Modeling the Stem Elongation Response of Poinsettia to Chlormequat

Stem elongation response to a single foliar application of the growth retardant chlormequat chloride [(2chloroethyl) trimethylammonium chloride] for poinsettia (Euphorbia pulcherrima Klotz.) was quantified. Growth retardant applications did not affect final leaf count or timing of visible bud, first bract color, or anthesis. There was a statistically significant effect of growth retardant concentration on stem elongation, with a range from 289 ± 15 mm (mean ± 95% confidence intervals) for the control plants to 236 ± 17 mm at 4000 ppm. The growth-retarding effect during the first day after the application was not significantly different between 500 and 4000 ppm, and concentration primarily affected the duration of growth-retarding activity. A dose response 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 set from plants receiving 0, 500, 1000, 1500, 2000, 3000, and 4000 ppm, with a resulting R2 of 0.99. Validation of the dose response model against an independent data set resulted in an r of 0.99, and predicted final stem length was within 12 mm of observed final length. A three-phase mathematical function has been used to model shoot elongation of single-stem poinsettia (Euphorbia pulcherrima Klotz.) (Fisher et al., 1996). In calibrating the model, aside from nyctoperiod and minor greenhouse temperature fluctuations, environmental factors were constant throughout the experiment. For a stem elongation model to be useful for simulation and practical application, however, it is necessary to incorporate the effect of perturbations such as growth retardant applications and temperature effects. Growth retardant chemicals are used widely in the commercial production of poinsettia and other flowering potted plants for height control (Ecke et al., 1990). A simulation model may be a useful horticultural tool because there is increasing pressure to minimize the use of growth retardants (Ludolph, 1992) and maximize their efficacy. Although many researchers have examined the impact of growth retardant applications on poinsettia final height and flowering (for example, Holcomb et al., 1992; Holcomb and Rose, 1992; Larson, 1967; McDaniel, 1986; McDaniel and Wilson, 1990), published data are not suitable for predicting short term changes in stem elongation rate. These data provide a basis for determining the effects of growth retardant type and concentration on final plant height. It is difficult to generalize these results, however, because growth retardant efficacy can be affected by a range of environmental conditions, including soil media composition (Barrett, 1982), timing (Gilbertz, 1992), nutrient status, application technique and frequency, chemical type, concentration, and temperature (Larson, 1967). The objectives of this project were to quantify the dose response of poinsettia to a single foliar application of chlormequat chloride growth retardant [(2-chloroethyl) trimethylammonium chloride] OC. HORT. SCI. 121(5):861–868. 1996. or publication 7 Aug. 1995. Accepted for publication 7 Mar. 1996. We ersity Outreach at Michigan State Univ. for providing funding for this e cost of publishing this paper was defrayed in part by the payment of es. Under postal regulations, this paper therefore must be hereby marked ent solely to indicate this fact. dress: Dept of Environmental Horticulture, Univ. of California, Davis, and incorporate the dose response into a three-phase model of stem elongation (f3-phase, Fisher et al., 1996). Separate data sets were collected for model calibration and validation. A methodology developed by Lieth and Reynolds (1986) for modifying the Richards function (Richards, 1959) to incorporate the effect of daminozide applications on chrysanthemum [Dendranthema grandiflora (Ramat.) Kitamura] was adapted to model a different chemical (chlormequat), species (poinsettia), and base function (f3-phase). Materials and Methods Modeling an environmental perturbation. In the Lieth and Reynolds (1986) approach, the Richards equation (Eq. [1]), dHu dt = kHu(A n – Hu ) (nA) [1] with the solution (Eq. [2]), commonly termed the Richards function, Hu = HoA H0 n + (A – H0 )e–kt n [ 2] was fitted to data from plants grown in constant conditions. In the Richards function, t represents time, A defines the upper asymptote of untreated plant height H u , and parameters H 0 , n, and k determine the lower asymptote and shape of the growth curve with respect to A (Table 1). The estimated values of A, H 0 , n, and k describe the asymptotic growth characteristics of H u in the absence of environmental change. The Richards function was modified by Lieth and Reynolds (1986) to simulate the response to an environmental perturbation. The response function was termed g(t), and the modified Richards equation was written as dH dt = dHu dt g(t) [3] where g(t) is a function that has a value ≥0. If g(t) = 0, there is no growth, a g(t) of 1 means that the treated plant grows at the same