Dynamic temperature control strategy with a temperature drop improves responses of greenhouse tomatoes and sweet peppers to long photoperiods of supplemental lighting and saves energy

Supplemental lighting is essential for year-round greenhouse crop production in regions with low natural light. Supplemental lighting to increase the daily light integral can be added via lengthening photoperiods or increasing light intensity. Light addition via long photoperiods is preferred because of the lower light-fixture costs in comparison with the use of high light intensity. However, long photoperiods cause photoinjury, such as leaf chlorosis in tomatoes and sweet peppers. This photoinjury has limited the yield increase from long-photoperiod lighting. In our previous study, we found that a temperature drop with dynamic temperature integration (TI) can promote photoassimilate translocation from leaf to fruit and thus has the potential to reduce photoinjury. Therefore, this study was initiated in 2013 with the aim of testing this temperature-control strategy to improve the response to long photoperiods, including both overhead high-pressure sodium (HPS) lighting and light-emitting diode (LED) intra-canopy lighting. The study was conducted over three winters (from 2013 to 2016) in eight greenhouse compartments, each with 50 m2 growing area. Two photoperiods (control and long photoperiod) of overhead HPS lighting and two temperature integration strategies (control TI with no temperature drop; dynamic TI with a temperature drop to 13.5°C for tomatoes or 15.5°C for sweet peppers at the end of the photoperiod) were applied in eight greenhouse compartments. Four intra-canopy LED photoperiods were applied to the four plots inside each compartment. Dynamic TI with a temperature drop significantly reduced photoinjury and increased fruit yield at long photoperiods for both tomatoes and sweet peppers. It also reduced heating energy use during cold winter months in the tomato experiments. Therefore, dynamic TI with a temperature drop is an energy-efficient climate-control strategy for improving the response of greenhouse tomatoes and sweet peppers to long photoperiods of supplemental lighting.