Effects of over-night lighting by red light emitting diodes on the growth and photosynthesis of lettuce under CO₂ enrichment

Previously, we reported that overnight lighting using red light-emitting diodes (LEDs) promoted the growth of some leafy vegetables.
The extended photoperiod increased the top dry matter of lettuce (Lactuca sativa L.) via the extended active photosynthesis time.
However, it is possible that continuous daytime conditions caused by overnight supplemental lighting (OSL) place stress on the plant via the excess photosynthesis.
Therefore, this study evaluated the effects of OSL from red LEDs on the growth and photosynthetic capacities of lettuce.
The combined effects of OSL and carbon dioxide (CO₂) enrichment were also examined to develop a way to increase lettuce production in greenhouses.
Lettuce seedlings were transplanted in a glasshouse at the 3-4 true-leaf stage and grown hydroponically for 3 weeks.
During the night (17:00-7:00), OSL was provided by red LEDs at a photosynthetic photon flux density (PPFD) of 300 (high) or 100 (low) µmol m^‑2 s^‑1. A constant net photosynthetic rate was maintained during OSL under both light intensities.
However, the maximum photosynthetic ratio was reduced by approximately 10% in lettuce grown under OSL, especially with the high light intensity.
Moreover, the maximal quantum yield of photosystem II in the dark-adapted state (Fv/Fm) tended to decrease in the lettuce grown under OSL. Conversely, under a short, continuous photoperiod with the same daily integrated PPFD, lettuce had the same photosynthesis level.
This suggested that OSL induced physiological stress via excess photosynthesis.
However, when we evaluated the effects of the carbon dioxide concentration during OSL on the photosynthesis of lettuce in a growth cabinet, a high carbon dioxide concentration (1,000 ppm) restored the maximum photosynthetic ratio and Fv/Fm value under OSL. The carbon dioxide enrichment reduced the photosynthetic stress on lettuce under continuous daytime conditions caused by overnight lighting.

International Symposium on Advanced Technologies and Management for Innovative Greenhouses: GreenSys2019

N. Fukuda, N. Kanesaka, L. Okushima, S. Sase

chlorophyll fluorescence, Fv/Fm, maximum photosynthetic ratio, supplemental lighting

https://doi.org/10.17660/ActaHortic.2020.1296.77