On the night transpiration of greenhouse roses under glass or plastic cover

Rose plants in pails were weighed at the beginning and end of several nights to determine the rate of transpiration under a range of air temperatures. The data were reduced to obtain the leaf resistance (both sides of the leaf in parallel), its mean being 2400 s m⁻¹ ± 30%. Once the night-time leaf resistance is known, the transpiration rate can be calculated from measured values of the leaf area index, the leaf temperature and the ambient temperature and dew-point temperature. If the leaf temperature cannot be measured, it can be estimated from the inside temperature and dew-point temperature, and the outside temperature. A calculated example shows that there is agreement within a factor of two between an estimate based on the present model and the rate of night evapotranspiration in greenhouses, as reported in the literature.