CANOPY SURFACE RESISTANCES TO WATER-VAPOR TRANSFER FOR 9 GREENHOUSE POT PLANT CROPS

Canopy surface resistances (r(s)) for nine ornamental species (begonia, cyclamen, gardenia, gloxinia, hibiscus, impatiens, pelargonium, poinsettia and schefflera) were deduced from measurements of stand evapotranspiration (E) and used with the Penman-Monteith equation to predict hourly rates of E. A multiplicative relationship was used to express r(s) as a function of solar irradiance (G) and vapour pressure deficit (D). For most of the species, r(s) showed a high correlation with solar irradiance, except for begonia which had a significant correlation between r(s) and D, and poinsettia which had a low correlation of r(s) with both G and D. Minimum leaf stomatal resistance (r(l,min)) and dark resistance, r(l,d) were deduced from r(s) formulae. Significant differences were found between species: r(l,min) ranged from 150 s m-1 (impatiens) to 400 s m-1 (gloxinia), whereas r(l,d) varied from 1500 s m-1 (cyclamen) to 4500 s m-1 (schefflera). Calculated hourly E rates, gave good agreement with the experimental data. These results show that such a method based on the Penman-Monteith equation and the estimation of r(s) can be used to predict the short-term evapotranspiration rate of ornamental crops.