Surface diffusive resistance of rooted poinsettia cuttings under controlled-environment conditions

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.