Supplemental lighting is often used to improve the growth and yield of greenhouse vegetables. However, the electricity required for supplemental lighting can account for as much as 30% of the recurring cost of operating a greenhouse (van Iersel and Gianino, 2017; Watson et al., 2018). Thus, reducing the cost of greenhouse supplemental lighting or increasing the productivity of crops would be beneficial to greenhouse vegetable growers. Lighting control approaches that efficiently drive photosynthesis and growth of greenhouse vegetable crops may improve the profitability of greenhouse vegetable production.
LED lights are becoming increasingly popular in horticultural applications for a variety of reasons, including their relatively high efficacy (Nelson and Bugbee, 2014). One unique feature of LED lights is that their light output can be controlled precisely and nearly instantaneously in real time, which is not possible with high-intensity discharge lamps (van Iersel et al., 2016; Weaver et al., 2019). Dimmable LED lights can be interfaced with quantum sensors and control systems, allowing for adaptive lighting control (van Iersel and Gianino, 2017). With adaptive lighting, supplemental light is provided so that the PPFD of sunlight and supplemental light combined reaches a specified threshold PPFD, and the lights are turned off if sunlight alone exceeds this threshold PPFD.
Photosynthetic light responses can generally be described as concave functions of PPFD. The efficiency of photosynthetic light use (moles of carbon fixed per mole of photons) invariably decreases as PPFD increases, and hence, photosynthetic gains per unit of applied photosynthetically active radiation will always be greatest at lower PPFDs (Aikman, 1989; Weaver and van Iersel, 2019). The decrease in photosynthetic light use efficiency at higher PPFDs is due in part to photoprotective processes that convert absorbed light energy to heat, rather than allowing it to be used for electron transport in the light reactions of photosynthesis. As PPFD is increased, proportionally higher amounts of absorbed light energy are dissipated as heat. This prevents light-induced damage of the photosynthetic apparatus via the action of complementary photoprotective processes, which include the xanthophyll cycle and molecular rearrangement of the chlorophyll antennae and photosynthetic reaction centers (Demmig-Adams et al., 2012; Horton, 2012; Rochaix, 2014; Ruban, 2015).
Because light drives photosynthesis more efficiently at lower PPFDs, we have hypothesized that crop growth will be improved if light is provided at lower PPFDs over a longer period of time (Weaver and van Iersel, 2019). This effect has been demonstrated in growth chamber experiments for several cultivars of lettuce (Lactuca sativa) (Koontz and Prince, 1986; Soffe et al., 1977), as well as for other vegetable species (Soffe et al., 1977) and strawberry (Fragaria ×ananassa) (Tsuruyama and Shibuya, 2018). Plant growth generally increases as the same daily amount of light is provided over a longer photoperiod. Growing greenhouse crops with longer photoperiods can also be advantageous because lower electricity prices may be available at night (Albright et al., 2000). However, similar results have not yet been reported for plants grown in a greenhouse, where lighting conditions are highly variable and likely cannot be as readily controlled.
In our experiments, we grew ‘Little Gem’ lettuce plants in a greenhouse with supplemental light provided to reach a minimum DLI of 17 mol·m−2·d−1 using an adaptive lighting system. We hypothesized that reaching the same DLI, with a longer photoperiod and lower average PPFDs, will result in increased growth for this romaine-type lettuce. Chlorophyll fluorescence measurements were used to test the hypothesis that photosynthetic efficiency decreases as PPFD is increased, as evidenced by a decrease in the quantum efficiency of photosystem II (ΦPSII). This is a unitless measure of the efficiency with which absorbed photons are used to drive the light reactions of photosynthesis.
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