Predicting rooting stages in poinsettia cuttings using root zone temperature-based models

Root zone temperature optima for root initiation and root elongation stages for rooting in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch 'Freedom Dark Red') cuttings was determined to be 28 and 26°C, respectively. Threshold temperatures where rooting development was slow (>24 days) or did not occur were ≤20 and ≥22°C. Time to visible rooting and postemergent root elongation was modeled based on cumulative daily mean root zone temperatures in growth chamber studies using a thermogradient table to provide simultaneous temperatures between 19 to 34°C. Time to root emergence at different root zone temperatures was best described using a nonlinear growth rate derived mathematical model, while postemergent root elongation up to 100 cm could be described using either a linear thermal time model or a nonlinear equation based on elongation rate. These temperature-based mathematical models were used to predict rooting in six greenhouse experiments. Using a root zone base temperature of 21°C, observed vs. predicted time to visible root emergence was highly correlated (r² = 0.98) with a mean prediction error (MPE) of 1.6 d. Observed vs. predicted root length using the linear thermal time model had a r² = 0.69 and an MPE of 14.6 cm, which was comparable to the nonlinear model with an r² = 0.82 and an MPE of 14.8 cm.