2018 ASHS Annual Conference

Photosynthetic photon efficiency [PPE, mol. of light output per KWh of energy used; mol·(KWh)^-1] is one of the important characteristics used in marketing grow light fixtures for indoor production (vertical farms or plant factories). However, a more useful efficiency measurement for growers is energy-use efficiency [EUE, grams of dry weight produced per KWh of energy used, g·(KWh)^-1], which is a product of PPE and light use efficiency (LUE, grams of dry weight produced per mol. of incident light, g·mol^-1). While PPE is driven by engineering technology, LUE is influenced by physiological responses of plants to incident light intensity and composition. Information on PPE of a fixture is generally available from lighting companies, whereas LUE is species specific and limited information exist on LUE of a species under a specific fixture. This is the major reason why PPE is generally used for comparing light fixtures. Using hydroponically grown leaf lettuce, we studied differences in EUE, PPE and LUE among four commercially available light fixtures (Philips-Greenpower, Fluence-RazrX, TotalGrow-TG1A Bulb, and Lithonia-T8) which differed in the intensity and composition of incident light. Our results indicated that significant differences existed among light fixtures in EUE, PPE and LUE. More interestingly, when data from all fixtures were pooled in regression analysis, there was no relation between EUE and PPE among fixtures; whereas a strong linear relationship was found between EUE and LUE in hydroponically grown leaf lettuce. Further analysis indicated that LUE was linearly related to leaf growth rate (light interception), which decreased exponentially with increasing fraction of blue light (light composition) in the incident light. Leaf growth rate generally increased with increasing fraction of red light in the incident light. Fraction of red light decreased with increasing fraction of green light among fixtures. While acknowledging that observed responses are likely specific to leaf lettuce, we conclude that plant physiological responses to light (or LUE), can be an important characteristic in determining optimal lighting fixture for commercial indoor production.