S06 - Session O2 - Effect of different light intensities on Lactuca sativa growth in indoor conditions.

Authors: Andres Mayorga-Gomez *, Marc van Iersel

Plant factories are becoming more popular because of their ability to produce pesticide-free food in or near urban areas. Plant factories do not require arable land and are water use efficient. However, they have high electricity costs related to air-conditioning and lighting. It is important to understand how efficient plants use the provided light to reduce electricity use associated with lighting. To quantify differences in plant growth and light use efficiency under different light intensities, 'Rex' lettuce ( Lactuca sativa ) was grown in a growth chamber with a temperature of 25 °C, relative humidity of ~50%, and a CO 2 concentration of 800 µmol·mol -1 . Groups of 15 plants were grown with photosynthetic photon flux densities (PPFD) of 125, 175, 225, 275, 325, and 375 µmol·m -2 ·s -1 provided by white LED lights with a photoperiod of 20 h, resulting in daily light integrals ranging from 9 to 27 mol·m -2 ·d -1 . Biomass accumulation increased by approximately 0.086 g/ plant for every 100 µmol·m -2 ·s -1 increase in PPFD ( R 2 = 0.95, P < 0.0001). However, plants grown under PPFDs of 325 and 375 µmol·m -2 ·s -1 developed severe tip-burn, making those PPFD levels unacceptable for lettuce production. Every increase in PPFD of 100 µmol·m -2 ·s -1 reduced light use efficiency (LUE, biomass/incident light) by 0.080 gmol -1 ( R 2 = 0.78, P < 0.0001). A possible reason for the decrease in LUE with increasing PPFD is the decrease in the quantum yield of photosystem II with increasing PPFD ( R 2 = 0.63, P < 0.01). The LUE and quantum yield of photosystem II were positively correlated (r(16)=0.62, P < 0.01 ). Our results suggest that there is a tradeoff between fast growth and high LUE. Additionally, lighting decisions cannot be made solely based on biomass production or LUE. Instead, the overall costs and revenues of the production system need to be considered.