Dynamic temperature control strategy with a temperature drop improved responses of greenhouse cucumbers to long photoperiods of supplemental lighting

Abstract

Supplemental lighting is essential for year-round greenhouse crop production in regions with low natural light conditions. Supplemental lighting for increasing 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 capital costs in comparison to the use of high light intensity. However, long photoperiods cause photo-injury such as leaf chlorosis in greenhouse fruiting vegetables such as cucumbers, and yield is no longer increasing with extended photoperiods. In our previous study, we found that a pre-night temperature drop with dynamic temperature integration (TI) can promote photo-assimilate translocation from leaf to fruit and thus has the potential to reduce photo-injury. This study was conducted from 2016 to 2017 with the aim to test this temperature control strategy for improving the response to long photoperiods including both overhead HPS (high pressure sodium) lighting and LED intra-canopy lighting. The study was conducted in 8 greenhouse compartments, each with 50 m2 of growing area. Two photoperiods (Control (17 h) and long photoperiod (20 h)) of overhead HPS lighting and two temperature integration strategies (control TI NDASH no temperature drop, and Dynamic TI with a pre-night temperature drop to 12.5°C at the end of photoperiod) were applied in the 8 compartments (2 replications). Three intra-canopy light treatments (control NDASH no intra-canopy LED, 17 h or 20 h of intra-canopy LED light) were applied inside each greenhouse compartment. The dynamic TI with a pre-night temperature drop significantly increased fruit yield at the long photoperiod but did not reduce photo-injury for greenhouse cucumbers. Therefore, dynamic temperature integration with a proper pre-night temperature drop can be used to improve fruit production in greenhouse cucumbers grown with long photoperiods of supplemental lighting.