Increasing Growth of Lettuce and Mizuna under Sole-Source LED Lighting Using Longer Photoperiods with the Same Daily Light Integral

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.

Canopy Size and Light Use Efficiency Explain Growth Differences between Lettuce and Mizuna in Vertical Farms

“…With increasing PPFD, CCI increased in both species ( Figure 4 A). In previous studies, CCI increased with lower PPFD [ 4 , 18 ]. In those studies, lower PPFDs were combined with longer photoperiods to maintain the same DLI.…”

“…Mizuna grew faster than lettuce at PPFDs ≥ 125 µmol m −2 s −1 ( Figure 7 A). A previous study conducted to identify the effects of photoperiod on leafy greens found much faster growth of mizuna compared to lettuce [ 18 ]. The difference in biomass between the two crops increased with increasing PPFD (and shorter photoperiods).…”

“…Chlorophyll production is a light-regulated process [ 22 ]. Therefore, the CCI increase in low PPFD treatments in prior studies was associated with longer photoperiods, which increases the amount of time available for plants to produce chlorophyll [ 18 ]. In our study, photoperiod was the same in all treatments and therefore the daily light integral was higher at higher PPFDs.…”

“…A previous indoor study with mizuna ( Brassica rapa var. japonica ) and oakleaf lettuce ( Lactuca sativa ‘Green Salad Bowl’) found much faster growth of mizuna compared to lettuce [ 18 ]. This growth difference must be the result of morphological and/or physiological differences between the two crops, but those underlying reasons were not explored in that study.…”

Continuous Lighting and High Daily Light Integral Enhance Yield and Quality of Mass-Produced Nasturtium (Tropaeolum majus L.) in Plant Factories

“…For instance, under the same DLI of 8.64 mol m −2 d −1 , the total FW and the total dry weight (DW) of hydroponic lettuce were heavier for the extended light period of 16 h than for the shorter light period of 12 h by 8.6% and 8.5%, respectively [8]. Weaver and van Iersel [9], who tested four different light periods (12,15,18, and 21 h) with the same DLI at 17 mol m −2 d −1 on lettuce grown in a greenhouse, found that the DW of lettuce was positively correlated with the photoperiod, and the highest DW was attained under 21 h. For lettuce and mizuna cultured under the DLI of 16 mol m −2 d −1 provided by white LED in a walk-in growth chamber, the shoot biomass of these crops grown under the photoperiod of 20 h was more than that of 10 h by 16.0% and 18.7%, respectively [10].…”