The effects of light quality on photosynthesis and transpiration of chrysanthemums during the vegetative and flowering stages are not known. Leaf and whole-plant CO₂ and H₂O exchanges of chrysanthemums during long-day (LD) and short-day (SD) photoperiods were measured under varying intensity levels of high-pressure sodium (HPS) and different monochromatic and multicolour light-emitting diode (LED) lights. All light sources induced leaf photosynthesis effectively, including green and orange LEDs. During both LD and SD, HPS, white, and notably orange light produced high rates of photosynthesis, whereas blue light had the lowest rates. In addition, there were only subtle changes in the response of leaf functions to light quality during flowering induction. Diurnal patterns of whole-plant net C exchange rate were similar under HPS and two commercial LED lights, one red–blue (RB) and one red–white (RW), during both LD and SD. The RB and RW LED lights were as effective at maintaining whole-plant C gain during the day as traditional HPS lights. However, in comparison to HPS, the RB and RW LEDs increased whole-plant transpiration and decreased water-use efficiency (WUE). A decrease in WUE under these LEDs was not evident from the leaf measurements, which emphasizes the importance of examining responses to light quality at the whole-plant level as well and not at the leaf level alone. In commercial production, the wavelength of supplementary lighting may heavily influence WUE and subsequent nutrient uptake, and ultimately growth and quality of cut flower crops.