environmental Changes
The phyllochron (degree-days/leaf) of wheat (Triticum aestivum L.) differs with planting dates and latitudes. An understanding of the causes for these differences is needed to model the development of wheat crop canopies. Stephens and Yamhill winter wheat cultivars were planted on a range of dates in outdoor pots and in field plantings to study the relationship of phyllochron to the daylength and mean air temperature at seedling emergence. Both cultivars had a nearly constant phyllochron for a given planting in both pot and field plantings until 600 cumulated degree-days (CDD), which was about the time of double ridge formation. Phyllochron values were different, however, for the different planting dates. At about 600 CDD the phyllochron increased in 18 of 20 cultivar-planting date combinations. Fifty to 60% of the variation in phyllochron prior to double ridge formation could be normalized by either the thermo/photo ratio at seedling emergence (mean temperature to photoperiod) or by the rate of change of photoperiod. These two methods have been suggested previously to normalize phyllochron for differences due to different planting dates. After 600 CDD the phyllochron was not related to thermo/photo ratio, the rate of change of photoperiod, maximum or minimum temperatures, photoperiod or its inverse, or the intensity of solar radiation. Thus, use of a constant value for phyllochron in dynamic crop models of wheat leaf development is not appropriate for all circumstances.
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