A method for instantaneous whole canopy water use efficiency measurement

Measurements of whole-canopy gas exchange - of CO₂ and H₂O - are important for agricultural and ecological reasons. The objective of this study was to investigate the use of a full-size greenhouse as an open-chamber system for measuring canopy-scale gas exchange and thus estimate whole canopy water use efficiency (WUE). The experiments were done in a fan ventilated greenhouse with a floor area of (15×24) m² in which mature pepper plants were grown. The fans were mounted at one end of the greenhouse and sucked ambient air through openings at the opposite end. As the air moved from one side of the greenhouse to the other, gradients of temperature, humidity and CO₂ developed along the structure. Photosynthetic water-use efficiency (also called intrinsic or instantaneous water-use efficiency), which is defined as the ratio of the rate of carbon assimilation (photosynthesis) to the rate of transpiration, was calculated using the differences in H₂O and CO₂ concentration between air outlet and inlet at different hours of the day. It is shown that the differences in H₂O and CO₂ concentration are closely related to the solar radiation intensity. The largest differences in H₂O and CO₂ concentration at air outlet were observed at midday when solar radiation was highest. WUE was roughly constant over most of the day. Early in the morning and late in the afternoon the WUE values were generally higher. A step change that increased ventilation by 100% increased WUE by about 50%.