MODELING THE STEM ELONGATION RESPONSE OF POINSETTIA TO CHLORMEQUAT

Stem elongation response to a single foliar application of the growth retardant chlormequat chloride [(2-chloroethyl) trimethylammonium chlo ride] for poinsettia (Euphorbia pulcherrima Klotz.) was quantified. Gr owth retardant applications did not affect final leaf count or timing of visible bud, first bract color, or anthesis, There was a statistica lly significant effect of growth retardant concentration on stem elong ation, with a range from 289 +/- 15 mm (mean +/- 95% confidence interv als) for the control plants to 236 +/- 17 mm at 4000 ppm, The growth-r etarding effect during the first day after the application was not sig nificantly different between 500 and 4000 ppm, and concentration prima rily affected the duration of growth-retarding activity, A dose respon se function was incorporated into a three-phase mathematical function of stem elongation of single-stem poinsettia to predict elongation of treated and untreated plants. The model was calibrated using a data se t from plants receiving 0, 500, 1000, 1500, 2000, 3000, and 4000 ppm, with a resulting R(2) of 0.99, Validation of the dose response model a gainst an independent data set resulted in an r(2) of 0.99, and predic ted final stem length was within 12 mm of observed final length.