The Ritchie wheat model is then converted to the Robertson (1968) Biometeorological Time Scale (BMTS) according to the following Ritchie-BMTS conversion table (first two columns), with corresponding approximate GDD accumulations (last two columns):

GDD heat units are calculated each day by averaging minimum and maximum daily temperatures and subtracting a base temperature of 0-degree Celsius (32-degree Fahrenheit). The accumulated GDD heat units drive the Ritchie wheat growth model, but plant growth during the emergence stage can be slowed down depending on the wheat variety and its sensitivity towards photoperiodism and vernalization.

Photoperiodism and vernalization are described below, with additional GDD information presented in the metadata section for the corn growth stage model.

Photoperiodism and Vernalization

Both photoperiod and vernalization influence leaf numbers and plant growth for winter and spring wheat varieties.

Photoperiod is the length of time a plant is exposed to daylight, whereby wheat is a long-day plant, meaning it develops more rapidly and makes fewer leaves during long days. Photoperiod in the wheat growth model is defined as the relative amount that plant development is slowed (and more leaves formed) during a shorter photoperiod than the optimum photoperiod, whereby the optimum photoperiod is assumed as 20 hours of daylight.

Photoperiod types are classified as PS, which require long days for timely flowering; or photoperiod insensitive (PI), which can be grown successfully in both long or short day environments. Photoperiod response is of interest to plant breeders in northern latitudes because new PI cultivars can grow faster, reduce risk of frost damage, and maintain or improve yields relative to PS wheat varieties (Dyck, et al, 2004). In all environments, PI cultivars tend to reduce tillering, reduce plant height, and reduce spikelets per spike, which correspondingly accelerates time to heading and maturity.

Vernalization also influences leaf numbers and it is the wheat plant’s response to relatively cold temperatures that must occur before reproductive growth will begin. Vernalization is assumed to occur at temperatures between 0 and 18 C, with the optimum temperature for vernalization in the range of 0 and 7 C and with temperatures between 7 and 18 C having less influence on the process. For winter wheat, the low temperature requirement for vernalization begins at germination and exposure to relatively low temperatures is required before spikelet formation can begin.

Spring-type winter cereals have little sensitivity to vernalization, which is the principal difference between spring and the winter wheat types. There are also intermediate, or facultative, wheat varieties that have varying degrees of sensitivity to vernalization.

Even though there is a genetic variability in sensitivity to vernalization, 50 vernalization days are assumed to be sufficient to completely vernalize all cultivars. Plant development is slowed (and more leaves formed) for each day of unfulfilled vernalization, whereby 50-days of vernalization is assumed to be sufficient for all cultivars.

Available Wheat Varieties within the Wheat Growth Model

The current winter and spring wheat growth stage models have the following wheat varieties and coefficients:

The proper wheat variety for a region should first be selected based on winter, spring, or facultative wheat varieties grown in the region. For example, farmers plant winter wheat varieties if cold temperatures during the growing season are typically below freezing after planting wheat. In contrast, farmers plant spring wheat varieties if normal temperatures typically remain above freezing throughout the entire growing season. Finally, facultative wheat varieties are typically planted in climates where temperatures may reach below freezing temperatures during some years, but not for all years.

In the past, photoperiod sensitive (PS) cultivars provided good yield stability, local adaptation, and high productivity in northern areas of North America and Europe, with plant breeders developing and providing more PI cultivars in recent years. Hucl (1995) reported that before the mid-1980s virtually all Canada Western Red Spring (CWRS) wheat cultivars were photoperiod sensitive (PS), but currently PI varieties comprise about 20 percent of CWRS wheat (Dyck, et al, 2004). In Europe, Foulkes (2004) reported that most wheat varieties grown in southern and mainland Europe are PI because of their wider adaptation, relative to PS cultivars, while varieties in the UK are mostly PS. Correspondingly, PI cultivars have been shown to have a 30% yield advantage over PS cultivars in southern European regions, 15% in mainland regions, and no yield advantage in the UK (Worland et al., 1994).

References:

Dyck, J.A., Matus-Cadiz, M.A., Hucl, P., Talbert, L., Hunt, T., Dubuc, J.P., Nass, H., Clayton, G., Dobb, J., Quick, J. 2004. Agronomic performance of hard red spring wheat isolines sensitive and insensitive to photoperiod. Crop Science, Nov-Dec, 2004

Foulkes, M.J., R. Sylvester-Bradley, A.J. Worland, and J.W. Snape. 2004. Effects of a photoperiod-response gene Ppd-Dl on yield potential and drought resistance in UK winter wheat. Euphytica 135:63-73.

Hucl, P. 1995. Growth response of four hard red spring wheat cultivars to date of seeding. Can. J. Plant Sci. 75:75-80.

Ritchie, J.T.1991. Wheat phasic development. p. 31-54. In Hanks and Ritchie (ed.) Modeling Plant and Soil Systems. Agronomy Monograph 31, ASA, CSSSA, SSSA, Madison, WI.

Robertson, G.W. 1968. A Biometeorological Time Scale for a Cereal Crop Involving Day and Night Temperatures and Photoperiod. Intern J. Biometeor. 12:191-223.

Worland, A.J. 1996. The influence of flowering time genes on environmental adaptability in European wheats. Euphytica 89:49-57.