Photochemical Characterization of Greenhouse-grown Lettuce (Lactuca sativa L. ‘Green Towers’) with Applications for Supplemental Lighting Control

Abstract

Plant light use efficiency decreases as light intensity is increased, and a better understanding of crop-specific light responses can contribute to the development of more energy-efficient supplemental lighting control strategies for greenhouses. In this study, diurnal chlorophyll fluorescence monitoring was used to characterize the photochemical responses of 'Green Towers' lettuce (Lactuca sativa L.) to photosynthetic photon flux density (PPFD) and daily light integral (DLI) in a greenhouse during a production cycle. Plants were monitored continuously for 35 days, with chlorophyll fluorescence measurements collected once every 15 minutes. Quantum yield of photosystem II (FPSII) decreased exponentially with PPFD, whereas electron transport rate (ETR) increased asymptotically to 121 mmol · m -2 · s -1 . Daily photochemical integral (DPI) is defined as the integral of ETR over a 24-hour period; DPI increased asymptotically to 3.29 mol · m -2 · d -1 with increasing DLI. No effects of plant age or prior day's DLI and a negligible effect of PPFDs 15 or 30 minutes before measurements within days were observed. Simulations were conducted using the regression equation of ETR as a function of PPFD {ETR = 121 [1 -exp(-0.00277PPFD)]} to illustrate methods of increasing photochemical light use efficiency for improved supplemental lighting control strategies. For a given DLI, DPI can be increased by providing light at lower PPFDs for a longer period of time, and can be maximized by providing light with a uniform PPFD throughout the entire photoperiod. Similarly, the DLI required to achieve a given DPI is reduced using these same methods.

Longer Photoperiods with the Same Daily Light Integral Increase Daily Electron Transport through Photosystem II in Lettuce

“…The asymptotic increase in ETR with increasing PPFD ( Figure 3 ) also is consistent with previous findings. Weaver and van Iersel [ 7 ] likewise found that ETR of ‘Green Towers’ lettuce responded to PPFD with an exponential rise to a maximum but reported a higher asymptote (121 µmol m −2 s −1 ). Zhen and van Iersel [ 10 ] found that the asymptote of the ETR curve depended on the light intensity plants received prior to the measurements, with plants grown in shade having lower ETR asymptotes than those grown in full sun.…”

“…The Φ PSII decreased linearly as PPFD increased ( Figure 3 ), which is consistent with previous studies. Weaver and van Iersel [ 7 ] reported that the Φ PSII of ‘Green Towers’ lettuce grown and measured in a greenhouse under natural light decreased exponentially as PPFD increased from 0 to ≈1500 µmol m −2 s −1 . These plants were exposed to a wider range of PPFDs than those in the current study, which may explain the differences in trends, linear versus exponential, between the two studies.…”

“…These plants were exposed to a wider range of PPFDs than those in the current study, which may explain the differences in trends, linear versus exponential, between the two studies. When comparing their data across the range of PPFDs used in this study (189 to 794 µmol m −2 s −1 ), they too saw a roughly linear decline in Φ PSII from ≈0.6 to ≈0.3; further declines in Φ PSII at higher PPFDs were small, with a Φ PSII of ≈0.22 at a PPFD of 1500 µmol m −2 s −1 [ 7 ]. Zhen and van Iersel [ 10 ] found that the Φ PSII of sweet potato ( Ipomea batatas ), lettuce, and pothos ( Epipremnum aureum ), grown in a greenhouse but measured in a growth chamber under precisely-controlled PPFDs, decreased exponentially with increasing PPFD, and the rate of decrease was species-dependent.…”

“…Under natural lighting conditions in a greenhouse, Φ PSII of ‘Green Towers’ lettuce decreased asymptotically from 0.81 to ≈0.22 as the PPFD increased from 0 to 1500 µmol m −2 s −1 [ 7 ], while in a separate study Φ PSII of ‘Little Gem’ lettuce decreased to only ≈0.35 at a PPFD of 1500 µmol m −2 s −1 [ 8 ]. For lettuce ‘Tiberius,’ grown under a PPFD of 200 µmol m −2 s −1 , Φ PSII decreased to ≈0.22 at a PPFD of 1400 µmol m −2 s −1 [ 9 ].…”

“…Under greenhouse conditions, without supplemental light, the DPI increased asymptotically with increasing DLI. Because those data were collected under natural sun light conditions, potential interactions between photoperiod, PPFD, and DLI could not be determined [ 7 ]. Our goal was to quantify the effect of PPFD and photoperiod on DPI, while maintaining a static DLI.…”