2018 ASHS Annual Conference

Most studies of photoperiodic effects on plant growth have used constant instantaneous photosynthetic photon flux densities (PPFD), which leads to different total daily light integrals (DLI) received in each photoperiod. Our objective was to quantify the effect of different photoperiods, all providing the same DLI, on crop growth. Because photosynthesis is more efficient at lower PPFD, we hypothesized that longer photoperiods with lower PPFD would result in faster growth than shorter photoperiods with higher PPFD. Mizuna (Brassica rapa var. japonica), 'Little Gem' lettuce (Lactuca sativa), and ‘Genovese’ basil (Ocimum basilicum) were grown from seed in a controlled environment chamber (20°C and 800 ppm CO₂) under six photoperiods (10, 12, 14, 16, 18, and 20 hr). White LEDs provided light and PPFD was adjusted so each treatment received a DLI of 16 mol·m^-2·d^-1. Mizuna, lettuce, and basil were harvested 30, 41, and 55 d after planting, respectively. Light interception, chlorophyll content, and quantum yield of PSII were positively correlated with duration of photoperiod in all three species. Mizuna plants grown with a 20 hr photoperiod had 10.9% greater light interception, 94.6% higher leaf chlorophyll content index, and 10.1% greater quantum yield near the end of the growing period than those grown with a 10 hr photoperiod. Lettuce plants grown with a 20 hr photoperiod had 11.4% greater light interception, 13.7% higher leaf chlorophyll content index, and 10% greater quantum yield than those grown with a 10 hr photoperiod. Mizuna and lettuce plants both also had greater shoot dry mass (28.1% and 18% greater, respectively) when grown with 20 hr photoperiods compared to 10 hr photoperiods. There was no apparent correlation between photoperiod and dry mass in basil. Basil plants grown with a 20 hr photoperiod had 13.7% higher leaf chlorophyll content index and 10% greater quantum yield than those with a 10 hr photoperiod. Lettuce plants grown under shorter photoperiods had notably yellower leaves, steeper leaf angle, and more upright growth than those in longer photoperiods. These results show that plants receiving the same DLI can have markedly faster growth when provided light over a longer photoperiod, but the effect appears to be species-specific. This is an important consideration when determining optimal lighting strategies for crop growth. Photoperiod, PPFD, and DLI cannot be studied isolation without accounting for simultaneous effects of the other two variables on plant responses.