Spectral Dependence of Electrical Energy-Based Photosynthetic Efficiency in A Single Leaf and Canopy under LED

Abstract

In plant factories using LED as artificial light source, photosynthetic efficiency of crops and electrical energy consumption are affected by spectral characteristics of the LED. And most of previous observations on the spectral dependence of photosynthesis and optical property have been limited to a single leaf so far. The objectives of this study were to investigate photosynthetic efficiency and the related optical properties of four cultivars of lettuce (two reddish and two green leaves) and to quantify the spectral dependence of photosynthetic efficiency at the canopy level for selecting the optimum spectrum of LED regarding the electrical energy consumption. Absorptances, photosynthetic efficiencies of a single leaf of the plants, and electrical energy consumption of LED were measured at 18 wavelengths with a narrow band of 10 nm from 400 to 700 nm. Anthocyanin and chlorophyll contents (SPAD value) were measured to explain the difference between each cultivars. Light penetrations into crop canopy were estimated by models. Light absoprtances over PAR range were similar among the green and reddish lettuce cultivars, while that around 550 nm (green region) was slightly higher in the reddish leaves. Photosynthetic rates per incident photon of a single leaf had two peaks at 650-660 and 400-410 nm and those per absorbed photon (quantum yield) had three peaks at 650-660, 400-410, and 540-560 nm. In the green region of spectrum, both photosynthetic rates per incident photon and those per absorbed photon were lower in reddish cultivars than green ones. The differences in color and photosynthetic rate between reddish and green cultivars were due to anthocyanin contents in leaves. Spectral dependence of light absorptance at canopy level was much weaker than that at a single leaf. Light absorptance of green light was almost the same as that of blue or red light. As the quantum yield and absorptance of green light at canopy level was not lower than that of blue or red light, the photosynthetic efficiency of green light at canopy level became higher than that at the single leaf. Due to the fact that light conversion efficiency in terms of electrical energy consumption was lower in green LED than red or blue one, therefore, photosynthetic efficiency considering electrical energy consumption was much lower in green LED than that of blue or red LED even at canopy level. These results could reflect the actual plant response to light spectrum more accurately at canopy level and give more information to optimize the artificial lighting strategies for energy-saving.