2019 ASHS Annual Conference

The cost of dehumidifying the humid air generated by plant transpiration in vertical farms contributes to the high power costs of HVAC in these systems. Reducing transpiration while maintaining photosynthesis should reduce HVAC costs, while maintaining yields. Under increasing light intensity, the transpiration rate increases linearly and photosynthetic assimilation rises to meet an asymptote. Since instantaneous water use efficiency (WUE) is defined as the transpiration rate divided by photosynthetic assimilation rate, under increasing light intensity, it theoretically reaches a maximum and then plateaus or declines. In this experiment, two different spectra of white light varying in the ratio of blue to red, and in the percentage of far-red light were used to perform photosynthesis-light response curves on ‘Green Salad Bowl’ lettuce and ‘Purple Wave’ petunias at 400 and 800 ppm CO₂. The instantaneous WUE curves reached a maximum or asymptote under both lights and in both CO₂ concentrations. Water use efficiency tended to be higher under the white light with a larger fraction of red and far-red light, compared to the light with more blue. It was also higher at higher CO₂ concentrations. Thus, water use efficiency, and thus HVAC costs, can be increased by increasing light intensity or CO₂ concentration, as well as by manipulating the light spectrum.