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Erin G. Wilkerson, Richard S. Gates, Sérgio Zolnier, Sharon T. Kester and Robert L. Geneve

Rooting stage, transpiration capacity, and relative water content were measured in cuttings every 5 days for 25 days. Cell divisions in phloem parenchyma were evident between 5 and 10 days after sticking, organized subcuticular root primordia were present between 10 and 12 days, and roots emerged between 12 and 15 days. Transpiration was measured in poinsettia (Euphorbia pulcherrima Willd. ex Klotzsch `Freedom Dark Red') cuttings under light or dark conditions at increasing vapor pressure deficit (VPDair) levels during different stages of rooting. Transpiration capacity did not increase until roots emerged on the cuttings. Light had a significant impact on transpiration rates only after roots emerged. Light was more significant than VPDair for determining actual transpiration. Between visible rooting (15 days) and 25 days, increase in total root length was linear (r 2 = 0.92) and significantly correlated with transpiration (r 2 = 0.98). Transpiration capacity increased after visible rooting, but did not significantly increase under non-misted conditions until cuttings were well-rooted and had a total root length >50 cm (18 days after sticking). Relative water content measured before and after entering the transpiration chamber confirmed that cuttings were only able to take enough water from the medium to continue sustained transpiration after 18 days. A cutting coefficient was developed from transpiration data to modify the misting interval for dynamic controlled misting. Greenhouse studies showed a 55% or greater reduction in water use with dynamic control compared to constant static or stepped down static control. Rooting performance was unaffected by misting interval. Foliar nutrition was significantly reduced in all cuttings after 7 days in the mist bench, but changes in foliar elemental content were not correlated with misting interval.

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Erin G. Wilkerson, Richard S. Gates, Sérgio Zolnier, Sharon T. Kester and Robert L. Geneve

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 ≥32 °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 (r 2 = 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 r 2 = 0.69 and an MPE of 14.6 cm, which was comparable to the nonlinear model with an r 2 = 0.82 and an MPE of 14.8 cm.