2018 ASHS Annual Conference

Photosynthetic responses to light intensity are generally asymptotic; light is used more efficiently to drive photosynthesis at lower light intensity. Thus, providing supplemental light at low intensities over longer periods should lead to increased photosynthetic gains, compared to an equivalent amount of light at higher intensities and shorter periods. To test this hypothesis, we used an adaptive LED lighting system in a greenhouse, which dynamically controls supplemental LED light intensity to reach, but not exceed, a specified light intensity. Using this system, ‘Little Gem’ lettuce (Lactuca sativa) plants were grown under a constant daily light integral (DLI) of 17 mol·m^-2·d^-1 provided over 4 different photoperiods; 12, 15, 18, and 21 hours. The average DLI in the control treatment (no supplemental light) was 7.89 ± 3.02 mol·m^-2·d^-1. Thus, the 4 treatments received slightly more than half of their light from the LED lights. Threshold light intensity was calculated as: Threshold PPFD (μmol·m^-2·s^-1) = 1,000,000 x [17 mol·m^-2 – Current DLI]/Time remaining (s)]. Hence, while each treatment received the same amount of light within each 24-hour period, extending the photoperiod allowed the same amount of supplemental light to be provided at lower instantaneous intensities. The study was terminated after 22 days. Dry weight increased quadratically with photoperiod (R² = 0.50, p=0.003), from an average of 0.53 g/plant with 12-hour photoperiods to 0.75 g/plant with 21-hour photoperiods. In the control treatment, average dry weight was 0.17 g/plant. Leaf chlorophyll content and leaf size of the fully expanded leaves increased linearly as photoperiod increased. Leaf size increased from 57.2 cm² in the 12-hour treatment to 68.2 cm² in the 21-hour treatment (p = 0.023), and chlorophyll content index similarly increased from 9.81 to 12.1 (p = 0.0015). Leaf area and chlorophyll content were higher in all supplemental lighting treatments than in the control (p < 0.0001). These results may be partly attributed to an increased photosynthetic light use efficiency as photoperiod increased and supplemental lighting was provided at lower intensities over longer photoperiods. However, morphological acclimation to photoperiod or light intensity also occurred, as plants developed larger leaves with higher chlorophyll content under longer photoperiods. In conclusion, providing supplemental light in a photochemically-efficient manner improves overall growth of this lettuce variety.