447 Predicting Development of Lilium (Asiatic Hybrid) using Thermal Units

Abstract

The objective was to develop a Thermal Units model to be used as a timing tool for two cultivars of Asiatic lilies: `Butter Pixie' and `Horizon'. Two-hundred-eighty-six plants of each cultivar were evaluated over a 2.5-year period. Environmental data were collected using thermocouples and data-loggers. Developmental events observed daily were: shoot visible out of the soil (VS), visible flower bud (VB), and open flower (OF). Rates of development (the inverse of the numbers of days to complete a given phenophase) increased with temperature up to a point. Thereafter, as temperature continued to increase, rate of development either slowed or declined. A piece-wise linear regression change point model was fitted to each data-set using S+ statistical package. This allowed the determination of the base temperature (Tb), optimal temperature (To), and the point of inflection (Ti). Tbs for the phenophase VS:OF of `Butter Pixie' and `Horizon' were – 0.4 °C and 3.0 °C, respectively. The resulting discontinuity of data prompted improvements in the thermal unit calculation formula. Using the new formula, thermal units were calculated. 1,102 °Cd and 833 °Cd had to be accumulated to complete the phenophase VS:OF for `Butter Pixie' and `Horizon', respectively. Predicted date of events were calculated and compared with the observed values. Subdividing the phenophase VS:OF into two (VS:VB and VB:OF) and using their respective Tbs and thermal units requirements reduced the error of prediction to 1.87 d from 2.13 d for `Butter Pixie' and to 1.86 d from 2.39 d for `Horizon'.