**5. Conclusions**

In conclusion, our data indicate that tomatoes grown under extended photoperiods under sole red supplemental light have improved fruit yield compared to those grown under a mixed lighting treatment. In fact, growth under a 23 h photoperiod with red supplemental light resulted in similar cumulative yield parameters to plants grown under a red or mixed light spectrum with 17 h lighting. By utilizing a low intensity and long photoperiod with energy-efficient red LEDs, a decrease in capital fixture cost and electrical costs can be realized. Under both red and mixed 23 h lighting treatments, physiological parameters indicated the presence of photoperiod-related injury at 54 DIT (8 January 2019) during the measurement period. However, during the subsequent production period, photoperiod-related injury was alleviated. Because no experimental parameters were altered, we propose that the increase in peak solar intensity or overall DLI from January to April could play a role in alleviating injury, but further study will be needed to confirm this hypothesis. Interestingly, the rootstocks used were observed to have an interaction with the light spectra and photoperiod length, which led to improved yield from TE plants compared to TK plants. These data suggest that both rootstocks and light spectral compositions/quality can impact photoperiod-related injury. With red light and proper rootstock, the negative yield impact from long photoperiod lighting can be eliminated.

**Author Contributions:** Conceptualization, J.L., B.G. and X.H.; methodology, J.L., B.G. and X.H.; validation, J.L., A.T., C.L. and J.Z.; formal analysis, J.L.; data curation, J.L., A.T., C.L. and J.Z.; writing—original draft preparation, J.L.; writing—review and editing, J.L., B.G. and X.H.; funding acquisition, X.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the Foundational Science Program of Agriculture and Agri-Food Canada, grant number J-002228.001.04, to X.H.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data reported can be found within the manuscript.

**Acknowledgments:** LumiGrow Inc. (Emeryville, CA 96408, USA) provided the Smart LED lighting system and helped to set up the lighting treatments.

**Conflicts of Interest:** The authors declare no conflict of interest.
