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Surface Diffusive Resistances of Rooted Poinsettia Cuttings Under Controlled-Environment Conditions

Zolnier, S., Gates, R.S., Geneve, R.L., Buxton, J.W.
Transactions of the ASAE 2001 v.44 no.6 pp. 1779-1787
Euphorbia pulcherrima, air, algorithms, canopy, environmental factors, equations, models, stomatal conductance, vapor pressure
The implementation of an evapotranspiration-based control algorithm for misting control during poinsettia propagation requires accurate estimates of resistances to the diffusive process at the canopy surface. The canopy surface resistance (r s ) and the canopy stomatal resistance (r c ) of fully rooted poinsettia cuttings in controlled environmental conditions, as affected by incident radiation (R i ) and air vapor pressure deficit (VPD air ), were determined from measurements. To create a model for r s , the Penman-Monteith equation was inverted and used with environmental measurements, while r c was estimated from porometer measurements. The canopy surface resistance was unaffected by VPD air under dark conditions, and a constant value of 363 ( ± 8) s m -1 can be applied over the range from 0.5 to 2.5 kPa. Under light conditions, it was found that the canopy surface resistance depends on incident radiation alone. A non-linear equation was fitted to the experimental data and showed that 93% of the r s variability was accounted for by R i . The canopy stomatal resistance estimated from porometry measurements was sensitive to both incident radiation and air vapor pressure deficit. Because of the lower precision of the porometer method used, only 71% of the r c variability was explained by R i and VPD air .