QUANTIFYING THE RELATIONSHIP BETWEEN PHASES OF STEM ELONGATION AND FLOWER INITIATION IN POINSETTIA

Stem elongation of poinsettia (Euphorbia pulcherrima Klotz.) was quant ified using an approach that explicitly modelled the three phases of a sigmoidal growth curve: 1) an initial lag phase characterized by an e xponentially increasing stem length, 2) a phase in which elongation is nearly linear, and 3) a plateau phase in which elongation rate declin es as stem length reaches an asymptotic maximum. For each growth phase , suitable mathematical functions were selected for smooth height and slope transitions between phases, The three growth phases were linked to developmental events, particularly flower initiation and the first observation of a visible flower bud. The model was fit to a data set o f single-stemmed poinsettia grown with vegetative periods of 13, 26, o r 54 days, resulting in excellent conformance (R(2) = 0.99). The model was validated against two independent data sets, and the elongation p attern was similar to that predicted by the model, particularly during the linear and plateau phases. The model was formulated to allow dyna mic simulation or adaptation in a graphical control chart. Model param eters in the three-phase function have clear biological meaning. The f unction is particularly suited to situations in which identification o f growth phases in relation to developmental and horticultural variabl es is an important objective. Further validation under a range of cond itions is required before the model can be applied to horticultural si tuations.