Lettuce Growth and Morphology in a Red-rich Low PAR Light Environment in a Greenhouse

Selective filtration of sunlight has been shown to affect the growth of plants. In this research a side-by-side comparison was done between two greenhouses, a conventional (control) greenhouse covered with double-layer acrylic panels and an experimental greenhouse covered with a glazing that creates a red-rich diffuse light environment while generating electric power through photovoltaic cells integrated into the glazing (Abengoa Solar US Patent pending). Two cultivars of lettuce (Lactuca sativa), ‘Rex’ and ‘Magenta’, were grown hydroponically for 60 days (November 27 to January 26, 2014).  Average daily light integral (DLI) over the plant canopy was 9.7±2.1 and 6.5±1.4 mol m^-2 in the control and experimental greenhouse, respectively.  When measured under a clear sky at midday, red (600-700 nm) photon flux over PAR (400-700 nm) was 38.5% and 64.4% and red to blue (400-500nm) photon flux ratio was 1.42 and 2.95 in the control and the experimental greenhouse, respectively. Average day/night temperatures were almost the same (22.8±0.9°C/17.9±0.4°C in the control and 23.6±1.2°C/17.9±0.3°C in the experimental greenhouse). Despite the lower DLI, initial plant fresh and dry weight in the experimental greenhouse was equal to or significantly greater than in the control, depending on variety. Thirty-three days after seeding (DAS), shoot fresh weight, shoot dry weight, and leaf area were 49%, 31%, and 55% greater respectively for ‘Magenta’ in the experimental greenhouse compared to the control. ‘Rex’ showed no significant differences in these variables.  Final shoot fresh weight (harvested yield) at 60 DAS was 14-29% greater in the control greenhouse, but leaf area was 25-29% greater in the experimental greenhouse. The enhanced initial growth of lettuce in the experimental greenhouse, despite the low DLI, was presumably due to greater canopy light interception caused by larger leaf area due to altered red-rich light quality. The higher yield in the control greenhouse compared to the experimental greenhouse at the final harvest in comparison to the seedling stage was likely due to canopy closure beginning around 41 DAS, diminishing the difference in canopy light interception, and exhibiting growth more directly based on available DLI. The red rich spectrum may also have contributed to a further increase in leaf photosynthesis, which will be investigated further. This study has shown that a red-rich light environment resulted in enhanced vegetative plant growth, a response that may be advantageous for some leafy vegetables and seedlings.