Abstract
Light recommendations for horticultural crops often focus on the optimal daily light integral (DLI) without regard to how that light is delivered throughout each day. Because photosynthesis is more efficient at lower photosynthetic photon flux density (PPFD), we hypothesized that longer photoperiods with lower PPFD results in faster growth than shorter photoperiods with higher PPFD and the same DLI. We quantified the effect of different photoperiods, all providing the same DLI, on photosynthesis and growth of two leafy greens. Mizuna (Brassica rapa var. japonica) and lettuce (Lactuca sativa) “Little Gem” were grown from seed in a controlled environment chamber (20 °C and 819 µmol·mol−1 CO2) under six photoperiods (10, 12, 14, 16, 18, and 20 h). LED fixtures provided white light and PPFD was adjusted so each treatment received a DLI of 16 mol·m−2·d−1. Mizuna and lettuce were harvested 30 and 41 days after planting, respectively. Longer photoperiods with lower PPFD increased light interception, chlorophyll content index, quantum yield of photosystem II, and aboveground biomass, but decreased instantaneous CO2 assimilation of lettuce and mizuna. Aboveground biomass increased 16.0% in lettuce and 18.7% in mizuna in response to increasing the photoperiod from 10 to 20 h. In summary, extending the photoperiod and lowering PPFD increases growth of lettuce and mizuna by increasing light interception and the quantum yield of photosystem II.
Highlights
Photosynthesis is the primary process plants use to accumulate biomass, and whole-plant photosynthesis is directly related to overall crop productivity [1,2]
Lower photosynthetic photon flux density (PPFD) combined with longer photoperiods increased chlorophyll contentduring indexthe (CCI), canopy light interception, and ΦPSII
The observed increase in growth under lower PPFDs and longer photoperiods is consistent with prior studies, which found that when daily light integral (DLI) is held constant, plants generally produce more biomass when grown under longer photoperiods [7,8,9,11,12,39,40]
Summary
Photosynthesis is the primary process plants use to accumulate biomass, and whole-plant photosynthesis is directly related to overall crop productivity [1,2]. In controlled environment agriculture, such as greenhouses and plant factories, natural sunlight is supplemented or replaced with electrical lighting to achieve adequate photosynthesis and growth. Light recommendations for crops often focus on the optimal daily light integral (DLI, total amount of photosynthetic light received by a plant in a day), without regard to how that light is delivered over time [4]. Finding the optimal way to provide supplemental or sole-source lighting, maximizing the efficiency with which plants intercept and use light to produce marketable biomass, has the potential to reduce input costs and increase yields and profitability [5]. Plants use light most efficiently for photosynthesis at lower light intensities [6], Agronomy 2020, 10, 1659; doi:10.3390/agronomy10111659 www.mdpi.com/journal/agronomy
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