Promotion of lettuce growth under an increasing daily light integral depends on the combination of the photosynthetic photon flux density and photoperiod

Abstract

In indoor farms, electric lighting controls pla growth and quality attributes, but incurs high capital and operating costs. The photon spectrum, photoperiod, and photosynthetic photon flux density (PPFD) can be optimized with other environmental variables to maximize harvestable yield considering production time and lighting costs. Previous studies have determined that increasing the photoperiod or PPFD can increase lettuce (Lactuca sativa) yield. However, it is unclear how, at multiple daily light integrals (DLI), combinations of a wide range of PPFDs and photoperiods affect growth and quality attributes of green- and red-leaf lettuce. We investigated the interaction between these factors on lettuce growth and quality attributes by growing lettuce ‘Rex’ and ‘Rouxai’ under DLIs from 6.9–15.6 mol m−2 d−1 using PPFDs of 120–270 μmol m–2 s–1 and photoperiods of 16–24 h d–1. Increasing the DLI increased shoot fresh and dry mass, leaf width and number, and chlorophyll concentration of both cultivars, regardless of the combination of the PPFD and photoperiod. Furthermore, increasing the DLI increased the red pigmentation of the red-leaf cultivar, ‘Rouxai’. Additionally, at a DLI of 15.6 mol m−2 d−1, lettuce grown under a lower PPFD and a longer photoperiod had greater fresh and dry mass than lettuce grown under a higher PPFD and a shorter photoperiod, while at a lower DLI of 10.4 mol m−2 d−1, this did not occur. We conclude that although increasing the DLI increased lettuce growth and improved quality attributes, the specific PPFD and photoperiod combination can have different effects on plant growth.