NIGHT-TIME TRANSPIRATION IN GREENHOUSES

High night-time humidity in greenhouses is characteristic of well insulated houses in mild weather. Using ventilation to remove excess vapour increases the energy requirement of a greenhouse in direct proportion to the amount of vapour to be removed which, in turn, increases linearly with canopy conductance. The night-time transpiration of several greenhouse crops was measured and their leaf conductance and resistance to vapour flux were determined by two methods: a weighing method was used for individual containers and a vapour balance method for a whole compartment. A few night-long comparisons between the two methods gave good agreement. A wide range of leaf resistances was obtained, from 500 to 2000 s m-1 for vegetables, from 2000 to 5000 s m-1 for most foliage and flower plants, with one crop reaching 20 000 s m-1. © 1990.