ACCURACY OF EQUATIONS PREDICTING THE PHYLLOCHRON OF WHEAT

Predicting the rate of leaf appearance, or phyllochron, aids in unders tanding and modeling grass development and growth. Nine equations pred icting the phyllochron of wheat (Triticum aestivum L.) were evaluated using field data from a variety of locations, cultivars, and managemen t practices. Each equation is referred to by the last name of the firs t author; if there is more than one equation by the first author, addi tional descriptors were included. The BAKER and KIRBY equations predic t the phyllochron based on changes in daylength following seedling eme rgence; CAO-TEMP and CAO-DAY use a curvilinear relationship with tempe rature and daylength, respectively; CAO-T&D uses the ratio of temperat ure to daylength; VOLK mathematically refines CAO-T&D; MIGLIETTA uses an ontogenetic decline in the rate of leaf appearance; and MIGLIETTA-D AY adds photoperiod effects to MIGLIETTA. No equation adequately predi cted the phyllochron. The r(2) values between predicted and measured p hyllochron for winter wheat and spring wheat cultivars, respectively, were BAKER (0.001, 0.486), KIRBY (0.002, 0.487), CAO-DAY (0.000, 0.174 ), MIGLIETTA-DAY (0.013, 0.008), MIGLIETTA (0.002, 0.405), CAO-TEMP (0 .100, 0.190), CAO-FIELD (0.078, 0.036), CAO-T&D (0.066, 0.030), and VO LK (0.119, 0.043). All equations predicted the phyllochron for spring wheat cultivars better than winter wheat cultivars. BAKER and MIGLIETT A showed no bias towards either over or underestimating the phyllochro n; KIRBY tended to overestimate the phyllochron; and the remaining equ ations were biased towards underestimating the phyllochron. Equations developed from field data had the greatest range of predicted phylloch rons. Based on multiple criteria, the BAKER equation best predicted th e phyllochron for the experimental data set. Other factors must be add ed to the equations to improve predictions. Much opportunity exists to improve our ability to predict the phyllochron.