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

Abstract

Controlled environment crop production recommendations often use the daily light integral (DLI) to quantify the light requirements of specific crops. Sole-source electric lighting, used in plant factories, and supplemental electric lighting, used in greenhouses, may be required to attain a specific DLI. Electric lighting is wasteful if not provided in a way that promotes efficient photochemistry. The quantum yield of photosystem II (ΦPSII), the fraction of absorbed light used for photochemistry, decreases with increasing photosynthetic photon flux density (PPFD). Thus, we hypothesized that the daily photochemical integral (DPI), the total electron transport through photosystem II (PSII) integrated over 24 h, would increase if the same DLI was provided at a lower PPFD over a longer photoperiod. To test this, ΦPSII and the electron transport rate (ETR) of lettuce (Lactuca sativa ‘Green Towers’) were measured in a growth chamber at DLIs of 15 and 20 mol m−2 d−1 over photoperiods ranging from 7 to 22 h. This resulted in PPFDs of 189 to 794 μmol m−2 s−1. The ΦPSII decreased from 0.67 to 0.28 and ETR increased from 55 to 99 μmol m−2 s−1 as PPFD increased from 189 to 794 μmol m−2 s−1. The DPI increased linearly as the photoperiod increased, but the magnitude of this response depended on DLI. With a 7-h photoperiod, the DPI was ≈2.7 mol m−2 d−1, regardless of DLI. However, with a 22-h photoperiod, the DPI was 4.54 mol m−2 d−1 with a DLI of 15 mol m−2 d−1 and 5.78 mol m−2 d−1 with a DLI of 20 mol m−2 d−1. Our hypothesis that DPI can be increased by providing the same DLI over longer photoperiods was confirmed.

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

“…sullivantii seedlings increased by 30 and 24%, respectively, as the photoperiod increased from 12 to 21 h [11], while lettuce shoot dry mass increased by 28% [12]. Under sole-source lighting, the effect of longer photoperiods, with the same DLI, on the growth of lettuce may be greater when that DLI is higher [13], consistent with the finding that the total daily electron transport rate through photosystem II responds more strongly to longer photoperiods when the DLI is higher [14].…”

“…Plants were grown under six photoperiods (10,12,14,16,18, and 20 h). The PPFD output from the light fixtures in each treatment was adjusted so that all treatments received the same DLI of 16 mol•m −2 •d −1 throughout the study (Table 1).…”

“…In addition, heat dissipation of absorbed light energy is upregulated in response to higher PPFD [20]; thus, a larger proportion of the absorbed photons are not used for photosynthesis, but instead their excitation energy is dissipated as heat or re-emitted as fluorescence light to avoid damaging the PSII machinery [20]. We have previously found that Φ PSII of lettuce varies little during different photoperiods with constant PPFD, resulting in greater daily electron transport through photosystem II under longer photoperiods with lower PPFD [14]. We did not calculate electron transport rates in the current study, because leaf absorptance values are needed to do so.…”

“…The first step in this process is the use of absorbed photons to drive electron transport in the light reactions of photosynthesis. The Φ PSII inevitably decreases while electron transport rate increases with increasing PPFD (Figure 4) [6,14,32]. Therefore, maximizing Φ PSII is not feasible, since it will unavoidably result in low electron transport rates and low photosynthetic rates.…”

“…Therefore, maximizing Φ PSII is not feasible, since it will unavoidably result in low electron transport rates and low photosynthetic rates. However, there may be opportunities to breed for cultivars with a higher Φ PSII [14], which may increase growth and the overall resource use efficiency in plant factories.…”